UNIVERSIDAD NACIONAL AUTONOMA DE MEXICO FACULTAD DE MEDICINA VETERINARIA Y ZOOTECNIA DIVISION DE EDUCACION CONTINUA CENTRO DE ENSEÑANZA PRÁCTICA E

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1 UNIVERSIDAD NACIONAL AUTONOMA DE MEXICO FACULTAD DE MEDICINA VETERINARIA Y ZOOTECNIA DIVISION DE EDUCACION CONTINUA CENTRO DE ENSEÑANZA PRÁCTICA E INVESTIGACIÓN EN PRODUCCIÓN Y SALUD ANIMAL Del 6 al 8 de febrero de 2002

2 DIRECTORIO UNIVERSIDAD NACIONAL AUTONOMA DE MEXICO Dr. Juan Ramón de la Fuente RECTOR Lic. Enrique del Val Blanco SECRETARIO GENERAL Mtro. Daniel Barrera Pérez SECRETARIO ADMINISTRATIVO Dr. Jaime Martuscelli Quintana SECRETARIO DE SERVICIOS A LA COMUNIDAD UNIVERSITARIA Dra. Arcelia Quintana Adriano ABOGADA GENERAL Dr. José Narro Robles COORDINADOR GENERAL DE REFORMA UNIVERSITARIA Lic. Néstor Martínez Cristo DIRECTOR GENERAL DE COMUNICACIÓN SOCIAL FACULTAD DE MEDICINA VETERINARIA Y ZOOTECNIA Dr. Luis Alberto Zarco Quintero DIRECTOR MVZ. MSP. Jorge Cárdenas Lara SECRETARIO GENERAL MC. Germán Valero Elizondo JEFE DE LA DIVISION DE EDUCACION CONTINUA MVZ. José Luis Dávalos Flores SECRETARIO DE PRODUCCION ANIMAL Dr. Javier Valencia Méndez DIRECTOR TÉCNICO DEL CEPIPSA

3 COORDINACIÓN ACADÉMICA MVZ. MC. Lorenzo Alvarez Ramírez COORDINACIÓN ADMINISTRATIVA DIVISION DE EDUCACION CONTINUA MVZ. Patricia Mejia Gutiérrez MVZ. Graciela Hernández Olvera EDITORAS MVZ. Graciela Hernández Olvera MVZ. Patricia Mejia Gutiérrez MVZ. Patricia R. Díaz Güemez

4 La reproducción parcial o total de los trabajos no podrá efectuarse sin la previa autorización por escrito del autor y citando estas memorias como referencia. La información contenida, así como estilo y ortografía en cada uno de los escritos es responsabilidad de los autores.

5 INCORPORATING SOCIO-SEXUAL CUES INTO THE MANAGEMENT OF REPRODUCTION IN THE SHEEP INDUSTRY 41 GRAEME B. MARTIN AND JOHN T.B. MILTON School of Animal Biology, The University of Western Australia, Crawley, WA 6008, Australia. Background The aim of scientists who work in the field of agriculture is to provide farmers with technology that will benefit them financially. In the context of the animal industries, technology for the management of reproduction includes artificial insemination and the manipulation of embryos, both of which are essential for genetic engineering and other biotechnologies. The problem is that these technologies are rarely very useful for the average goat or sheep farmer anywhere in the world, including Mexico. These techniques are very expensive and are suited only to specialist breeders (studs) or intensive industries, and not to the extensive operations that dominate most systems of animal production. We must also remember that extensive, lowcost systems are often the most effective under the constraints of the environment. For example, genetically engineered sheep may be able to grow faster and produce large amounts of very fine wool, but it is most unlikely that they will be able to do this in areas where there is only low quality forage or where high quality pasture is only available for half of the year. The laws of energetics cannot be reversed we cannot get something for nothing the amount of production we get from an animal will always depend on the quantity and quality of the feed we can give it, irrespective of whether the animal has normal genes or engineered genes. So, what can we tell you in regard to the breeding of farm animals? In fact, there is a lot to discuss as you may have already realized from the content of this postgraduate course. Agricultural and biomedical researchers at universities and government scientific organizations all around the world have spent over 100 years trying to understand how our farm animals reproduce. Now is the time to start taking advantage of all of that knowledge, not just the most recent, high-profile discoveries, and we can use this knowledge to modify the management of our flocks. We can introduce new approaches that are simple and that do not require complex equipment. One such approach is to make much greater use of the socio-sexual cues (including pheromones) that our farm animals were themselves using to manage their own reproduction, long before they were domesticated (Martin 1995). In this article, we are proposing for discussion a new system for managing the reproductive program of sheep flocks based on our knowledge of the role biostimulation in the induction of ovulation and the induction of the mother-young bond. It was originally formulated for Merinos in which the production of wool was financially more important than the production of meat. However, it can be adjusted to suit alternative production systems for sheep, and also to goats that are bred for meat and fibre. Indeed, such flexibility has proven important for Australian Merino breeders when the price of wool has fallen and they needed to switch to a more even balance between meat and wool production. In a system in which fine wool is a major product, we need to consider two conditions: 1) the price of the wool is high enough for wool production to be as profitable as lamb production (the management strategy for reproduction may emphasize the maintenance of flock size rather than maximizing the number of lambs produced); 2) the animals will continue to be managed extensively (low input, low output). In addition, we need to consider the following principles:

