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Tips for Improving Out-of-season Reproduction

13 January 2021

Ohio State University

By now, 2020 fall lambing is a task of the past with lambs weaned and either sold at the sale barn or retained for feeding. Before winter and spring lambing floods our minds, now is an appropriate time to take a minute to review the fall lambing season.

For most, fall lambing in 2021 is in the far distance; however, timely planning now will foster future improvements.

This article provides tips for improving out of season reproduction in sheep.

Lamb supply is seasonal in nature and is explained largely by the seasonal nature of sheep reproduction, yet demand for product is present year-round. In order to meet market demand, deficits in supply are met by imported product and by domestic lamb feeders who hold lambs to extend the season of supply. In some parts of North America, the holding of lambs has hurt product quality, as lambs entering the market are often overly fat and mature.

Another strategy to fill in gaps in supply and to avoid product quality concerns is through out-of-season lamb production. Out-of-season production allows lambs with the optimal degree of maturity to be harvested throughout the year. This strategy, when combined with a decrease in the production birth interval, is referred to as accelerated production. Accelerated production has the potential to increase production efficiency and simultaneously remove the constraint caused by seasonal supply.

For accelerated production or other modes of out-of-season production to be successful, one must find a way to overcome the seasonal constraint on sheep reproduction. Fortunately, there are breeds that are less seasonal which have been selected to breed out of season.

Most of these breeds, however, still exhibit a degree of seasonality in their reproductive performance and exhibit a lower conception and ovulation rate in the less optimal mid-winter to mid-summer breeding period. Working with this relatively aseasonal genetic base, it is possible to manage this population to improve reproductive outcomes in the less optimal breeding season without the use of exogenous hormones.

Improved out of season reproduction in relatively aseasonal sheep can be realized by:

  • Improving the nutritional management of both rams and ewes
  • Improving breeding management practices
  • Screening for male fertility
  • Enhancing male biostimulation, often call “the ram effect”.

Ram effect:

The sight, smell (via pheromones) and breeding behavioral cues of rams can have a stimulatory effect on ewes that are “lightly” in . Some refer to this period as the “transition period between the non-fertile season and the fertile season. This effect is commonly referred to as “the ram effect”. Ewes that are lightly in anestrus include both those from highly seasonal breeds about to enter or exit the typical anestrus period (period of infertility present from February to July in the Northern hemisphere) and those from less seasonal breeds during the entire anestrus period.

The ram effect is strongest when ewes have been isolated from rams for at least 30 days. When ewes are isolated from males for extended periods, the reintroduction of males can either induce behavioral estrus a full 17-day cycle later, or cause ewes to exhibit a “silent” heat at 4-6 days following exposure, where the ewe ovulates but does not exhibit behavioral estrus. This silent heat is followed by behavioral estrus 17 days later. This creates 2 modes of behavioral estrus within a flock: one at 17 and one at 23 days following ram exposure.

Due to slight variations in the timing of estrus and in the length of gestation in sheep, one sees a concentrated birth period of approximately 7 days in length. The ram effect allows both the induction and synchronization of estrus. Most producers like the induction effect, but some would prefer not to synchronize ewes as it concentrates the lambing period to the degree that it can tax both facility and labor resources. An option to alleviate this might be to divide the ewe flock in half and stagger both teaser and fertile ram introduction to each group by a week.

Most typically, vasectomized rams (non-fertile rams often called “teaser rams”) are used to create this effect. They exhibit normal mating behavior, yet are not able to successfully impregnant the few ewes exhibiting behavioral estrus, resulting in better synchronization of breeding by the fertile rams. We are learning that most ewes, including those thought to be relatively aseasonal, stop cycling mid-winter to mid-summer and go into a light anestrus that can be rescued by ram introduction. If teaser rams are not used, and fertile rams are simply turned in during this period, the ram effect is still at play. It simply manifests in the majority of ewes settling on the second heat cycle following the fertile ram exposure.

If teaser rams are used, the process is “jump started” a cycle, with most ewes lambing during the first cycle following fertile ram exposure. We do not know if the use of teasers rams necessarily improves ovulation rate or conception rate over that observed using fertile rams that settle ewes in the second cycle following exposure. Some feel that use of teaser rams “spares” the serving capacity of fertile rams, since the fertile rams are not actively chasing as many ewes over an extended period to detect estrus, thereby conserving energy and improving conception rates. What we do know is that the ram effect is a major factor in natural mating during what is considered the out-of-season period, it synchronizes mating, and is able to jump start breeding.

Recommended teaser ram coverage is 2-3%, with rams introduced 14 days prior to the introduction of planned fertile rams.

