3.1 Performance objectives
3.3 Concepts of reproductive physiology
3.4 Reproduction in genetic improvement
3.5 Physiological constraints
3.6 Practical applications
3.8 References and reading materials
Module 3 is intended to enable you to:
Several attempts have been made to improve the productivity of cattle in sub-Saharan Africa (SSA). Understanding the physiology of reproduction is essential to achieve this goal. Most important is finding potential points where intervention in the life cycle of the cow or bull can result in more efficient performance and hence higher productivity.
Long dry periods result in reduced milk production and hence economic losses. The number of calves born can be reduced because the periods optimal for conception are missed. In addition, the cost of treatment of reproductive disorders can be high or beyond the reach of smallholder farmers. This module will deal with reproductive management in cattle to achieve the desired goal.
It is important you learn the anatomy of the organs that are involved in the reproductive physiology of the cow. This information will help you to understand the various processes involved in reproduction. The ovary and uterus are shown in Figure 3.1. For detailed descriptions of these parts consult veterinary books in your library.
Figure 3.1. Reproductive tract of a cow showing uterus and ovaries.
Most cattle in SSA breed all year round. It is important to understand the basics of the reproductive stages so that management can be applied to attain high productivity. We have divided the life of the cow in four cycles. It is important to understand these cycles because this knowledge is important to formulate strategies to achieve higher productivity.
The three main phases in the lifetime of a cow are pre-puberty (before cycling), puberty (initiation of cycling) and the reproductive periods. This module will describe these periods briefly.
Pre-puberty. This is the period before cycling starts. During this period the ovaries are small. Incomplete cycles where ovulation does not occur are detected first. The onset of ovulatory cyclic activity follows and is a gradual process. The luteinising hormone (LH) can be detected in blood plasma at normal adult basal levels and peaks as early as 6 weeks of age and continues up to puberty. In the bull calf, the size of the testicles indicates the approach to puberty and production of viable semen. The changes are associated with androgen rather than LH.
Puberty. Puberty (initiation of cycling) may be defined as the time when the reproductive organs become functional. In the female, it is defined as the time when the first functional oestrus occurs and the earliest age at which reproduction can occur. Usually genotype or breed, nutrition, season and other environmental factors (e.g. climate) determine the age at which puberty is attained. As a consequence, large variations occur between and within breeds. The age at first calf delivery may also vary as a result.
Undernutrition results in delay of the onset of puberty in heifers. Heifers that are well fed grow faster and attain puberty at an earlier age.
Reproductive period. Breeding activities in cattle can continue for many years. A cow is born with a full supplement of ova; at the age of 15 to 20 years, a cow will have no primordial follicles. Testicular size is a good indicator of sperm production and reproductive activity and is easy to estimate (Figure 3.2). Males produce sperm continuously. Even though efficiency of sperm production peaks at 11 months, total production continues to increase with increase in testicular size. Both cow and bull reach senescence at about age 15 years but this is of no practical value, as cattle are usually not kept to this age.
Figure 3.2. Measuring testicular size.
Even though cattle in the tropics can breed all year round, the efficiency of cows and bulls is affected by many environmental factors.
Oestrus, or heat, is defined as the time when the female is receptive to the male. It occurs in cycles.
The length of the oestrous cycle is 20 days in heifers and 21 days in cows. Oestrus is short, 630 hours, but it varies among breeds and the range is considerable. The first day of oestrus is usually designated as day zero. Ovulation occurs after the end of oestrus.
The events in oestrus follow a specific timed sequence (Figure 3.3).
Figure 3.3. Events in the oestrous cycle of the cow.
Luteal phase. A corpus luteum is formed under the influence of pituitary LH. The function of the corpus luteum is to secrete progesterone, which reduces the amount of the hormone oestrogen produced. As long as the corpus luteum is functional, oestrogen is unable to trigger formation of the follicle (a large fluid-filled sac containing the egg). By day 18 the corpus luteum degenerates due to prostaglandin (released from the uterus) and goes through a regression phase. Associated with corpus luteum regression is a decline in progesterone (Figure 3.3).
Follicular phase. Following the decline in progesterone, an increase in oestrogen (mainly oestradiol-7) takes place and peaks before the onset of oestrus (Figure 3.3). Oestrogen stimulates the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus. GnRH stimulates the release of FSH and LH from the pituitary gland. LH rises to a peak at the beginning of oestrus. FSH and LH stimulate the development of follicles in the ovary. One follicle predominates which secretes oestrogen and triggers a wide range of anatomical and physiological changes in the cow [see Signs of oestrus, p. 25]. All the changes ensure that if the cow is mated, the sperm will stay alive until one of the spermatozoa will effect fertilisation. The follicle ruptures and releases the egg (ovulation).