6 a) it may not be necessary to get 100% of the ewes to lamb, but only to get sufficient lambs for flock replacement plus a number of extra animals for the market; this decision will depend on the marginal returns for each commodity; b) the flock will be mated some time between the middle of Spring and the beginning of Summer (ie, lambing between early autumn and early winter), so that we can take best advantage of our socio-sexual cues. The Synchronized Breeding Program The Program is aimed at a very short period of mating (4-6 days) and a very compressed lambing (6-10 days), so that there are short periods of intensive management. In the rest of this article, we will describe each step of the management scheme, the reasons for introducing it, the potential hazards and the potential benefits. For the sake of convenience, the Program is divided into 3 Phases: Phase A Synchronized Mating Phase B Pregnancy Diagnosis Phase C Synchronized Lambing These are shown in Figure 1, with an example of how the program could be structured and operated. Phase A Synchronized Mating The success of this program depends entirely on our ability to synchronize the oestrous cycles of the ewes. There are several hormone preparations on the market which can be used to do this, but generally they are too expensive to buy and too labour-intensive for anybody except stud breeders. However, the Merino and many other breeds of sheep and goats show the Male Effect (Martin et al 1986; Walkden-Brown et al 1999). This topic is, of course, the main focus of this postgraduate course, so we will not discuss it in detail here. However, to follow our proposed breeding program, several aspects of the phenomenon must be kept in mind: the females stop ovulating in the spring, but when males are re-introduced (after a period of isolation), the females ovulate within 3 days. This first ovulation is often silent, but a series of normal oestrous cycles follows. Since all the females receive the same stimulus at the same time, they all ovulate within a few days of each other, and we have a simple and cheap method for inducing synchronized cycles. 42

7 43 Time Operation Number of Animals Phase A Day 42 Day 14 Day 7 Day 1 Day Day 18 Day 22 Day 34 Day 44 Select and feed rams Inject wethers with testosterone Inject wethers with testosterone Inject progesterone; introduce T-wethers Ovulation rate 1.5 (feed or hormones?) Introduce rams, with marking harnesses Remove rams Re-introduce T-wethers, with marking harnesses (new colour) 35% marked ; remove marked ewes not marked? 35 marked Phase B Day 58 Scan unmarked ewes 60 pregnant 5 nonpregnant 40 nonpregnant? Phase C Day 120 Day Day 160 Day 163 Day 170 Day Day 185 Separate ewes with singles from ewes with twins Embryo mortality Put ewes into lambing area Supplementary feed for 7 days Lambing Perinatal losses Ewes out of lambing area 30 ewes with singles 30 ewes with twins 5% 20% 28 born 48 born 10% 20% 26 survive 38 survive Ewe wethers? Day 240 Wean lambs 64 Weaned 64% of ewes in flock 99% of ewes that mate Figure 1: the breeding program described in this article, including an example of a schedule and numbers showing the performance of the animals and the areas of reproductive wastage. The animal numbers are for a Merino flock in which wool production is the highest priority. Notes: a) marking means that the ewes have been detected in oestrus by males with marking harnesses; b) ewe wethers are ewes that leave the reproduction flock for this year and become wool producers only (they receive the same management as wethers); c) control of ovulation rate may not be necessary if the breed is prolific, but it is easy to include a feeding regime or hormone treatments. Now we can consider each step in the procedure (Figure 1): 1) Pre-mating preparation of rams Checking the sexual organs of rams for signs of abnormality before mating is a good practice, no matter what breeding scheme you use. Another good practice is to give the rams a high quality feed supplement (we use lupin grain) for 8 weeks before you expect them to work. This increases the size of the testicles so they produce more sperm and can impregnate more ewes (Martin and Walkden-Brown 1995).

