How do sheep reproduce

The retina of the eye is stimulated by the presence of light. When light is abundant, an excitatory pathway is active inhibiting melatonin release. Because of genetic differences, different breeds of sheep maintain different levels of photoresponsiveness.

Melatonin is a modified amino acid secreted by the pineal gland that communicates information about environmental lighting to various parts of the body. Melatonin has important effects on maintaining biological rhythms, and has important effects on reproductive function.

Light that is exposed to the retina is first relayed to the suprachiasmatic nucleus of the hypothalamus, an area of the brain well known for coordinating biological clock signals Fibers from the hypothalamus descend to the spinal cord and project back to the pineal gland.

Thus it converts signals from the sympathetic nervous system into hormonal signals Bowen. Seasonal changes in day length have profound effects on reproduction in sheep, and melatonin is a key player in controlling such events.

During the non-breeding season, the gonads become inactive, but as breed season approaches, the gonads must be rejuvenated.

Photoperiod is the most important cue to allow animals to determine what season it is. The pineal gland is able to measure day length and adjust secretion of melatonin accordingly.

In species with longer gestation times, such as sheep, the anticipation of spring must occur much earlier, and seasonal changes in the fall act as a stimulus. Short photoperiods stimulate gonadol activity. Melatonin is released during the night, so as days shorten and nights lengthen in the autumn, the duration of the nocturnal melatonin peak is increased. Conversely, after the winter solstice, the photoperiod increases and the duration of the melatonin signal falls Essential Reproduction, Seasonal breeding in sheep is a serious challenge for the industry.

It reduces the effectiveness of accelerated lambing programs, restricts integrating lambing into other farm activities, and limits access to favorable seasonal markets Notter. Potential genetic improvements are possible by selection to reduce seasonal breeding.

Hybrid vigor must be achieved using a terminal-sire-crossbreeding program. There are breeds that are less seasonally bred such as the Dorset that make for good candidates for a breeding program to reduce seasonal breeding.

These breeds will increase fertility but will reduce lamb growth rates and carcass value Notter. Bowen, R.. Schoenian, "Sheep ". Senger, P. Pullman: Current Conceptions, Inc. Notter, D. Biology of Reproduction 78, Johnson, M. Essential Reproduction. Blackwell Publications, Regardless of this breed-related variation in the length of the breeding season, all breeds are most fertile in the fall, and anestrus is an unlikely problem associated with regular annual breeding.

Estrous periods that occur in the fall are longer and more intense, and maiden ewes have a shorter and less intense estrus than mature ewes. The optimal time to breed ewes naturally or artificially is in the first half of the estrous period, or 12—18 hours after the onset of estrus. Ewes show no overt signs of estrus, and heat detection requires the presence of a ram, a teaser ram made infertile by either vasectomy or epididymectomy , or a testosterone- or estrogen-treated wether.

The age of puberty of ewe lambs varies greatly and is influenced by breed, nutrition, presence of the ram, and season of birth. Well-developed ewe lambs with a body condition score of 3—3. Two exceptions exist: lambs born in the fall to a spring breeding will reach puberty in the spring and be less fertile to an induced estrus, and some breeds of sheep are slow maturing eg, range breeds and may not be fertile at 7—8 months of age.

However, breeding of ewe lambs early is encouraged, because this can be a major determinant of increased productivity and profitability for the overall flock, especially meat breeds.

Ewes that breed as lambs are able to produce more lamb crops in their lifetime than those first bred as 2-year-olds. Ewe lambs should be separated from ram lambs by 5 months of age to avoid unwanted pregnancies, and from all market lambs to avoid overconditioning from a diet too high in energy. Over-conditioned ewe lambs are less fertile and produce less milk than those fed to achieve a body condition score of 3.

Follicle development and ovulation rates are major determinants of fertility. Ovulation rate is a polygenic trait showing marked breed difference and moderate heritability 0. Selecting replacement ewes born as twins or triplets will slowly increase prolificacy within a flock, but using more maternal breeds is recommended because their offspring will also have desirable traits of better mothering and milk production.