Conception. Mating may take place followed by conception. The luteal phase of the cycle starts again (Figure 3.3). In this case the embryo develops and the corpus luteum does not regress but continues to secrete progesterone and remains active throughout the pregnancy. If there is no mating or fertilisation fails to occur, progesterone secretion ceases abruptly on day 17 or 18 of the luteal phase. This is brought about by the uterus releasing uterine luteolysin which initiates the regression of the corpus luteum. The regression is followed by growth and maturation of another follicle and the cycle starts again (Figure 3.3).
Example: How to assess progesterone during oestrous cycles
The enzyme-linked immunosorbent assay (ELISA) technique can be used to assess progesterone levels during the oestrous cycle (Boland et al. 1985). Mukasa-Mugerwa et al. (1990) assessed progesterone with a commercially available ELISA technique in 20 Ethiopian Menz ewes. In their experiment the oestrous cycle averaged 17.2 ± 1.0 days (range 1520 days). Progesterone values were under 1.0 ng/ml from 2 days before to 4 days after oestrus. Hormone concentrations rose steadily to peak at 5.05.6 ng/ml on days 10 to 14. This was followed by a rapid decline to 3.0 ng/ml (53% of day 14 peak value) on day 15; 0.8 ng/ml (15% of peak value) on day 16; and 0.2 ng/ml (3.7% of peak value) on the day before oestrus. It was concluded that ELISA method can be used for progesterone determination in Ethiopian sheep, and also, that progesterone level of under 1.0 ng/ml are indicative of either anoestrus or the follicular and early luteal phases of the oestrous cycle.
The major signs shown by a cow on heat are:
Figure 3.4. Hetrosexual/homosexual behaviour: A major indicator of oestrus in cows.
Cycling in animals is affected by physiological, pathological and environmental factors (e.g. high temperature) (Figure 3.5). Prolonged suckling, pregnancy and mummification (foetal death without abortion or resorption) delay oestrus leading to a condition known as anoestrus. Diseases that affect the ovary can lead to a long anoestrus or failure to come to heat.
Figure 3.5. Possible causes of anoestrus in cows.
The production of spermatozoa by a bull is a continuous process. Semen can be collected in successive harvests. The spermatogenic cycle takes 60 days.
Mating in the heifer is highly sensitive to oestrogen. The egg is normally shed about 10 hours after the onset of oestrus. Oestrogen causes the egg to travel down the oviduct and assists the sperm travel up the oviduct. The union of egg and sperm takes place in the oviduct.
Pregnancy is established after fertilisation. It is defined as the time span between the implantation of a fertilised ovum in the uterus and the expulsion of the foetus and its associated membranes at term. It has three phases:
Gestation is the interval from fertile service to process of calving (parturition). During parturition the uterus contracts, foetus is delivered and placenta is expelled. Parturition should be completed in about 8 to 12 hours in cattle.
Lactation, the final phase of the reproductive cycle of the cow, is the formation and secretion of milk to nourish the young. Man domesticated cattle to produce enough milk for calves and for human consumption. The duration of lactation varies depending on nutrition. The yield of milk in cattle increases steadily until it reaches a peak at 8 or 9 weeks and then declines over the remainder of the lactation period. The composition of milk also changes during lactation. Once colostrum (the first milk) changes to milk, there is a small decline in protein, fat and lactose until peak production. After the peak, these constituents increase slightly until the cessation of lactation.
Post-partum is the period following parturition in which lactation starts and reproductive cycles are established. The number of days from parturition to first ovulation averages about 20 days and time to first oestrus averages about 34 days. Ways to manipulate the reproductive cycle to shorten the period between calving and first oestrus include:
Nutrition manipulation. Low nutrition in beef cattle, that reduces weight after calving, prolongs the period to first oestrus. Supplementation is essential to alleviate this constraint. This treatment must be economical for smallholder farmers to accept it.