8 2) Preparation of T-wethers The synchrony of the oestrous cycles following the ram effect is not perfect, so we will be choosing only those ewes which are well synchronized for our breeding program. To stop the others from conceiving outside our chosen time period, we use wethers for the initial stimulation of the ewes. Wethers don't act like rams because an essential piece of their anatomy is missing, so we inject them with the male hormone, testosterone (hence Twethers ). This makes them act and smell like rams, so they can stimulate the ewes to ovulate (Signoret et al 1982). It is sufficient to give a single injection (500 mg testosterone ester in oil) one week before the T-wethers are to be introduced into the ewe flock (KP Croker, personal communication). This treatment is simple and inexpensive and is a better alternative to the use of vasectomized rams. 3) The "Male Effect" At the same time that the T-wethers are introduced, the ewes must all be given a single injection of progesterone (20 mg in oil). The ewes are anovulatory so they are missing this hormone that normally helps to maximize the synchronizing effect of the T-wethers because it prevents abnormal corpora lutea and short cycles (Lindsay et al 1982). The T- wethers stay with ewes for 18 days on the morning of Day 18, when the ewes start coming on heat, the T-wethers are taken out and replaced with the rams (see below). The degree of synchrony that can be expected is shown in Figure 2. 4) Flushing the ewes If we decide not to aim for 100% pregnancy, we need to increase the number of lambs born per ewe we need a high proportion of twins (but not triplets because the postnatal mortality is too high). In Western Australia, we use supplementary feeding with lupin grain (500 g per day) because it has two advantages: the grain is relatively cheap and it also feeds the animals. Other grain, such as corn, will be equally effective. This approach is cost-effective because only about 6 days of feeding is necessary (this gives us 50% twins in Merinos), but it only works with synchronized cycles because the feed must be given over Days 8-14 of the oestrous cycle. Hormone treatments are also possible. Fecundin is an effective product and is very reliable for inducing twin ovulations in ewes in good condition, but might be too expensive for this program. An alternative that should be considered is unpurified PMSG, simply obtained by harvesting blood serum from a pregnant horse (Gherardi and Lindsay 1980; Donrov et al 1998). Some scientific help for the farmers is necessary because the dose will need to be tested in the laboratory. 44

9 Ewes mated (%) Proposed "Mating Period" Rams 1% 3% 6% Days after T-wethers introduced 45 Figure 2: the pattern of oestrous activity in three flocks of ewes, mated with 1, 3 or 6 rams per 100 ewes. Because of the synchronizing treatment, most of the ewes come into oestrus on Days after the T-wethers were first introduced. It is evident that with 1% (and possibly 3%) rams, there are not enough rams to mate with all of the ewes. From Wilkins (1990). 5) Mating On the morning of Day 18, the T-wethers are replaced with the rams that have been fed nutritional supplements. It is very useful here to have marking harnesses on all rams so we can see how well they are working and quantify the success of the T-wethers. These rams will be very busy normally, they would expect about 6% of the ewes to come into oestrus each day, or about 24% over 4 days. If it is successful, our synchronizing program will provide at least 65% in oestrus in those same 4 days, and this is one reason that the rams must be in the best possible health. The other problem is that we need extra rams to get a good pregnancy rate 3% is quite adequate for a normal mating in a flock that has not been synchronized. The rams can find all the ewes once, because as many are marked with 3% rams as with 6% rams (Figure 2), but it seems that there is simply not enough time for the rams to cover the ewes more than once, and that mating more than once is necessary for maximum fertility (Figure 3). The rams are removed at the end of Day 21. In a program that is designed for meat production, rather than wool, the rams can be left with the ewes for a longer period (Table 1) so that more ewes will have an opportunity to conceive. The disadvantage is that lambing will be spread over a wider period, increasing the difficulty and costs of supplementary feeding before lambing and the labour for monitoring the animals after lambing (see below). This is not worthwhile if the ram effect is very efficient (as it is in the example in Table 1) but would be worthwhile if the breed used does not respond very well to the stimulus. In some cases, it may be more efficient to recycle the ewes that do not mate, along with the ewes that do mate but do not conceive, into a second synchronization program (Figure 1). This would provide two periods of intensive lambing for more precise and efficient management.

