- The Male Effect in sheep and goats
|Last update: February 8, 2013 07:52:12 AM|
THE MALE EFFECT IN SHEEP AND GOATS
J.A. van Rooyen
Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg (EC), 5900
E-mail: Johan van Rooyen
The male effect refers to the effect of the introduction of the male on the oestrus cycle of the seasonally anoestrus female. This effect is a neuro-hormonal response that is utilised by flock managers to increase the ovulation rate in females and shorten the breeding season. The introduction of males to the female flock will elicit this response. Flock managers use vasectomised males (teaser rams) as a common practice in small-stock farming.
Improvements in reproductive rates in small ruminants is commonly achieved by the use of synchronisation protocols involving the use of progesterone impregnated vaginal devices and the use of gonadotropins, mostly Pregnant Mare Serum Gonadotropin (PMSG) (Radostits et al., 1994). This practice is increasingly being questioned on the basis of consumer rejection of the use of exogenous hormones. The unwanted side-effects of the use of PMSG include ewes carrying four or more lambs resulting in ewe mortalities and reduced viability of lambs. The recent concept of production described as the ‘clean, green and ethical’ method seeks to find natural alternatives to achieve the same objective of improved reproductive rates by natural means (Martin et al., 2004). The use of vasectomised males to create the male effect and the use of lupins and other focussed feeding practices to improve the ovulation rate in females and semen quality in males as well as improved management, nutrition and genetic selection to improve offspring survival form the basis of this concept that aims at improving the acceptability of products from livestock.
The male effect, which is present in sheep as well as goats, has been reported extensively since the first paper was published in 1944 (Pearce & Oldham, 1984; Rosa & Bryant, 2002; Martin et al., 2004). The presence of rams creates olfactory, behavioural and visual stimuli which result in an increase in the secretion of luteinizing hormone which stimulates folliculogenesis and ovulation (Pearce & Oldham, 1984; Radostits et al., 1994). Although it was initially believed that pheromones played a pivotal role in the male effect, subsequent work has proved that visual and behavioural stimuli are equally as important as the smell (Martin et al., 2004).
Delgadillo et al. (2009) reviewed the commonly held beliefs about the male effect and came to the conclusion that many dogmas may not be true. Complete isolation of females from males, even to the extent of considering the direction of the prevailing wind is not necessary. Ewes recognise novel rams and respond to them even if they are running with other rams. It is widely held that the male effect is not of much use in cyclical females. It has, however, been found that the introduction of teaser rams can even override the effect of luteal progesterone resulting in a more robust response in sheep. It is also partially true in goats.
Another widely held belief is that continuous contact is necessary for the male effect to take place. Although some studies indicate a response after only a few hours of exposure, the overwhelming evidence is in favour of continuous exposure for 17 days rather than a short-term exposure every 17 days to create oestrus synchronisation (Delgadillo et al., 2009). Ewes and does that are seasonal breeders can respond to the male effect in the deep anoestrus period if a male is introduced that is ‘in season’. This effect in the male can be simulated with day length manipulation combined with melatonin treatment. The perception that the male effect is purely the result of smell has also been proven incorrect. Ewes respond to behaviour, smells and the novelty of the ram.
Adult males are more efficient at creating the male effect and this has been proven to be the result of an ‘improved’ smell (Ungerfeld et al., 2008). Flock managers should therefore ensure a constant supply of new young teaser males to prevent the situation where all teasers have to be replaced at once by young ones.
Testosterone treated wethers can substitute for teaser rams for identification of ewes in oestrus and the ‘ram effect’ has also been reported. There is some uncertainty about the mechanism and effectivity of testosterone treated wethers in respect of their pheromone production. Treated wethers may be of value if used together with vasectomised males and are believed to assist in breaking up harem groups, thereby ensuring higher levels of exposure of ewes to males (Reeve, 1984).
Ronderib Afrikaner teaser rams are superior to Merino rams in the resulting ovulation rates in Merino ewes. The ovulation rates were 28.9% and 14.4% higher in the spring and autumn respectively (King, 1994). Ronderib Afrikaner rams have a number of unique characteristics in their own response to the presence of ewes. Their basal Luteinising Hormone (LH) levels are lower than that of Merinos but their response to the introduction of ewes is much higher. It is believed that the pheromone production and the smell of Ronderib Afrikaner rams is the key to their superior ram effect. Observations on Ronderib Afrikaner rams indicate that they exhibit some behaviour patterns in common with goats such as the soiling of the ventral abdomen with urine which leads to the characteristic ‘goat ram smell’ during the breeding season.
In sheep the introduction of a ‘novel’ ram results in an ovulation in the ewe within 30 to 72 hours and again on the sixth day (Radostits et al., 1994; Rosa & Bryant, 2002; Gelez & Fabre-Nys, 2004; Delgadillo et al., 2009). It is, however, usually recommended that teaser rams are introduced 15 to 17 days before mating, as shorter periods result in lower percentages of ewes coming into oestrus (Bedos et al., 2010). Entire rams introduced for short periods (2 to 4 days) before breeding were less effective in eliciting the ram effect when compared to vasectomised rams that were introduced for 17 days pre-breeding (Kenyon et al., 2008).
