RG Holroyd1 and G Fordyce2
1Queensland Beef Industry Institute, Department of Primary Industries, PO Box 5545, CQMC, North Rockhampton, Qld 4702, Australia
2Queensland Beef Industry Institute, Department of Primary Industries, PO Box 976 Charters Towers, Qld 4807, Australia
The paper reviews methods of improving fertility in extensive and semi-extensive conditions in the dry tropics of northern Australia. Basic issues such as sustaining the pasture resource and effective cattle control are prerequisites for efficient implementation of management options. Management options can be classified at 3 levels and should be incorporated sequentially into breeding herds to improve fertility.
These options include;
Weaning rates in herds where these management strategies have been implemented in a structured way have consistently exceeded 80% in a range of seasonal conditions.
Most of tropical Australia is dry tropics. Native pastures occupy most of northern Australia’s rangelands and these pastures, in which perennial tropical grasses are the key plants, occur mainly on low fertility soils and grow mainly in the summer period (November to April). Grazing animals commonly suffer protein and energy deficiencies in the dry season (April to November) and mineral deficiencies, particularly phosphorus throughout the year (1). Annual productivity of cattle is constrained by variable rainfall and extremes of temperature. There are a number of distinct native pasture communities and there is a large variation in annual performance both between and within each of these communities. For example McLennan et al. (2) showed that annual liveweight gains of steers, grazing northern black spear grass pastures, averaged about 100 kg in unsupplemented animals with an annual range of 41 to 154 kg.
Cattle production in this region is characterised by large properties, low stocking rates, low management costs, and low productivity compared with many temperate production systems (Table 1). However, as costs increase and cattle values fluctuate, management must change to increase productivity and maintain profitability. On-going research is continually producing options that enable this to be achieved.
Table 1. Physical and productivity averages of cattle properties in the dry tropics of Australia (north of the Tropic of Capricorn), excluding the vast desert areas of the Northern Territory and West Australia (3).
Brahman or Brahman cross
No. of cows
No. of cattle
89% of cattle
Breeding on the property
Seasonal mating if used
Mating heifers before 2 years
Heifers used as replacements
Bulls used per cows mated
96% of properties
79% of cattle
56% of properties
75% of heifers
43% of properties
12% of properties
14% of properties
31% of properties
57 calves per 100 cows mated
820 per property
30% of herd
37% of total sales
The increasing demands of the live-export trade to Asia and the Middle East and for cattle for the domestic feedlot industry has seen turn-off age reducing for male castrates and for females surplus to breeding requirements. This in turn has increased the proportion of breeding cattle on many properties in the region. Females are a higher risk group of animals to variations in seasonal conditions. Application of good management and husbandry practices to all classes of female cattle is of increasing economic importance in north Australian beef herds to increase reproductive rates and reduce female mortalities
There are a number of ways of defining reproductive performance and commonly used indices include pregnancy rates, calving rates, branding rates and weaning rates. All of these indices have their place and all have deficiencies in obtaining meaningful comparative data from herds. Pregnancy rates are useful with short mating periods but don’t take into account subsequent prenatal, perinatal and postnatal losses. Calving, branding and weaning rates are, respectively, the number of calves born (both alive and dead), branded (at time of fire branding and castration) or weaned as a proportion of cows mated the previous years. Calving rates are often difficult to determine in extensive areas because of property size and often adverse seasonal conditions. Branding rate is the common index of fertility in extensive herds but it suffers from the inaccuracy that, although calf numbers are accurately recorded, cow numbers since mating are usually estimated because of mortalities, animals missed at musters and escaping into other paddocks. Weaning rate is the best index of fertility as it represents the period that the calf is dependent upon the cow for survival. It’s accuracy suffers from the same problem as the calculation of calving rates (3). In reality in extensive herds, use of several of these criteria such as pregnancy rates and branding rates is meaningful to assess herd fertility providing the weakness of each criteria is recognised.
A realistic target for weaning rates for adapted cattle in the dry tropics in average or better rainfall years is a minimum of 80 calves weaned for each 100 cows mated. This may be as high as 90% in extremely good seasons with excellent management. However a minimum weaning rate should be of 70% across a range of years. Where we have been able to get accurate records, we have a number of herds that are capable of achieving these levels. For example, at Swans Lagoon Research Station in the dry tropics of north Queensland, a demonstration of efficient herd management in a herd of about 1000 5/8 Brahman cows has produced a 15-year average branding rate of 83%. This management system was based on continuous mating, conservative stocking rates: 2 annual musters; weaning all calves 100 kg and over at each muster; crisis supplementation; vaccination for botulism and leptospirosis, initial mating of females at 2 years of age; culling at 9 years of age and selection of replacement bulls and heifers using objective performance data.(4)
Herd simulation models are valuable tools in establishing viable objectives and optimal management practices for beef herds. These models put into an economic perspective, the relative importance of manipulating reproductive rates, growth rates and survival rates for particular markets. Such an example is given in the following web address. www.dpi.qld.gov.au/breedcowdynama/ .(5). Outputs from these models give good guides to changes in herd structure and viability of enterprises with implementation of new management practices. Using these models, the implementation of improved management practices that increased branding rates from 50% to 72% on an extensive property improved gross margins from $18 to $36 per adult equivalent (J Boorman, pers coms). However there is a ceiling on increasing reproductive rates in some production systems. Increasing weaning rates from 80% to 90% in semi-extensive areas reduced gross margins mainly because the enterprise sells a greater proportion of females as lighter weight heifers rather than as heavier weight cows (R Cheffins, pers coms). These examples highlight that each business should be evaluated in relation of current market strategies.