High levels of soluble protein can cause early embryonic death via increased serum urea nitrogen concentration. Overfeeding of energy to ewes in good body condition can cause decreased fertility. The sudden introduction of novel rams or teasers "ram effect" to anestrous ewes can induce the onset of ovarian cyclicity during the transition season, most often 4—6 weeks before the onset of the ovulatory season in seasonal breeds. However, it is less effective in synchronizing estrus once ewes have started to cycle.

Responding ewes commonly ovulate within 48 hours of ram introduction but usually are not receptive, ie, the estrus is silent. In ewes with a silent estrus, ovulation is followed by the formation of either a normal or a short-lived 5—6 days corpus luteum CL.

After regression of a short-lived CL, ewes ovulate without displaying signs of estrus and commonly form a normal CL. The PMSG causes heat and ovulation. Some research projects have also induced sheep to cycle out of season with a treatment of progesterone followed by the sudden introduction of rams ram effect. Conception is the point in time when the sperm fertilizes the ova. Conception rate is usually considered to be the number of ewes that lambed compared to the number of ewes exposed to the ram.

Using this broad definition of conception rate, more factors can influence the results, because both fertility and embryo loss can be included in the outcome. Nawaz and Meyer reported the performance of ewes of different breed crosses in Oregon between and Ewes were mated for the first time at 18 months of age.

Ewes were group mated on pasture to Hampshire rams for 42 days starting the third week in August after exposure to teaser ram for 2 weeks. The ram-to-ewe ratio was with rams periodically replaced with fresh rams Table 1. There was no significant difference in conception rate between the breeds shown when using the ram effect and exposing ewes in season for several cycles. Casas et al. The 5-year project was designed to produce 1, F1 ewes that lambed at 1, 2 and 3 years of age.

Ewes were group mated by Suffolk rams for day periods that began August 5, October 15 and December Ewes bred in August were exposed to a teaser ram for 17 days prior to breeding. Table 2 shows that the ewe lambs had significantly lower conception rates compared to 2- and 3-year-old ewes. They also appear to have had a shorter season than their older counterparts.

In a follow-up study, Casas et al. Ewes were exposed to a teaser ram for 16 days before breeding. Rams were light treated 6 weeks of 8-hour days prior to breeding and semen tested.

The results are shown in Table 3. Overall conception rates are much lower in May than in March. There is a significant difference between breeds in the May conception rates. It is interesting to note that the Romanov cross ewes bred as well in May as they did in March. Source: Nawaz and Meyer Source: Casas et al. Notter evaluated conception rates of crossbred Rambouillet Finn ewes from in Maryland further south than Ontario, so theoretically better for out-of-season breeding.

Ewes were bred on an accelerated lambing schedule in August, November and April, exposed to Suffolk rams for 45 days for each breeding. Table 4 shows the accelerated lambing results for a system of three lambings in 2 years. Table 4 also shows significantly lower conception rates out of season compared to in season. The November conception rates are lower than expected rates for ewes bred annually in November. It appears that ewes being bred on an accelerated lambing schedule may have lower conception rates in season.

Lunstra and Christenson synchronized ewes with progestogen sponges for 16 days and injected them with PMSG on day The results for the two groups were not statistically different, showing that synchronization worked as well mid anestrus as in late anestrus Table 5.

Conception rates were highest among ewes that exhibited estrus within 72 hours of the PMSG injection. In the same study, ovulations were actually counted at hours after the onset of estrus. This showed that The authors speculated that when ewes are treated to breed out of season, there may be greater asynchrony between the time of heat, LH surge and ovulation, particularly in those ewes that don't show heat within 72 hours of PMSG injection.

This could be one reason for the difference in the number of ewes in heat and ewes actually lambing. Target conception rates are calculated based on either how many ewes produce lambs or how many ewes are pregnant at scanning. Table 6 targets are based on how many ewes produce lambs.

Many factors can affect target conception rates. These will differ depending on the age of the ewes, lambing interval, breed and production system.