BRAHMAN NEWS MARCH 2012 ISSUE #174
Developments in reproductive biotechnology have provided the opportunity to increase the numbers and availability of high genetic merit seedstock.
However, to obtain the maximum benefits of these 'hi-tech' programs it is important to acknowledge the key herd management and biological factors that influence reproductive performance.
For instance, with the increased use of MOET/IVF programs it is important to appreciate that a number of complex interactions that affect puberty, oestrus, ovulation and conception.
This article looks at some of the basic factors that affect the reproductive behaviour of females.
The oestrus cycle of a Bos indicus female ranges between 17-24 days. Within this range various factors affect the cycle length including season, nutrition, disease, age and the production status of the cow.
Cycle lengths are longer and the incidence of silent heats greater during the environmental and nutritional stress periods that occur in subtropical-tropical climates.
"High humidity and high temperatures increase silent herds"
Irregular cycles are more frequent when stock experience excessive weight loss at calving and during early lactation. Irregular cycles are also highly related to low condition scores pre and post calving.
Diag 1: Schematic representation of the oestrus cycle
The duration of oestrus in Bos indicus cattle ranges between 10 to 15 hours but can vary between 2 to 26 hours. Variations in the length of oestrus are due to within breed peculiarities, environmental and social factors.
During oestrus, Bos indicus cattle usually stand for service for a relatively short period of time with mounting activity greatest at daybreak.
"Most oestrus activity occurs at night especially at dawn"
Differences and fluctuations in the intensity of oestrus are most often associated with nutritional and environmental factors. Other factors that affect the intensity of oestrus include respiratory disorders, lameness, cystic ovaries, and changes to the herd structure which upset the social order.
The oestrus cycle consists of four phases; proestrus, oestrus, metestrus and diestrus.
The proestrus-oestrus period of the cycle is usually referred to as the follicular or oestrogene phase whereas the metestrus-diestrus stage is the luteal or progestational phase (Diag 1).
The reproductive cycle is controlled directly by hormones from the ovary and indirectly by hormone from the hypothalmus and the pituitary gland.
Under the stimulation of follicle stimulating hormone (FSH) from the pituitary gland, the ovary produces increasing quantities of oestrogens. The first phase of the reproductive cycle is referred to as the 'building up' phase. During this phase the ovarian follicle surrounding the ovum increases in size. Oestrogens absorbed from the follicles enter the blood stream promoting an increase in cell growth in the uterus in preparation for oesteus, ovulation and subsequent pregnancy.
This is the period of sexual activity in the female and is largely determined by the levels of blood oestrogen.
"Oestrus is shortly followed by ovulation"
Just before ovulation the follicle is enlarged and the enclosed ovum undergoes maturation changes. Oestrus terminates about the time the follicle ruptures or ovulation occurs. At this time the ovum is expelled from the follicle to be captured by the upper part of the fallopian tube.
This is the post ovulatory phase during which period there is a decrease in oestrogen and an increase in progesterone levels (Diag 1).
During metestrus the cavity left by the rupturing of the follicle begins to reorganise and this newly organised structure (the corpus luteum or yellow body) secretes progesterone.
This is the period where the progesterone from the corpus luteum has a marked influence on the uterus in preparation for the embryo to implant. If pregnancy occurs the corpus luteum remains intact. If the ovum is not fertilised or the embryo fails to implant and pregnancy does not occur ,the corpus luteum regresses.
With the regression of the corpus luteum there is a new wave of ovarian follicles develop and a new proestrus period is initiated (Diag 1).
Stock behaviour provides a general pattern of visible events which can act as a guide (Table 1).
"The most reliable sign of oestrus is standing to be mounted"
Whilst cows may be attractive to the bull for several hours either side of oestrus, the number of mountings and penetration by Bos indicus bulls can be 10 times less than Bos taurus bulls. Bos taurus bulls have been observed mounting 35-50 times over a longer duration.
Table 1: Common oestrus behaviour
|Obvious Signs||Less obvious signs|
|Licking and rubbing||Restless and nervous|
|Sniffing vagina and urine||Reduced grazing|
|Mutual chin resting||Bellowing|
|Mounting other cows||Vaginal mucous discharge|
|Standing to be mounted||Swollen vulva and frequent urination|
Due to the shorter heat periods, less mounting and diurnal activity, Bos indicus stock are sometimes called "shy" breeders . Unlike Bos taurus bulls, Bos indicus matings are characterised by 1-3 mountings which are frequently at night time.
Changed social interactions due to mixing of groups, the number of cows, introducing strange cows or strange surroundings can affect oestrus behaviour.
The number of cows on heat at any one time can affect the frequency and intensity of heat activity. Mixing groups of cows or confining stock has been shown to reduce oestrus behaviour. Bullying of stock (eg heifers) can reduce heat activity.
Table 2: Effect of number and times of observation on heat detection
|NUMBER OF OBSERVATIONS||OBSERVATION TIME||DETECTION RATE (%)|
Mucous discharge (quantities and/or consistency) at oestrus varies between stock with the greatest amounts of discharge occuring when the cow is mounting, lying down or is urinating. It can be easily missed, especially at night and is therefore (despite popular belief) a less reliable indicator of oestrus.
The intensity of signs of oestrus appears to be related to conception rates. Stock displaying moderate-strong heats generally have better conception rates than those showing weak heats. Excessive oestrus behaviour (eg nymphomania) is often associated with low conception rates due to the incidence of cystic ovaries.