Fordyce (6) described a number of steps that need to be addressed by cattle producers for efficient herd productivity in northern Australia. Basic issues such as sustaining the pasture resource and effective cattle control are key criteria for the efficient implementation of secondary levels of management.
Sustaining the pasture resource is important for our northern rangelends as beef production from native pastures will continue to be, by far, the most important use of these rangelands in the foreseeable future. Correct stocking rates, appropriate paddock sizes, use of burning and spelling of paddocks are vital for sustainable production at an acceptable cost.
Effective cattle control is being able to segregate different classes of cattle, keeping them where you want them and being able to recover them when you want to at a minimum cost and effort. It allows for more effective implementation of other husbandry and management practices. Important segregation groups are cull females, small weaners, older weaners, heifers, first-calf cows, mature cows, immature and cull bulls and steers.
These can be classed at 3 levels (6) depending upon a) their value in improving herd production efficiency, first-level options are generally more important; and b) the stage at which they can or should be incorporated into existing management, first level options before second-level options, etc.
First-level management options
Appropriate genotype, supplementation, weaning and weaning management, and botulism control.
Second-level management options
Heifer management, selecting breeding cattle, efficient culling, disease control and bull soundness.
Third-level management optionsControlled mating, spike feeding, dry season segregation of cows and artificial breeding
From the basics through all levels of management, development should be in stages to ensure that resources necessary for any strategy are in place before it is attempted and that the pay-off from each stage of development finances subsequent stages (7). Each of these will be addressed in the following sections.
In the dry tropics of northern Australia, the appropriate genotype has a balance between suitable productive traits for particular markets (eg growth rate, meat quality) and adaptive traits such as parasite resistance and heat tolerance. Unfortunately there is a degree of mutual exclusivity between production potential and environmental adaptation (8). Therefore optimum genotypes have a blend of both with the compromise defined by the production objectives and the level of stress that exists; the latter is related to level of management as much as to the seasonal conditions and type of country. Studies have shown that in the dry tropics that, on average ½ to ¾ Bos indicus is ideal (9). The ¾ cross is more appropriate under more harsh conditions as they have better growth rates (10, 11) (Fordyce et al 1993a; 1993b) and hold condition better in prolonged dry seasons (12). Thus cattle with higher Bos indicus levels are very suited for harsh environments with lower levels of management because of their greater survival ability.
Table 2. Estimated effects of management on production parameters in a typical north Australian breeder herd
|Growth||Fertility||Survival||Reduced costs||Increased return/cost|
|Heifer||First calf||Mature cow||Calf loss||Brand. rate||Bulls||Weaner||Cow/Heifer|
|Sustaining pasture resources||++||++||++||++||++||++||+||++||++||++||++|
|Weaning and weaner management||+||++||++||++||++||++||++||-||++|
|Control of reproductive disease||+||+||+||++||+||+||-||+|
|Selecting breeding cattle||+||+||+||+||+||+||+||+||+||+||+|
|Dry season segregation of cows||+||-||+|
|++ Significant benefit in most years; + Significant benefit in some years; small benefit in most years; Blank: Generally no overall impact; - Opposite effect in many years|
The main breed types in use are high-grade Bos indicus such as the Brahman with a limited number of Sahiwals, and stabilised Bos indicus crosses such as the Santa Gertrudis, Droughtmaster, Braford and Brangus. As well there is limited use of Sanga genotypes such as the Africander and Belmont Red (½ Africander, ¼ Hereford and ¼ Shorthorn). As well there have been limited introductions of Boran and Tuli.