Example: Effect of nutrition on reproductive activity in cows after a long anoestrus period
Body condition is a major determinant of oestrous activity. This is especially important for female draft animals because their bodies are already stressed by work. Zerbini et al. (1996b) examined 12 low body condition, non-milking, non-cycling (depletion state) F1 crossbred dairy cows (Friesian × Boran and Simmental × Boran) in the Ethiopian highlands. The cows were fed natural grass hay and offered mineral lick plus 3 kg concentrate (800 g noug cake, 150 g wheat middlings, 30 g salt and 20 g bone meal per kg concentrate) and 7 hours per day natural grazing. The results of this study indicated that healthy cows which had been underfed to the extent that no reproductive events occurred (i.e. ovulation and oestrus) for a period of more than 2 years, rapidly regained reproductive ability under improved nutrition. All cows became pregnant after a relatively short period of repletion and subsequently all calved normally. Despite the exceptionally long period of anoestrus, the cows were able to resume ovarian activity and to cycle within 3 months of dietary repletion.
Remove suckling inhibition. Management of the post-partum period depends on the rationale for raising the cow. If we are raising the cow to produce milk for marketing, then removal of the calf becomes necessary. Nursing delays oestrus and ovulation. Table 3.1 shows the effect of rate of suckling on oestrus. Reduced suckling resulted in more cows experiencing oestrous. Neither calf nor dam weight at weaning was affected by reduction in suckling.
Example: Influence of suckling on resumption of post-partum ovarian function
The effect of suckling on post-partum ovarian function was monitored by determining plasma progesterone weekly (Mukasa-Mugerwa et al. 1991). Enzyme immunoassay was used in 16 Small East African Zebu (Bos indicus) cows which were maintained with a fertile bull in Debre Birhan, Ethiopia. The results indicated:
Table 3.1. Effect of suckling on first calf heifer.
Normal daily suckling
Suckling once a day
Post-parturition interval (days)
Oestrus at 90 days (%)
Cow weight at weaning (kg)
Calf weight at weaning (kg)
Inadequate nutrition is a major constraint to productivity of cattle in SSA. Nutrition stress can influence a cow in the following ways:
Example: Effect of nutrition on onset of puberty and conception
The dry season in SSA is associated with poor quality feed. Tegegne et al. (1992a), working in Ethiopia during the dry season, fed Boran and Boran × Friesian crossbred heifers a diet containing 1.5 kg/head per day of a 2:1 mixture of wheat bran and noug cake (Guizotia abyssinica) which contained 16% crude protein and 10.8 MJ/kg dry matter energy. The results led to the following conclusions:
High ambient temperature imposes adverse effects on conception rates in cattle. It shortens the duration of oestrus. In hot weather most cows come to heat during the night when no one observes them, thus missed heats are common in the summer. High temperature can also prolong anoestrus in lactating cows. A simple measure to lessen the effect of high temperature is to provide shade and air movement. High temperatures also affect the fertility of the bull by affecting both the libido of the bull and the quality of semen collected for artificial insemination (AI).
Embryo transfer is the process in which a breeder allows a cow to become temporarily pregnant, recovers the embryo in early pregnancy, and transfers it to the reproductive tracts of another cow to complete gestation. This section of the module will briefly describe the rationale for embryo transfer and the steps to do it. For details of the process, refer to listed reference material.
1. Superovulation, synchronisation and fertilisation
Superovulation. This technique aims at increasing the yield of viable ova. Injections of pregnant mare serum gonadotropin (PMSG) or follicle-stimulating hormone (FSH) are given to stimulate the growth of additional follicles. This is followed by injection of luteinising hormone (LH) to induce ovulation. It is recommended that the treatment start 812 days into the oestrous cycle and corpus luteum degeneration.
Synchronisation. To succeed in embryo transfer, it is essential to synchronise the donor and recipient reproductive phases. This ensures the right environment for the embryo. Recipients should be in oestrus one day after the donor. Oestrus can be either natural or induced (using appropriate hormones).
Fertilisation. Superovulated donors are inseminated often with more sperm per inseminate. This is to safeguard against damage to sperms during transportation.
2. Embryo collection, evaluation and culture
Ova can be removed from the animal surgically or non-surgically. The non-surgical method is safer and preferred. Currently, catheters (e.g. Foley catheters) are used to recover embryos. After collection, ova are examined under the microscope with a magnification 10× to 15×. A fine pipette is used to remove them into a culture medium for evaluation. Morphologically normal ova are selected to use in embryo transfer.
Implantation. Usually ova of more than eight cells are transferred. This process is done non-surgically. There are many techniques for implantation of the ova in the recipient reproductive tracts (see the list of references for details).
Pregnancy. The highest pregnancy rates are achieved when one embryo is transferred into each uterine horn of the recipient cow.