Accurate heat detection and reproduction management is essential for short calving intervals and suitable calving rates. It is particularly important to the outcomes of AI, MOET and IVF programs and monitoring herd fertility and the reproductive soundness of bulls.
"Poor heat detection compounds infertility problems"
Heat detection should be simple, practical and accurate. Successful heat detection requires that the cows are well identified, correct records are maintained and there is sufficient time for observation by experienced staff.
"Observation 3-4 times/day improves detection by 80 per cent"
Table 2 shows that more observations at the most appropriate times improves heat detection dramatically.
Note: Failure to detect heat properly means:
Diag 2: Best time to breed
The biggest problem in heat detection can be that most of the heat activity (70 per cent) occurs between evening and morning. This coupled with short standing heats and reduced mountings of Bos indicus cows emphasises the importance of observation management by experienced personnel.
A number of factors influence the rate of maturity and subsequently the age at which heifers reach puberty.
Puberty in Bos indicus stock is generally later than Bos taurus stock and Bi x Bt crosses.
"Puberty is affected by numerous genetic and management factors"
In recent years it has become clear that puberty is not a clear cut phenomenon as previously supposed.
Puberty involves the transition from a state of ovarian inactivity to one in which there are regular ovulations and conception is possible. It involves a complex interplay of endocrine, management, genetic and environmental factors.
"Puberty is not synonymous with ovulation"
Heifers may not be capable of successful conception until after several oestrus cycles. Puberty is a gradual, rather than an acute event and only occurs when the genitalia secrete sufficient hormones with full development of the reproductive organs and endocrine system.
That is, in some heifers first oestrus (conveniently, the first indication of puberty) is not followed by ovulation and a number of so called "silent" oestruses may occur before the start of persistent cyclic ovulations. Under natural conditions heifers are usually not capable of successful reproduction during this period of silent oestruses. This period is referred to as non pubertal oestrus (ie NPO) or adolescent infertility.
NPO is significantly longer in nutritionally challenged environments and more common in heifers that have failed to reach suitable growth targets.
"Weight relative to body size affects the onset of puberty"
In a given environment, heifers that have the ability to reach early puberty at a suitable weight for age, cost less to rear to breeding and calving age and subsequently contribute to the business earlier than late maturing heifers.
Estimates for the age at puberty for Bos indicus cattle in subtropical and tropical environments range from 12 to 24 months depending on genetics, management and environmental factors.
Heifers reach puberty at a certain weight for age and skeletal development. Variations occur amongst strains of Bos indicus stock and even between families of females.
In suitable environments most heifers reach puberty at 30-40 per cent of their adult body weight. In poor environments heifers may not reach puberty or develop persistent cyclic ovulations until they reach a higher percentage of their adult weight despite adequate frame scores.
Estimates of heritabilities of age at puberty, first conception and first calving are generally low, 0.4, 0.15-0.25 and 0.10-0.20 respectively, indicating that these traits are highly influenced by environmental and management factors. Nevertheless faster growing-early maturing heifers are more likely to reach puberty at a younger age than high frame score heifers.
Studies suggest that the sire and dam and their genetic interactions affect age and weight at puberty and outbreeding as compared to close breeding, impacts positively on the onset of sexual maturation.
The major factors controlling the onset of puberty are body weight relative to skeletal size and age. Unless heifers reach desired target weights they are less likely to have successful reproductive cycles and long term pregnancies.
"Target weight to age is related to nutrition"
Under developed heifers generally have extended prepubertal periods and a greater incidence of NPO. Heifers of a suitable size in a prepubertal state respond positively to an increase in intake. Alternatively, cycling heifers whose nutrition is severely restricted quickly display anoestrus even though they have exceeded the critical pubertal bodyweight.
Heifers on suitable planes of nutrition not only reach puberty earlier, they are more fertile and have stronger ovulations. Studies show that the ovaries and genitalia grow faster with heifers that have suitable weight/growth rates, than heifers on restricted diets (low quantity/quality of dry matter).
Well grown heifers cycle earlier and withstand the stresses of gestation, calving and lactation and conceive earlier post partum than less developed heifers.
"Well grown heifers should be bred as early as physiologically possible"
Provided the heifers have reached suitable weight for age targets, further growth and mature weight is not compromised by early calving in nutritionally sound environments.
Heifers calving late due to over and under feeding, have been found to have lower lactation yields, longer first dry periods, longer calving intervals, require more services for conception and are more prone to calving problems, than well managed heifers.
The possible drivers of sexual maturity and puberty involve an increase in pituitary gland output, an increased size and activity of the ovaries and the development of the endocrinie system.
The development of the endocrine system results in the secretion of reproductive hormones from the hypothalamus which stimulates the anterior puberty gland to release sufficient levels of reproductive hormones (LH & FSH) to allow oestrus and ovulation to occur.
Maturation of the sexual internal organs initiates cyclic hormone patterns necessary for follicular growth and oestrogen and LH production.
Studies show that the reproductive hormone levels (LH, FSH, oestrogen, progesterone) are low and varied in prepubertal heifers. Sexual maturity allows transient rises in these hormones, the increased sensitivity of the ovaries and prevents (overrides) inhibitory blocking mechanisms to allow oestrus and ovulation.
"Puberty is the result of complex physiological and environmental interactions"