Brahman crosses have greater herd productivity than Sahiwal crosses mainly because of higher growth rates, lower calf losses and better temperament (10, 11, 13). The more fertile Africander crosses have only expressed higher herd productivity in more favourable environments (14). Very little data exists on the productivity of the recently arrived Boran and Tuli apart from studies with their use in crossbreeding programs. The studies of Frisch et al. (15) compared the total productivity of straightbreds, and 2-way cross cows and steers generated from breeds of African (both Sanga and zebu), European (both British and continental) and Indian origins. This work aimed to identify genotypes best suited to particular combinations of environments, production systems and markets. Significant individual and maternal heterosis for production traits was generated in crosses between any of the cow breeds. The productivity of all F1 cow genotypes exceeded that of all straightbred cow genotypes, in some cases by more than 20%, without the need for additional inputs. On pasture and without intervention to control parasites, not all crossbred steers had higher growth rates than Brahman contemporaries. In feedlots, crossbred steers generally had higher growth rates and better feed efficiencies and produced more tender meat than Brahman contemporaries. These results demonstrate that all of the breed groups have a role in improving productivity in tropical Australia (15). It is a question of finding the right genotype for the particular environment, management system and target market.
Rotational crossbreeding systems sustain a higher level of heterosis than inter se matings of the average genotype. However the long term success of various rotational crossbreeding relies on good cattle control, very good management and a high level of commitment. A limitation of this system in extensive areas in the dry tropics is availability of Bos taurus bulls sufficiently adapted to the environment (16). Sections of industry, particularly the larger pastoral companies, are creating their own composite breeds based on a mixture of Bos indicus, Bos taurus and Sanga genotypes with the aim of generating cattle for premium beef markets.
An issues being addressed by the Co-operative Research Centre for Cattle and Beef Quality, an initiative of the Australian Government, is to increase the knowledge, in northern Australian environments, of genetic relationships between components of herd profitability in order to improve feed efficiency and beef product quality without unduly comprising breeder herd performance and adaptability. In other words, if industry selects and uses sires for beef quality attributes such as retail beef yield percentage and intramuscular fat percentage, does it compromise female fertility and survival? This project is in its infancy but involves a number of pastoral companies and research organisations across northern Australia (17).
Liveweight of the breeding cow is the key driver of fertility in northern Australia. Fertility of Bos indicus or Bos indicus cross cows is limited primarily by the time required for resumption of ovarian activity. Influencing cow liveweight and body condition coming into the mating season is important for fertility and this can be manipulated primarily in two way;
The relationships between weight/condition and fertility are well researched. Simply, heavier/better conditioned heifers and cows have higher pregnancy rates. Most Brahman or Brahman crossbred lactating cattle will cycle within 3 months of calving if they are in forward store-prime condition and are on good feed; ie, they will generally have better than an 80% pregnancy rate. For typical first-calf heifers, this is around 380 kg; for older breeders, the weight is about 420 kg.
The effect of liveweight on female fertility has been examined in a large number of studies. For example, Figure 1 demonstrates the relationship between liveweight and start of mating weight for Droughtmasters (50% Bos indicus) in the dry tropics of North Queensland (18). The relationships tend to be linear with lactating first calf cows and with mature cows either curvilinear (18) or linear (19). In these studies the effects of liveweight on fertility were most noticeable in lighter than in heavier cows.
Figure 1. Relationship between start of mating liveweight of lactating first-calf cows (o) or mature breeders (·) and subsequent pregnancy rate. Date derived from 50% Bos indicus females in the dry tropics of north Queensland (18).
Dixon (20) summarised all of the available data sets from northern Australian herds and concluded that for cows less than 340 kg at start of mating, it would be reasonable to expect a 5% increase in pregnancy rate for each additional 10 kg increase in liveweight at mating. For cows greater than 340 kg liveweight at mating, for each additional 10 kg, the response is more likely to be a 3% increase.
Some principles developed by a number of researchers in northern Australia and summarised by Dixon (20) are;
Figure 2. The relationship between the reduction in liveweight loss of non-lactating pregnant breeders during the dry season due to feeding urea-based supplements and the increase in pregnancy rate during the following wet season
Feeding cattle to achieve target weight/body condition is at a cost. To minimise feeding costs but maximise returns (efficiency) options include
For high breeding efficiency, feeding must be targeted at enhancing ovarian function, particularly at critical times.
There are different approaches for supplementary feeding of nitrogen supplements to breeders during the dry season, strategic supplementation or crisis supplementation.
Strategic supplementation is the provision of long term, low levels of nitrogen supplements mainly based on urea. This is as home-made dry licks, roller drums, commercial or proprietary blocks or water dispensers. The general aim is to provide 25-30 g urea per breeder per day during the early dry season and 45-60 g urea per breeder per day during the late dry season (20). During severe dry seasons, urea-based supplements substantially reduced loss of breeder body reserves during the dry season. However in years when there was out-of-season rain sufficient to grow at least a “green pick” during the dry season, there was no response to urea-based supplements, presumably because breeders are able to meet their requirements for rumen degradable protein from pasture.