Example: Practical application of embryo transfer in sub-Saharan Africa
Although the embryo transfer technique has potential, its application in SSA is limited to research stations. Government and large commercial enterprises may use it to import exotic breeds. For example, frozen embryos of NDama cattle from West Africa have been transferred into zebu cattle in Kenya, East Africa.
One of the limiting factors in developing embryo transfer in SSA is control of the technique to induce donors to shed several ova instead of one through superovulatory treatment (a regime of hormone treatment). Breed, age, physiological condition, season and nutrition affect it. A number of gonadotropin treatments have been used in superovulation programmes. Tegegne et al. (1994) studied the response in Boran cattle (Bos indicus) after treatment with either pergovet (human menopausal gonadotropin) at 1050 or 1350 IU of pulset (porcine gonadotropin) at 500 or 1000 IU. The results showed that both perovet and pulset could effectively be used to superovulate Boran cows. Although both hormones resulted in good embryo yield, pergovet tended to yield more embryos in Ethiopian Boran cows. Pergovet given at 1050 IU and Pulset at 1000 IU resulted in 19.4% and 39.0% more transferable embryos than at 1350 and 500 IU, respectively.
Reproductive failure leads to low productivity in cows. In this module we shall cover more common causes of reproductive failure.
When oestrus is not manifested in a cow during lactation for an extended period, this is termed lactational anoestrus. Even though lactational anoestrus is not a disease, it is a sign of temporary depression of ovarian activity. It can be caused by (Figure 3.5):
Failure to ovulate may be due to failure of the follicle to release the egg due to cystic ovaries. Follicles with cysts grow and regress but fail to ovulate.
Fertilisation may fail due to:
Fertilisation in cows may take place successfully but the embryo does not complete its cycle to delivery. Embryo mortality in cattle usually occurs during rapid growth and differentiation of embryo. This can be due to:
Embryonic mortality can be determined in the following ways:
Abortion is defined as the spontaneous expulsion of the foetus. Abortions can be caused by diseases such as brucellosis or by genetic, hormonal or nutritional factors. It also occurs in some heifers when bred immediately after puberty.
Perinatal mortality refers to the death of the offspring shortly before or during parturition. It also includes mortality within the first 24 hours after parturition. In cattle it can range from 5% to 15% of births. Perinatal mortality can be caused by old age or poor nutrition of the cow. Neonatal mortality refers to the death of the offspring during the first few weeks of life. Nutritional deficiencies in the calf result in this disorder, e.g. disturbance in selenium metabolism may cause the disorder.
Failure in the fertility of the male can be due to:
Infectious diseases in cattle can cause abortion. The following are diseases which generally cause abortion: brucellosis, bluetongue virus, foot-and-mouth disease, rinderpest, tick-borne fever, bovine petechial fever, trypanosomosis. For details see Hafez (1980).
Example: Reproductive problems in crossbred cattle in central Ethiopia
Crossbred cattle, the progeny of zebu and HolsteinFriesian cattle are mainly used for milk production in Ethiopia in dairy farms around the major cities, e.g. Addis Ababa, Gondar and Jimma. The major constraints to production facing these cattle are inadequate nutrition, poor management and genital diseases. Tekelye Bekele et al. (1991) studied reproductive problems facing these cattle. They found that calving intervals were much longer than the optimum 360 days expected from dairy cows. This was due to the extended calving to conception interval owing to anoestrus. Longer calving intervals were also caused by stress due to tick-borne diseases such as heartwater and babesiosis. Abortion rate ranged from 1.7% to 20.2%. The researchers concluded that cattle in the dairy farms under study had low reproductive efficiency because of inadequate oestrus detection, poor artificial insemination services, semen deterioration owing to lack of liquid nitrogen, insufficient feed supply and diseases.
It is essential to know when to improve reproductive management in cows. The first opportunity for intervention is during the period between birth and first conception (interval 1 in Figure 3.6). We can reduce the period to first conception by adequate nutrition, breeding and proper health management. Failure to detect heat can lead to delayed first conception. The second opportunity for intervention is to reduce the interval between delivery and subsequent conception. This opportunity offers itself in a cyclic manner. During this period (interval 2 in Figure 3.6) heat detection is vital. Other factors that can help in reducing interval 2 include good nutrition and calf removal.
Figure 3.6. Intervention points for improving reproductive performance in cows.