In the non-breeding season ewes return to anoestrus after one or two ovulations (Rosa & Bryant, 2002). About 50% of Merino ewes introduced to teaser rams in October in the southern hemisphere became anovular after 50 days (Rosa & Bryant, 2002). It may therefore be advisable to introduce teasers about 7 days before the breeding season in spring, gradually increasing the period to 17 days in autumn. The first heat after introduction of teaser rams in spring is usually a silent heat in sheep (Gelez & Fabre-Nys, 2004). Ewes become anovular after one or two cycles during spring mating, suggesting that photoperiodicity overrides the ‘ram effect’. It is suggested that ewes become accustomed to the presence of rams (Murtagh et al., 1984).
In goats the buck effect induces oestrus in does after the ‘sudden’ introduction of the male and it has been reported that 80% to 100% of the does can be induced if they are well fed and healthy. However, the conception rates remain low until one month prior to the natural mating season. The first 3 months post-partum is also less successful owing to the suppression of the oestrus cycle by lactation. The initial cycles are short as a result of the lack of a preceding luteal phase and progesterone priming. The cycles following the first short cycle will be normal (Howe, 1990).
Goat ewes ovulate shortly after introduction of bucks and again on day five. About 60% of goats will exhibit heat with the first ovulation (Gelez & Fabre-Nys, 2004). Teaser bucks may be shared between flocks of does by daily rotation. Four hours of exposure per day for 15 days was as effective as longer or continuous exposure in producing oestrus in does (Bedos et al., 2010).
Standard recommendations on the use of the male effect cannot be formulated owing to the variation between seasons, breeds and management practices. It is, however, clear that the following observations can form the basis of a management protocol that can be adapted to suit the local requirements:
- Ewes respond to novel rams
- Exposure need not be continuous
- Exposure for 15 days in autumn is more effective than a shorter exposure
- Ronderib Afrikaner rams are more effective than other breeds
- Testosterone treated castrates are less effective than vasectomised rams.
Bedos, M., Flores, J.A., Fritz-Rodriquez, G., Keller, M., Malpaux, B., Poindron, P. & Delgadillo, J.A., 2010. Four hours of daily contact with sexually active males is sufficient to induce fertile ovulation in anestrus goats. Hormones and Behaviour 58, 473-477.
Delgadillo, J., Gelez, H., Ungerfeld, R., Hawken, P. & Martin, G., 2009. The 'male effect' in sheep and goats - Revisiting the dogmas. Behavioural Brain Research 200, 304-324.
Gelez, H. & Fabre-Nys, C., 2004. The “male effect” in sheep and goats: a review of the respective roles of the two olfactory systems. Hormones and Behaviour 46, 257-271.
Howe, P., 1990. Management of goat reproduction for optimal productivity. In: Thompson, K.G. (Ed.), Refresher Course for Veterinarians, University of Sydney, Sydney, pp. 91-103.
Kenyon, P., Morel, P., Morris, S. & West, D., 2008. A note on the effect of vasectomised rams and short-term exposure to entire rams prior to the breeding period on the reproductive performance of ewe lambs. Applied Animal Behaviour Science 110, 397-403.
King, P.R., 1994. The effect of Ronderib Afrikaner rams on the fecundity of Merino ewes. Ph.D. Thesis. University of Stellenbosch.
Martin, G., Milton, J., Davidson, R., Banchero Hunzicker, G., Lindsay, D. & Blache, D., 2004. Natural methods for increasing reproductive efficiency in small ruminants. Animal Reproduction Science 82-83, 231-246.
Murtagh, J., Gray, S., Lindsay, D., Oldham, C. & Pearce, D., 1984. The effect of the presence of 'rams' on the continuity of ovarian activity of maiden Merino ewes in spring. In: Lindsay, D.R., Pearce, D.T. (Eds.), Reproduction in sheep, Cambridge University Press, Cambridge, pp. 37-38.
Pearce, D. & Oldham, C., 1984. The ram effect, its mechanism and application to management of sheep. In: Lindsay, D.R., Pearce, D.T. (Eds.), Reproduction in sheep, Cambridge University Press, Cambridge, pp. 26-34.
Radostits, O., Leslie, K. & Fetrow, J., 1994. Herd Health: Food Animal Production Medicine. Saunders, Philadelphia.
Reeve, J., 1984. Management of males for ram induced breeding of crossbred ewes in spring. In: Lindsay, D.R., Pearce, D.T. (Eds.), Reproduction in sheep, Cambridge University Press, Cambridge, pp. 35-36.
Rosa, H. & Bryant, M., 2002. The 'ram effect' as a way of modifying reproductive activity in the ewe. Small Ruminant Research 45, 1-16.
Ungerfeld, R., Ramos, M. & Gonzales Pensado, S., 2008. Ram effect: Adult rams induce a greater reproductive response in anestrus ewes than yearling rams. Animal Reproduction Science 103, 271-277.
Grootfontein Agric 13 (1)