Where strategic urea-based dry season supplements reduce breeder mortality due to under-nutrition, they are highly profitable. Where increased weaning rates are the only benefit, the return from urea supplementation is marginal. However there are other benefits which should be considered. These include reduced risk and greater management flexibility, there tends to be a tighter calving pattern leading to lines of weaners which are heavier and more similar in liveweight, breeders are in better body condition thus potentially more valuable as culls. Economic returns from urea-based supplements are highly sensitive to supplement input costs. With efficient water medication systems, the supplement costs may be less than half the cost of dry lick systems, and one quarter the cost of lick block systems. Even when the increased economic return depends entirely on increased weaning rates, urea supplementation using water medication can be highly profitable (20).
Crises or survival supplements are short term, higher levels of nitrogen and energy supplements based mainly on true proteins such as cottonseed meal and molasses as an energy source. These types of supplements are used to avert mortalities in years where there is a delayed seasonal break, thus allowing a decision on supplementation when seasonal conditions are known.
Protein meals such as cottonseed meal (40% protein) are fed at about 0.35% of body weight; about 1.4 kg/day for 400 kg cows (21). Molasses mixes fortified with 8% urea are provided ad-lib. and cattle limit themselves to about 0.5% of body weight; about 2 kg/day for a 400 kg cow (22).
Protein levels in tropical pastures drop rapidly in the wet season. Supplementation of lactating first calf cows and mature cows over the wet season period with both protein and energy concentrates has increased cow weight and body condition and increased calf growth as a result of higher milk yields. However wet season supplementation has had little effect on either post-partum anoestrus or pre-weaning conception rates (23, 24, 25).
A widespread problem in Australia's tropics is phosphorus deficiency. Major improvements in growth and fertility are achieved through supplementing with phosphorus and using management, particularly weaning, which reduces requirement. Because of much greater relative deficiencies of other nutrients in the dry season, phosphorus supplementation is mostly restricted to the wet season. Supplement requirements in deficient areas depend on level of deficiency, age, and reproductive status, but usually is about 5-10 g/day for lactating cows (26). Deficiencies of nutrients other than nitrogen, sulphur, phosphorus and energy are generally localised.
Weaning management is a critical component of overall management to achieve high breeding efficiency in the dry tropics. Weaning improves body condition of cows, thereby increasing fertility and probability of dry season survival. Good weaning management is a key factor in achieving average weaning rates of 80-85% in the dry tropics (27).
The general recommendation for the dry tropics is to wean down to 3 months of age at each muster (28). This is about 100 kg but is usually less than 100 kg at later musters in the dry season and drought. With restricted mating periods, the minimum weaning age may be higher at the first weaning in each year, but generally all calves should be removed at the time of bull removal from the herd. It is important that weaning be carried out at least twice yearly in continuously mated herds. The optimum times are towards the end of the growing season when cows are on a maintenance plane of nutrition and about 4 months later in the dry season before significant numbers of cows calve (29). Weaning only once a year in continuously-mated herds may lead to more cow deaths because cows which conceive as a response to weaning do not have a calf which is old enough to wean the following year. This results in more cows lactating in the dry season with a consequent higher survival risk (29).
The suckling stimulus and poor nutrition, particularly during lactation, are the major reasons for delayed post-partum cycling. Bos indicus cows are more sensitive to these effects than Bos taurus.
In cows, weaning achieves weight preservation, eg, 25-40 kg (27, 30) which reduces the management costs of cows. In herds where sound early weaning practices have been introduced, early weaning markedly reduces the need to survival feed cows in extended dry periods. We often use the term “early weaning”. It is a misleading term as it suggests that all calves are weaned at a younger age or that the time of weaning is earlier in the year. Most calves are still weaned at the same age and often at the same time of the year as under traditional management but up to 20% of calves are weaned younger (down to 3 months of age and 100 kg). This group requires special attention.
The short-term trigger responses to weaning in empty cows increases pregnancy rates in these cows by about 50%, with most conceptions in the 2 months after weaning (27, 30). Empty cows in poor condition at weaning will not cycle until they have received at least 30 days of above-maintenance nutrition. The weight and body condition advantage is often carried into the next mating season, and may substantially improve conception rates in that year although this is not always the case (31). This is the most important fertility advantage rather than having empty cows conceiving as a result of weaning them now and being poor survival risks the following year.
Mothering ability may also be poorer in low-conditioned cows. One study (G. Fordyce, Unpublished) showed a trend for late-weaned cows, which entered the drought in poorer condition, to lose a higher proportion of calves between confirmed pregnancy and weaning (13% v 7%).