We have described the signs of heat. But how do we identify a cow which is on heat? The following are tips to help you use the heat signs more efficiently:
There are special methods and aids to help detect heat:
There can be errors in detecting heat. It is costly to wrongly diagnose a cow on heat because it leads to wrong management (e.g. AI) decisions. In general in SSA 36% of the cows presented for breeding are not in heat. These cows had ovarian and uterine changes indicating they were not on heat but still the farmers will judge them as being on heat. Incorrect detection of heat leads to irregular heat cycles.
Errors in detecting heat occur by:
False or irregular heats are caused by:
Example: oestrus synchronisation using a prostaglandin analogue in Ethiopia
Tegegne and Franceschini (1993) used 32 Boran × Friesian heifers in Ethiopia to evaluate oestrus synchrony using a prostaglandin analogue, Prosolvin. Plasma samples were obtained periodically and used to determine progesterone levels. Oestrus was shorter in treated heifers than in controls. Pregnancy rates were 91%, 73% and 60% in heifers that received full and split doses of Prosolvin, and in the control group, respectively.
To successfully manage cattle fertility, it is important to diagnose whether pregnancy has occurred or not. The following are common methods to diagnose pregnancy:
In temperate climates, the target body weight at first mating (which is critical minimum weight) for Bos indicus is 280300 kg at 2427 months of age. This cannot be widely applied to tropical heifers. Target weight for tropical cattle should be developed taking into consideration the nutritional plane that farmers can afford.
In many cases farmers focus more on managing the fertility of heifers and give little attention to the fertility of bulls. In this module we will briefly focus on bull fertility aspects as they affect productivity of cattle in SSA.
Bull fertility is dependent upon total sperm production which is highly correlated with testis size. The best measure for testis size in live animals is the scrotal circumference. Why are the testes so important? They produce sperm which is vital in fertilisation and hence fertility of the herd. The testes also produce sex steroids (male sex hormone testosterone) which are vital for the reproduction process.
Bulls with small scrotal circumferences may end up with poor fertility and hence are recommended for rejection for breeding. It is important to note that for any given age there is a standard scrotal circumference. This simple measure can contribute significantly to bull fertility improvement research. Scrotal circumference can be measured on the live animal using either a flexible plasticised cloth or metal tape while testes length and diameter can be measured using caliphers.
The reproductive performance of bulls is influenced by the genetic constitution of the breed, climate, nutrition, age and disease. It is important to manage the fertility of bulls because they graze alongside cows in the animal agricultural systems of SSA. Often little attention is paid to bulls, as the myth is all bulls are good bulls! For Bos indicus herds, bull fertility can influence herd fertility.
Fertility problems in Bos indicus bulls include:
Selection criteria are now available for growth rate and reproductive traits. For bull fertility, some of the criteria are:
Heritability for testis size in Bos indicus bulls is high (estimates 0.550.65). Thus one of the important selection criteria for improved fertility is testis circumference. The relationship between live weight and scrotal circumference of bulls at puberty in different breeds is given in Table 3.2.
Table 3.2. Age, live weight and scrotal circumference of bulls of different breeds at puberty.*
Example: Comparison of sperm reserves in two breeds of zebu bulls
The bull plays an important role in influencing herd fertility and contributes to genetic improvement. Gonadal and extragonadal sperm reserves were determined in Small Highland Abyssinian Zebu (SHAZ) and Boran bulls (Tegegne et al. 1992b). Mean scrotal circumference was 27.9 and 30.7 cm for SHAZ and Boran bulls, respectively. Daily sperm production did not differ between SHAZ (2.89 × 109 sperms) and Boran (3.67 × 109 sperms) bulls. Corpus sperm reserves were higher in Boran than SHAZ bulls. The researchers concluded that:
Farmers use oxen for draft to plough land. The oxen are fed all year round but their work output is utilised for only part of the year. The use of the cow for draft is more efficient because:
ILRI and the Ethiopian Agricultural Research Organization (EARO) conducted joint research on the performance of crossbred dairy cows (Friesian × Boran and Simmental × Boran) used for draft. The main treatments were varying workloads and supplements (Zerbini et al. 1996a).
Farmers are sceptical about using their cows for draft work. For farmers to adopt this technology, it is therefore essential to determine the optimum workload that dairy cows can undertake. Feed and body energy stores have to be channelled to different productive tasks such as milk production and/or pregnancy and work. Research in the joint EAROILRI project found that in the Ethiopian highlands work efficiency of the cow increased from about 7% to 26% as the workload increased to its maximum (Zerbini et al. 1996a).