10 46 % of Ewe Flock % Rams Mated Pregnant Figure 3: the effect of ram percentage on proportion of ewes mated and conceiving on Days 18-21, in the experiment shown in Figure 1. A similar proportion of ewes were mated by 3% and 6% rams, but 3% rams were insufficient for maximum pregnancy. From Wilkins (1990). Table 1: the outcome of a study on a farm in South Australia in which the ram effect and nutritional supplements were combined in a program of managed reproduction for Merino sheep. The study was done in late spring with two groups of 250 ewes (only 80 ewes per group were used to study ovulation rate). The program was isolation of ewes from rams for 4 weeks, introduction of 2% teasers for 2 weeks (Days 0-14), then 3-4% rams for 6-8 weeks. Feed was abundant mature clover-grass pasture with the addition of lupins (500 g/day on Days 12-26) in the supplemented group. The study was repeated on a second farm with the same basic outcome. Adapted from Nottle et al (1992). Control Lupin Ewes marked, Days % 14% Ewes mated, Days % 98% Ovulation rate (Day 28) Pregnant to first ovulation 90% 91% Lambs born per ewe mated Lambs marked per ewe mated Phase B Pregnancy Diagnosis We use two steps in the pregnancy diagnosis to sort the animals into classes for special management. 1) Detecting Non-pregnant Ewes The T-wethers are harnessed with markers (using a different colour to that put on the rams) and put back into the flock on Day 34, 12 days after the rams have been removed, and are left there for 10 days. They should still work well, but it is wise to give an extra injection of testosterone. They will mark the ewes that come into oestrus because they are not pregnant in our Merino flocks, there is normally about 35% of these and they can be separated from the main breeding flock. They can then re-enter the breeding

11 program or, as we do in a wool production system, they can now be managed in the same way as wethers ( ewe wethers ) since their only function for the rest of the year is to produce wool. An important advantage of removing these ewes is a reduction in the cost of scanning. 2) Scanning for Single- and Twin-producing ewes The ewes that were not marked by the T-wethers might a) be pregnant; b) be still in anoestrus (ie, they did not respond to the ram effect); c) have ovulated but failed to conceive and then returned to anoestrus; or d) have conceived but then lost their embryo(s). Only the pregnant animals require special treatment for the rest of the program, and these have to be detected by ultrasound scanning so they can be removed. This process can be done with a rectal probe, as early as 30 days after the rams are removed, or with an external probe 25 days later. The rectal probe is still not commonly used but is important because it provides flexibility in planning (eg, the decision about whether to attempt again to mate the non-pregnant ewes). In part, the scanning is a verification of the efficiency of the T-wethers for detecting nonpregnant ewes, so the extra cost may appear unnecessary. However, in some seasons, there might be large proportions of ewes that do not ovulate in response to the male effect, or that were already ovulating (ie, they were not in anoestrus) when the T-wethers were first introduced. These ewes would not be synchronized so the scanning is also a backup a guarantee that sufficient lambs will be conceived. The ewes that are found to be nonpregnant at scanning are added to the nonpregnant flock (as rebreeders or ewe wethers ). A major bonus of the scanning is that we can count the number of fetuses being carried by each ewe, so that we know exactly how many lambs to expect (and can re-mate the ewe flock if there has been some sort of disaster); and we can separate the ewes with a single fetus from those with two fetuses. This would allow us to provide more care, feed and shelter to the ewes that will produce twins. These animals play a major role in our strategy because they make up for the lambs that we lost by not aiming for 100% pregnancy, and they can only do this if we increase their chances of producing healthy, viable twins. This is the key in our system, and we will go into it in more detail in Phase C. Phase C Synchronized Lambing This part of the program has three main aims: a very short, but intensive period of work for lambing (say, 6 days); maximum lamb survival; maximum lamb growth. The strategy presented here achieves these to the best of our ability at present, but it is the main area in which we can improve efficiency. In Australia, for example, up to 11 million lambs die each year and most of those die in the first 2-3 days after they are born. This is the critical period. These deaths are caused by problems in both the mother and the young (Lindsay et al 1990): the Merino ewe is slow to form a solid bond with her young; many Merino ewes do not realize that they have 2 lambs; the Merino ewe leaves the birth site readily, especially if the grazing is poor in the immediate area, and then she can't find her way back; some Merino ewes produce only small amounts of colostrum and milk at birth; the Merino lamb is slow in learning to recognize it's mother. Effectively, this is another example of biostimulation chemical and other sensory 47