At weaning calves lose a high quality supplement; although under poor nutritional conditions, the dams of some calves may cease to lactate, leaving small calves at high risk. The minimum weaning age is therefore the best compromise between requirements for the cows to sustain high calf output and survive and the costs and infrastructure available for feeding calves. The major cost of weaning, especially in very young calves is the extra nutrition which they require and the infrastructure and management necessary to carry this out. Six-month-old calves are relatively easy to manage; nutritional management should aim to keep them at maintenance or better. The cost of weaning increases for younger calves as they need to be kept growing by at least 0.2 kg/day to prevent stress-related disease during periods of undernutrition and to compensate for the age disadvantage within their year group.
The growth rate of calves post-weaning is critical to the success of weaning programs. Holroyd et al. (32) found that young calves weaned towards the end of the growing season and not provided with adequate supplements to maintain growth were 54 kg lighter than contemporaries weaned 3 months later. Over the following 3 years, those weaned early achieved 75% compensation for this difference. Weaning calves younger and providing appropriate nutrition does not delay turn-off age (at the same weight) as compensation for any growth penalty due to weaning younger occurs in subsequent years (33).
Table 3 shows recommended supplementation of weaners. Weight is a more important determinant of supplement requirement than is age. This is because, as the dry season progresses, weaners of the same weight are older, at the same time, available pasture is decreasing in quality. About 100 kg and 150 kg are the probable weights at which calves can change from a complete ration to a true-protein-based ration and from a true protein- based supplement to an inorganic protein-based supplement respectively and maintain satisfactory growth and survival rates (34,35). To avoid weight loss in the first dry season post weaning and thus avoid delays in reaching puberty, 100-150 kg heifers should be supplemented with energy and true protein, rather than NPN sources.
Suitable supplements include:
Growth rates of young calves, which are supplemented appropriately, match that of unweaned calves during the dry season (30) as dams have low milk yields when grazing poor quality pastures, growth rates of suckling calves may average as low as 0.3 kg/day.
Temporary weaning (48-72 hours) has frequently received publicity as a method of inducing cyclicity in lactating cows. A number of trials have shown that increased weaning rates following 48-hour weaning is almost a chance event. However detailed studies show that reversal of the suckling stimulus may not be achieved for at least 5 days after weaning. This supports the general conclusion from temporary studies that consistent increases in pregnancy rates are achieved only when used in conjunction with progesterone priming (3). Thus temporary weaning is only applicable in overcoming the problems associated with artificial breeding programs using lactating cows. It has no place in extensively managed herds as it promotes conception in cows that then become greater survival risks the following year.
Table 3. Feed requirements of weaners during the dry season (16)
Weight and age
|Growth objective||Pasture quality||Yard feeding|
|Urea + TP blocks
|Hay 1.5-2.5 kg/day|
30 g urea daily
|Dry licks + TP
|200 g/day||Cottonseed or
0.5 kg daily
0.5 kg daily
hay 1.5 kg daily
|300 g/day||Calf pellets
16-18 % CP
1-2 kg daily
16-18 % CP
1-2 kg daily
1.5 kg daily
|Good grass:||Feed which has good leaf and colour|
|Poor grass:||Feed which has little leaf or colour|
|Urea + TP blocks:||Blocks which contain urea and 5-10% true protein|
|Dry licks:||Urea-based licks|
|Dry licks + TP:||Licks which also contain a protein meal, eg, cottonseed meal|
|M8U||Molasses with 8% urea (completely dissolved)|
|MUT||Molasses with 5% urea (completely dissolved) and 10% true protein meal|
Botulism caused by the toxins from Clostridium botulinum types C and D is one of the most economically important infectious diseases of northern Australia. Major precipitating factors are phosphorus deficiency and drought leading to bone chewing and carrion eating. Phosphorus supplementation will reduce the prevalence of the disease. Despite the extensive use of vaccines, botulism is still an important cause of mortality. A problem has been the practice of starting vaccination programs on a single-dose resulting in ineffective long-term protection. This situation is further complicated by extended periods between booster vaccinations, low mustering efficiency and poor vaccination techniques (37). Most outbreaks in recent years have been in young cattle that have not been adequately protected. Diagnosis is based on history, clinical signs and by exclusion of other diagnoses. A botulism ELISA has been used to provide supporting evidence of disease exposure of vaccination failure (38). Only bivalent vaccines (Type C and D toxoids) are available in Australia and effective prevention requires all at-risk cattle to be vaccinated annually. A vaccine with a new adjuvant is now available which uses only a single dose for 12 months protection
In well-managed herds, an accepted level of reproductive wastage from early pregnancy to weaning is about 10%. This usually falls into the categories of 3% prenatal, 3% perinatal and 3-4% postnatal (39). Heifers and first calf cows are the groups most likely affected by reproductive diseases, with older cows having developed some degree of immunity through previous exposure. There are a number of important diseases in northern Australia affecting fertility.