Screening for ram fertility:

Rams should be screened for fertility prior to any breeding season to reduce risk of breeding failure. This becomes relatively more important during out-of-season mating. The minimal screening method is to palpate the ram’s testicles to record their firmness and to detect the potential presence of inflammation or scar tissue. Fertile ram testes are firm and never soft, nor are they hard to the touch. The presence of a distinct epididymis should also be present at both the head and tail of the testes.

Careful palpation screening can remove about 65-75% of infertile rams, but identification of the remaining 25-35% of subfertile/infertile rams requires a breeding soundness exam. This procedure requires collection of semen, typically via electric ejaculation, and semen examination for gross semen motility, functional morphology and indications of male reproductive tract infection. Rams with more than 80% normal semen morphology and strong waves of gross motility are the ones needed for the more challenging out-of-season breeding period.

Ensuring ram fertility:

Another method to improve male fertility is exposure to a lighting protocol. Photoperiod manipulation has been demonstrated to improve both ram and ewe fertility. However, the feasibility of placing the entire ewe flock under photoperiod control is less practical than applying control to a smaller population of rams. A light protocol that has been shown to improve ram fertility is a 120-day cycle consisting of: 30 days of 16 hr. light/8 hr. dark, followed by 30 days of 8 hr. light/16 hr. dark, followed by 30 days of 16 hr. light/8 hr. dark, followed by final round of 30 days of 8 hr. light/16 hr. dark followed by introduction of rams to ewes. Rams maintained under this protocol for 4 months appear to maintain a high state of fertility thereafter for the breeding season.

Ram coverage:

Ram coverage is recommended to be higher during the out-of-season breeding period. This has the effect of creating greater biostimulation (ram effect). Coverage of 3-5% is recommended with no estrus synchronization or 4-8% with estrus synchronization methods. A rule of thumb is to keep ram service to less than 6 ewes per 24-hour period. Many successful farmers perform out-of-season mating with a large mating group consisting of introducing a relatively large group of rams to a large group of ewes.

It is not clear if the dynamics of ram competition in a group mating scheme are beneficial or detrimental to flock conception, however. One can theorize that the negative influence of a subfertile, yet active male will be minimized in a group breeding scheme. However, does the enhanced competition ultimately harm or improve reproductive outcomes? Further research needs to be done to answer this question.

Nutritional management of rams:

Well-fed, fine-wool rams have enhanced fertility over those fed a maintenance diet during the out-of-season period. Therefore, it is recommended to increase the plane of ram nutrition to 2 times their maintenance energy intake for the 4-week period prior to mating so that rams are certain to reach a target condition score of 3.5 at mating. It is also recommended to group rams according to nutritional needs during this period, with rams under 2 years, and especially those under 12 months, grouped separately from mature rams.

The working model to explain the beneficial impact of nutrition on male fertility during the out-of-season period is that the signals that enhance testosterone production and semen quality are amplified when rams are in positive energy balance (gaining weight) and able to overcome the negative signals present that lower fertility during the out-of-season period.

Nutritional management of ewes:

At Michigan State University, we have initiated a series of studies seeking to optimize ewe nutrition to improve reproductive outcomes in accelerated production. The major aim of this effort is to improve reproductive outcomes in general but especially during the out-of-season period. This work was performed in an accelerated production program that uses a natural mating system without the use of exogenous hormone therapies.

So far, we have learned that flushing ewes to provide 2 times maintenance energy intake for the 3-week period prior to ram turn-in results in profound improvements in ovulation rate compared to maintenance-fed ewes during both the Spring (38% increase) and Fall (31% increase) mating periods. During the Spring period only, we also observed subtle improvements in conception rate in well-fed ewes, who conceived at 94% compared to maintenance-fed ewes conceiving at 82%.

We also studied the impact of underfeeding ewes during this period to 50% of maintenance, but interestingly, we saw no impact of undernutrition on reproductive outcomes relative to the ewes fed to 100% maintenance energy requirements. This tells us that supplemental feeding and weight gain (positive energy balance) is very important in optimizing reproductive outcomes during any breeding season, but especially so during the out-of-season period. We are now extending these studies to look at the impact of ewe nutrition prior to the flushing period (further back into the previous lactation) on several aspects of flock productivity in accelerated production.

In short, we highly recommend feeding ewes well for the 3-week period prior to mating in accelerated production. We have evaluated the change in income over feed costs by feeding ewes well for the 3-week pre-breeding period in accelerated production using a production simulation model under market conditions present in Michigan, USA in October 2018. This analysis revealed a remarkable 55% improvement in income over feed costs with improved pre-breeding nutrition.

Reproductive outcomes can be improved in sheep that are relatively aseasonal by screening ram fertility and enhancing the male biostimulation (ram) effect. Placing both rams and ewes in positive energy balance prior to the breeding season improves reproductive outcomes in a cost-effective matter regardless of season, with possibly more dramatic results during the out-of-season period.