12 information is exchanged between mother and young to establish a bond between them (Nowak et al 2000). The strategy that we have described here is designed to prevent this process from being disrupted through simple and inexpensive management practices. 1) The Lambing Area A good lambing area will fill three major requirements: a) sufficient feed for the ewe so she can graze within about 20 metres of the birth site and not lose her lambs; b) shelter to reduce the chilling effect of wind; and c) protection from predators. Tall grass could accomplish the first two of these, but this may be difficult to find, so a mix of pasture, shrubs and trees (or specially-built shelters for small flocks) is perhaps the best option. In our situation, fodder trees are a good option the ewes could eat the tree and the rows of trees would serve as wind breaks and also isolate ewes from each other and help prevent them from stealing each others lambs, and then abandoning them when their own arrives (a common problem in synchronized lambings). Most farms should be able to provide a good lambing area because it needs only to be large enough to sustain the ewes for 2-3 weeks, from about a week before lambing is due to start until about a week after the last lamb is born. Another option is to provide this sort of special treatment only for the ewes bearing twins, where the losses are liable to be most serious. The high rate of mortality in twins has traditionally been seen as a major drawback in producing them but, even with Merinos, our experience suggests that the losses can be reduced to below 20% with very little extra input and remember, we are not advocating that they be given this attention all their lives just for 3 working weeks. 2) Strategic Feeding Obviously, good nutrition after lambing is essential for maximizing milk production and lamb growth. However, depending on the time of lambing, it may not always be possible to provide high quality pastures in this period or, more especially, throughout pregnancy. We are therefore looking for periods when a short, economical burst of a supplement will benefit lamb survival and growth. Studies by Lindsay et al (1990) have shown that: supplements in the early stages of pregnancy might increase the size and efficiency of the placenta; this should increase the birth weight of the lambs and perhaps the production of milk by the ewes; feeding a supplement during the last 7 days of pregnancy could double the amount of colostrum available at birth; this would provide a vital boost to the chances of lamb survival, particularly amongst twins born in bad weather; studies with Corriedales in Uruguay have recently shown that this can be achieved by feeding corn (500 g per day; G Banchero Hunzicker, personal communication). 3) Lambing a time for calm! This should be self-evident. The lambing ewes should be surveyed quietly, preferably on foot and from a distance, and without the help of the dogs or motor vehicles. Small lambing areas would make this a simple and pleasant task. 4) Back into the Real World Depending on the size of the lambing paddock and the resources it contains, the ewes and lambs can be moved after a week or so, but if they are moved early they should be moved slowly and for only short distances. 5) Weaning Our estimates suggest that this system should regularly produce a weaning rate of 65% of the ewes introduced into the system at the very beginning, or 113% of the ewes that lamb. With this figure in mind, it is easy to work out how many ewes are required in the first place to produce sufficient lambs for flock maintenance (where wool is the primary 48

13 product) or for a balance of wool and meat production. 49 Weighing up the Costs and Benefits Many of the costs and benefits cannot be assessed in financial terms in an article such as this, because they will depend on the management of individual farms (eg, the cost of extra rams), or on the choice of options available to accomplish a particular part of the scheme (eg, nutritional supplements, scanning), or on the value that is placed on labour. Therefore, we will list some items below that need to be considered when assessing the value of the program. The Extra Costs i) Progesterone and T-wethers In our situation, the wethers come from the farm s wether flock (on most Australian farms, about half of the sheep are wethers) and go back into it when the mating program is finished. The cost of the testosterone and progesterone treatments is very small so it is simply a matter of determining the expense of maintaining a small group of animals that have been castrated before puberty. ii) Nutrition or hormones for twin ovulations The additional costs here are for the hormone treatments (there is a wide range of options and so a wide range of costs) and the for the feed supplements at mating and just before lambing. iii) Extra rams iv) Ultrasonic scanning The total cost is reduced by using the T-wethers to detect many non-pregnant ewes. v) Lambing paddock The Extra Benefits i) Sheep numbers maintained while maximizing wool production. The basis of this is our contention that dry ewes can produce as much wool per hectare as wethers. This clearly is an important benefit only in wool-producing systems. For a meat-producing system, we would shift the emphasis to getting a greater proportion of ewes pregnant and perhaps increase the emphasis on ovulation rate. ii) Flexibility in planning wool or meat production Running a large ewe flock, with only a small proportion of them lambing, provides the capacity to increase the sheep numbers rapidly, if a new management strategy requires it, or for switching the emphasis from wool to lamb production if the wool price was to fall dramatically. iii) Short lambing period This is a major selling point for our system. Lambing can be assigned well in advance to a particular week during which the workload would be heavier than normal. iv) Lambs of uniform age This may be important where there is an emphasis on meat production and a high price is paid for a group of animals of the same age. v) Predictable production By the time the ewes have been scanned, the farmer will know very accurately how many lambs he will have in 4 months time. This allows the planning of strategies for that time and also allows a second attempt to mate the flock, if there has been a breakdown in the system. Conclusions This scheme has been developed from the results of over 30 years of research into the problems of reproductive wastage in Merino flocks in Australia. The fact that it appears to be based on simple concepts and ideas, and is not "high tech", belies the sophistication of the