Bovine ephemeral fever is the most important viral disease affecting productivity in northern Australia. Biting midges (Culicoides spp.) and some species of mosquitoes transmit it. These insects are most active in summer and autumn months and this determines disease prevalence. The disease classically manifests itself as fever and lameness lasting for about 3 days ("3-day sickness"). Death is from exposure and dehydration if the affected animal becomes recumbent particularly in extreme environmental conditions (37). Older steers and bulls seem to be more prone to the condition. Abortions may occur during the second and third trimesters (40). Midpiece abnormalities in spermatozoa have been associated with the febrile reaction of ephemeral fever (41). Diagnosis is confirmed by paired serology. Following natural infection, cattle generally remain immune for at least 2 years. It is recommended that at least bulls are vaccinated to prevent temporary infertility but vaccination of commercial breeders appears not to be warranted.
Akabane virus infection is recognised as the major cause of the congenital abnormalities, arthrogryposis and hydranencephaly. The main vector is Culicoides spp. Seroprevalence is higher in cattle at higher stocking rates. The impact of Akabane on reproductive losses in northern Australia is far from understood. Typically following an epizootic of Akabane virus infection, there is sequentially an increase in abortions and stillbirths. This is followed by the birth of calves unable to stand (encephalopathy), then calves with varying degrees of arthrogryposis (one of more limbs affected and may be associated with dystocia), hydrancephaly (blind ‘dummy’ calves) and microcephaly (‘dummy calves’). In one detailed study, 18% of infected females produced an abnormal calf and the highest incidence of abnormalities occurred following infection of pregnant cows between the 3rd and 6th month of gestation (40). After infection cattle develop strong immunity most probably persisting for life. There is no evidence of cross-immunity between Akabane and other arboviruses such as Aino. There is no vaccine available.
Bovine pestivirus (Bovine viral diarrhoea virus), with the development of improved diagnostic techniques, has now been associated with a spectrum of significant reproductive loss such as fertilisation failure, embryonic mortality, abortion, foetal mummification, congenital abnormalities and stillbirths. These conditions occur following infection of susceptible females between mating and mid-gestation. Most infections occur as a result of close contact between a persistently infected (PI) carrier and susceptible cattle. PI carriers arise from transplacental infection of susceptible females pregnant between 25 to 125 days of gestation. About 1% of cattle are PI (40). Changes in herd management can create greater opportunities for this virus to cause losses. Segregated heifer management, cell grazing, intensive AI programs and intensive weaner management create opportunities for production losses associated with the virus. Pestivirus is less likely to remain endemic in well managed herds as PI carriers are poor performers, usually quickly identified and culled (42). We have no commercially available vaccine in Australia.
Campylobacter (vibriosis) and trichomoniasis are common infectious venereal diseases causing repeated return to service associated with embryonic mortality, abortions in mid gestation and pyometra. Branding rates in infected heifer groups can be 20 % units less than non-infected groups. In all-year round mated herds, the disease is often not recognised (37). Campylobacter is diagnosed with an IgA ELISA on vaginal mucus. All bulls should be given primary and secondary vaccinations prior to first mating and then vaccinated annually at the final round of mustering. Maiden heifers may also be vaccinated prior to mating. Trichomoniasis is diagnosed usually from preputial washings. Culling both aged and infected bulls and seasonal mating can reduce the impact of these diseases.
Leptospirosis is widespread but the prevalence tends to be low to moderate within herds. Leptospirosis has been associated with increased prenatal and perinatal losses in some herds. Recent work would suggest that L. hardjo is not abortigenic (42). Responses to use of bivalent vaccines have been variable. Vaccination of weaner heifers 4-6 weeks apart then a booster prior to mating is the recommendation. Subsequent annual boosters should be given in the second round prior to calving.
Puberty: There is a large variability in first oestrus in Bos indicus and Bos indicus cross heifers. This has management implications for maximising fertility of this class of animal at first joining. Generally about 80% of heifers weighing 275 kg will conceive (43). Many heifers reach first oestrus at well below 200 kg; equally, many have not reached first oestrus at 400 kg.
Key outcomes from recent Swan's Lagoon research (44) on time of first evidence of oestrus in Brahman cross heifers are:
Yearling mating: In extensive herds, many heifers are unintentionally mated as yearlings because of lack of bull control. Many producers prefer to only mate yearlings over 300 kg as they consider that if smaller heifers conceive, their survival chances are low, and the chances of being stunted are high. It may be better to mate heifers as yearlings, even if they are only 200 kg because cycling yearlings, if not mated early in the year, will almost certainly conceive later in the year. This leads to out-of-season calves, higher costs to manage these, and a high chance of mortality (45).