Zerbini et al. (1996a) found that in working cows, glucose is an important energy source for the muscles; it is also important during reproduction. Fatty acids were found to be important for the synthesis of milk fat as well as reproductive hormones. Non-esterified fatty acids (NEFAs) and ß-hydroxybutyrate were found to increase in working cows whereas glucose, magnesium and inorganic phosphorous decreased.
Zerbini et al. (1996a) found a greater loss in body weight in working non-supplemented cows than in working supplemented ones. Glucose was decreased in non-supplemented cows, which stimulated the release of NEFAs from adipose tissue (150% increase in the plasma). Fatty acids were mobilised from the fat depots even in well-fed animals. This suggests that NEFAs are the principal fuel for the muscle tissue of working dairy cows. Reduction in NEFAs in poorly fed working cows explains the low conception rate. Energy restriction affects reproductive performance at the hypothalamic or pituitary level. This could be due to inhibition of gonadotropin-releasing hormone (GnRH).
Work stress has an effect on fertility. Thus when breeding dairy cows are intended to perform work, the incidence of anoestrus should be studied. Zerbini et al. (1996b) found:
The economic implications indicated that the value of work more than compensated for the small reduction in milk production and longer calving interval. Working cows must be supplemented to ensure adequate nutrition. When working crossbred cows are supplemented, the greater return to investment was due to higher value of work output, in spite of the higher feed costs.
Boland M.P., Foulkes J.A., MacDonnel H.F. and Sauer M.J. 1985. Plasma progesterone concentrations in superovulated heifers determined by enzymeimmunoassay and radioimmunoassay. British Veterinary Journal 141:409415.
Cole H.H. and Cupps P.T. 1977.Reproduction in domestic animals. Academic Press, New York, USA. 665 pp.
Esslemont R.J., Bailie J.H. and Cooper M.J. 1985. Fertility management in dairy cattle. Collins, London, UK. 143 pp.
Gordon I. 1983. Controlled breeding in farm animals. Pergamon Press, Oxford, UK. 436 pp.
Hafez E.S.E. 1980. Reproduction in farm animals. Lea and Febiger, Philadelphia, USA. 627 pp.
Mukasa-Mugerwa E., Zere Ezaz and Viviani P. 1990. Plasma concentrations of progesterone during oestrous cycles of Ethiopian Menz sheep using enzyme immunoassay. Small Ruminant Research 3:5762.
Mukasa-Mugerwa E., Tegegne A. and Franceschini R.R. 1991. Influence of suckling and continuous cowcalf association on the resumption of post-partum ovarian function in Bos indicus cows monitored by plasma progesterone profiles. Reproduction Nutrition Development 31(3):241247.
Payne W.J.A. 1990. An introduction to animal husbandry in the tropics. Longman, New York, USA. 881 pp.
Seidel G.E. 1984. Applications of embryo transfer and related technologies to cattle. Journal of Dairy Science 67(11):2786–2796.
Tegegne A. and Franceschini R.R. 1993. Evaluation of luteolysis and oestrus synchronization using a prostaglandin analogue (Prosolvin) in Boran × Friesian crossbred heifers in Ethiopia. Journal of Applied Animal Research 4:107114.
Tegegne A., Entwistle K.W. and Mukasa-Mugerwa E. 1992a. Effects of dry season nutritional supplementation on growth, onset of puberty and subsequent fertility in Boran and Boran × Friesian heifers in Ethiopia. Theriogenology 37:10171027.
Tegegne A., Taddele Y. and Kassa T. 1992b. Gonadal and extragonadal sperm reserves in two breeds of mature Zebu (Bos indicus) bulls in Ethiopia. Tropical Agriculture (Trinidad) 69(3): 243246.
Tegegne A., Franceschini R.R. and Sovani S. 1994. Superovulatory response, embryo recovery and progesterone secretion in Boran (Bos indicus) cows after treatment with either Pergovet or Pluset. Theriogenology 41:16531662.
Tekelye Bekele, Kasali O.B. and Tsion Alemu. 1991. Reproductive problems in crossbred cattle in central Ethiopia. Animal Reproduction Science 26:4149.
Zerbini E., Wold A.G. and Gemeda T. 1996a. Effect of dietary repletion on reproductive activity in cows after a long anoestrus period. Animal Science 62:217223.
Zerbini E., Gemeda T., Wold A.G. and Tegegne A. 1996b. Effect of draught work on the metabolism and reproduction of dairy cows. In: Phillips C. (ed), Dairy science. CAB (Commonwealth Agricultural Bureau) International, London, UK. pp. 145168.