14 system without intensive research, we would never have reached the level of understanding of the physiology and behaviour of our sheep that allows us to develop such schemes, let alone propose them as serious alternatives to current practices. Whether our ideas are of practical benefit in Mexico is a question we cannot answer, but the point of describing it here is not to encourage all Mexican breeders to adopt it, but to generate some discussion about alternative ways to manage reproduction. However, like Australians, I am sure that Mexicans would like a low-cost system for producing lambs and a system in which there is sufficient flexibility to follow changes in markets. Acknowledgements The research that led to these concepts and the formulation of the program were the result of the efforts of a team of people which includes the author, Professor David Lindsay, Dr Ian Purvis, Dr John Wilkins, Dr Chris Oldham, Dr David McNeill, Dr Trish Murphy, Dr Raymond Nowak, Dr I Gede Putu, all from The University of Western Australia. A great deal of the work was funded by the Australian Meat & Livestock Research & Development Corporation in their more sensible period. References Donrov, Ts., Batsaihan, D. & Ley, W.B. (1998). Gonadotropin extraction from pregnant mares' serum and effect of PMSG preparation on the fertility of Mongolian native ewes. Small Ruminant Research 28, Gherardi, P.B. & Lindsay, D.R. (1980). The effect of season on the ovulatory response of Merino ewes to serum from pregnant mares. Journal of Reproduction & Fertility 60, Lindsay, D.R., Cognié, Y. & Signoret, J-P. (1982). Méthode simplifiée de maîtrise de l'oestrus chez la brebis. Annales de Zootechnie 31, Lindsay, D.R., Nowak, R., Gede Putu, I. & McNeill, D. (1990). Behavioural interactions between the ewe and her young at parturition: a vital step for the lamb. In: Reproductive Physiology of Merino Sheep Concepts and Consequences (Eds.: CM Oldham, GB Martin & IW Purvis) Chapter 15, pp [School of Agriculture (Animal Science): The University of Western Australia, Perth]. Martin, G.B. (1995). Reproductive research on farm animals for Australia some long-distance goals. Reproduction, Fertility and Development 7, Martin, G.B., Oldham, C.M., Cognié, Y. & Pearce, D.T. (1986). The physiological responses of anovulatory ewes to the introduction of rams a review. Livestock Production Science 15, Martin, G.B. & Walkden-Brown, S.W. (1995). Nutritional influences on reproduction in mature male sheep and goats. Journal of Reproduction & Fertility Supplement 49, Nottle, M.B., Kleemann, D.O., Grosser, T.I. and Seamark, R.F. (1992). Use of the ram-effect and shortterm lupin feeding to increase ovulation rate in Merinos. Animal Production in Australia 19, 200. Nowak, R., Porter, R.H., Lévy, F., Orgeur, P. & Schaal, B. (2000). Role of mother-young interactions in the survival of offspring in domestic animals. Reviews of Reproduction 5, Signoret, J-P., Fulkerson, W.J. & Lindsay, D.R. (1982). Effectiveness of testosterone-treated wethers and ewes as teasers. Applied Animal Ethology 9, Walkden-Brown, S.W., Martin, G.B. & Restall, B.J. (1999). Role of male-female interaction in regulating reproduction in sheep and goats. Journal of Reproduction & Fertility Supplement 54, Wilkins, J.F. (1990). Contribution of embryo loss to reproductive performance in Merino ewes. PhD Thesis, University of Western Australia. 50