In studies of mating yearlings, (about 14 months of age at joining), pregnancy rates have ranged from 25 - 63% in Brahman (46) and 33% in Brahman cross heifers (47). However annual mortalities increased to 4.5% and about a third of these were calving related (46, 47). This is compared to an annual average of 2% in heifers mated as 2-year-olds. Pregnancy rates of these yearling heifers, as lactating first calf cows is variable and seasonally and nutritionally dependent but have ranged from (46 - 85%). Yearling mating may achieve increases in lifetime calf output of 0.2 - 0.4 calves in heifers over 150 kg at weaning. Rearing a calf at 2 years of age only appeared to stunt heifers by 4.5 years of age if they failed to miss a calf in a year (47)
Heifer management: Under continuous mating, initial lactation of heifers often occurs during the dry season because of lack of cattle control. These animals have a high mortality risk, and a poor chance of re-conceiving whilst lactating. Segregation of heifers to provide preferential nutritional management is a cost-effective way of improving fertility. A recommended heifer management system is based on the principle that a heifer is a female up to 3.5 years of age, irrespective of reproductive performance (46).
The major features of recommended management for heifers are:
The timing of initial mating of heifers is critical. In the dry tropics, initial lactation should be timed to coincide with the best nutritional conditions. A short seasonal mating should be used starting in January-February. This coincides calving with onset of first rain. Bulls should then be removed at the first weaning round in April/May to give a 3-4 month maximum mating period.
For 1-year-old heifers the mating period should also be restricted to prevent 2-year-olds calving in the mid-late wet season, thus reducing the potential for dystocia. For 2-year-old heifers, if they are not pregnant by the end of the restricted period, they should be culled for low fertility, except in years following very poor nutritional conditions when delays in puberty result in unacceptably low overall conception rates. For 3-year-olds, they are transferred to the cow herd at the end of mating and then subjected to mature cow mating practices.
The primary benefits of the proposed heifer management system are (45):
The proposed system has additional costs but these costs are offset by extra female sales. Computer simulation models forecast that the investment is recovered within two years of commencing the new management practice. As well the models forecast that profitability of extensive herds will be significantly improved through adoption of improved young female management practices. This is estimated to increase gross margins by $5,000/1,000 head of cattle (45).
The objective of heifer and cow culling is to remove those females with low fertility as fertility is repeatable (12) and some fertility traits are heritable (48). After about 8 years of age, cows with deteriorating dentition have poorer ability to forage and a higher survival risk under drought conditions (49). Culling should be based on reproductive records and visual assessment for physical normality and traits of other economic importance.
Pregnancy diagnosis can be used to increase efficiency of culling by identifying low-fertility cows up to a year earlier. Successful adoption of pregnancy testing requires accurate technique. Trained, experienced veterinarians can achieve close to 100% accuracy using rectal palpation when cows are at least 6 weeks into pregnancy.
Sale of females surplus to requirements for breeding is an important source of income. Often culled females are not suitable for sale because of poor body condition. Prevention of pregnancies by surgical spaying (ovariectomy) is widely used especially where segregation and cattle control are problems, to improve their sale value. There is no consistent evidence that spayed females grew better than entire females.
Traditional spaying involves removal of the ovaries through an incision in the left flank or by passage spaying through an incision in the dorso-cranial aspect of the vagina. Flank spaying has been the predominant method used but results in hide damage and carcase trim and has been opposed by welfare groups. However in recent years there has been a dramatic increase in the use of the Willis dropped-ovary technique. Compared to flank spaying, it has many advantages for the extensive cattle industry. These include higher numbers can be spayed per day, minimal surgical complications, no hide damage or carcase trim. It avoids the need to use electroimmobilisers and is a more humane and aesthetically acceptable means of spaying (50). The instrument and the technique are described in detail by Habermehl (51).
The Willis dropped ovary technique is a significant advance in pregnancy control in the extensive cattle industry with real benefit to the welfare of the animal and the industry. However the complete elimination of spaying is the ultimate aim. Research on other methods of fertility control has been on-going. Use of intrauterine devices has shown not to be effective (52). GnRH agonist bioimplants placed subcutaneously in the dorsal surface of the ear show promise. Female cattle treated with GnRH agonist have a downregulated anterior pituitary and cannot initiate preovulatory surge releases of LH. Ovulation is blocked and cattle remain in anoestrus. Ovarian activity returns to normal within a relatively short period after discontinuation of treatment (53)
Selection criteria when breeding or purchasing replacement bulls and heifers must be consistent with future production objectives. The economic value of each trait is dependent on its effect on production efficiency and market value, its heritability, and its measurement accuracy. Selection emphasis on different traits should be balanced because selection for extremes for a trait, eg, growth rate, almost invariably has undesirable effects on other important traits such as dystocia and must be avoided. Use of EBVs for various traits can minimise this.
In most well managed herds, between 50-60% of heifers are required as replacement breeders particularly when efficient culling procedures are in place.
Growth rates are best assessed using objective measurements. The best available method for improving growth rates is to use EBVs (similar to EPDs). This is roughly twice as efficient as using weight ratios or raw weights. However, apart from purchasing replacement bulls, use of EBVs is still not an option at this stage for the vast majority of commercial herds.
Important aspects in selection of breeding cattle in the extensive herds in the dry tropics include:
This is discussed in detail in the accompanying paper of this proceedings (60).
The main points with bull selection and management for extensive herds in the dry tropics are:
Controlled or seasonal mating is the practice of leaving bulls with cows for a number of months of the year rather than having bulls with the cow herd all year round. The major objective is to match the period of peak nutritional demands of the lactating cow with the peak nutrient in the pasture. In the dry tropics, controlled mating aims to prevent dry season lactation rather than restrict the calving period.
There are many advantages in controlled mating including:
However in extensive areas there are a number of advantages of continuous mating. More calves may be born throughout the year in continuous mating but the spread of calvings is much greater. Seasonal conditions can move peaks of conceptions. In continuously mated herds, conceptions are related to rainfall in the previous 2 months. (61). Therefore about 2/3 of conceptions occur in the optimum period. With controlled mating, late rainfall may significantly reduce conception rates.
In the dry tropics, mating commences from November to January onwards for 5-7 months. This causes the start of calving in the following year to coincide with the start of the wet season. Good quality pasture is matched with peak nutritional needs of the lactating cow. Calves are then weaned at the end of the growing season. Where mating periods have been for only 3 months in the dry tropics, weaning rates can be 10% lower than continuously mated herds because of the variability of the onset of the wet season.
The implementation of controlled mating can’t be done without temporarily reducing weaning rates, unless there is already a distinct calving peak in the herd. It assumes that other management practices are already in place such as sound weaning management, dry season supplementation and separate heifer management as outlined. The two essential and simultaneous steps are ensuring that heifers are initially mated for 3-4 months and that bulls are removed from the herd during the dry season for a gradually increasing period of time in successive years.
There is a range of physiological states of cows within a herd during the dry season. This means that there are a range of classes of animals with different survival risks and reconception rates. Those with the highest risk of death and lowest probability of reconception can be segregated in the early dry season for preferential management such as supplementary feeding. The early dry season is the best time to segregate cattle as few young calves have to be mothered-up.
Reproductive status, particularly lactation in the mid-dry season, is a better indicator than body condition in predicting survival rates of different classes of females. The data of Fordyce et al. (47) (Table 4) demonstrates the effect of body condition and pregnancy status on survival of 3 to 7 year old Bos indicus cross cows during a drought. Cattle were observed in August with survival feeding commencing in November and ceasing with useful rain in January
Table 4. Effects of body condition and pregnancy status of 3-7 year-old Bos indicus cross cows in August on their probability of survival in the ensuing dry season (49)
|Body condition score|
|Stage of pregnancy||Poor - ≤3||Backward - 4||Store - 5||Forward - 6||Prime - 7|
|1st trimester (1-3 months)||0.58||0.79||0.87||0.92||0.98|
|2nd trimester (4-6 months)||0.43||0.67||0.78||0.86||0.97|
|3rd trimester (7-9 months)||0.33||0.58||0.70||0.80||0.96|
Segregation is not viable under all conditions. To make it worthwhile, it must be performed by at least mid-way through a dry season while there is still plenty of standing feed available and cows are still in reasonable condition
Spike feeding is the feeding of a high quality supplement to late pregnant heifers or cows for a short period in the late dry season. Feeding should be for about 50 days, starting 6-8 weeks before calving is due to commence. The strategy increases conception rates in lactating cows in the following year by an average of 15% and increases lifetime calf output by 0.3 calves on average (62). Spike feeding reduces post-partum anoestrus intervals via a mechanism which is not dependent on the effects on weight or body condition (63).
Artificial breeding (AB), mainly from artificial insemination, is not widely used in the extensive beef industry. AB is generally used to generate a better genetic base in a herd by using semen from high value bulls. AB is generally not used to improve reproductive efficiency but to produce higher value or more efficient progeny; for example for producing steers from bulls that have high EBVs for retail beef yield or marbling.
The major limitation to the use of AB is one of economics. The cost of a calf produced from AB is about 3 to 6 times that produced from natural mating. This is because of the additional infrastructure, additional labour and cattle handling, veterinary and drug costs that are required to run successful AB programs.
At present the use of AB in the extensive beef industry is mainly restricted to the stud sector. Large scale genetic assessment schemes such as Group Breedplan cannot be successfully conducted without the use of AB to provide sufficient link sires (64). The need to provide these genetic linkages, plus improvements in synchronisation programs will see the gradual increase in use of AB in the extensive beef industry. However it is unlikely that it will ever exceed 10% of all matings unless there are some major reductions in costs and improvements in reproductive performance.
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