Kerala, India
Nutrition directly impacts reproductive performance. Ewes maintained in acceptable condition before breeding typically produce more lambs if they are flushed, or given the opportunity to gain weight before and during the breeding season. They can be flushed with rested pastures or through supplementation. Begin flushing three weeks before breeding and, if feasible, continue through the first cycle (approximately 17 days).
Flushing ewes proves most effective when they are mated early in the breeding season. Since ovulation rate approaches maximum during mid-season, flushing at this time is less beneficial. Flushing results are quite variable. Sometimes, when farm flock ewes are already on elevated nutrition levels before breeding season, flushing may not affect ovulation or lambing percentage.
Nutrition affects total lifetime sheep productivity by influencing mature size. Well-developed ewes consistently have higher lamb crop percentages than smaller ewes. Fat ewes, however, are typically less fertile, do not respond to flushing, and may experience more embryonic death loss.
Ewes grazing on legume pastures, such as alfalfa and clover, may occasionally be less fertile. Under certain conditions, the estrogen content of these legumes relates to reproductive disorders. Breeding dates may be delayed and conception rate reduced when ewes are on pastures with high estrogen content. However, legume estrogen content declines during later maturity stages.
Effect of Lambing and Lactation
Both lambing and lactation suppress estrous cyclicity in ewes. Generally, the postpartum anestrous phase lasts throughout lactation, even though the uterus typically returns to normal two to three weeks after lambing. Most ewes lambing in late winter or spring do not exhibit estrus until the following breeding season. However, ewes lambing in fall usually exhibit a fertile heat four to eight weeks after lambing, or approximately two weeks after weaning.
Effect of Ram
Infertile, diseased, or uninterested rams often cause poor lambing rates. The average number of ewes that can be mated to a ram are as follows: well-matured ram lambs, 15 to 30 ewes; yearlings to five-year-old rams, 25 to 50 ewes. However, in many low-rainfall areas of New Mexico, the average number of ewes per ram may be 30 to 40 percent lower than these values. These rates depend on season, temperature, sex drive, and body condition. Rams six years and older in good physical condition may still be suitable for pasture or hand breeding.
Rams vary in their sexual behavior. Some rams mate repeatedly with the same ewes, even though several other ewes in heat are present. Some rams prefer black-faced or white-faced ewes when both groups are in the same flock.
Temperature has a pronounced effect on ram semen quality. Rams may be completely sterile or show reduced fertility during late summer as a result of heat. If temperature exceeds 90°F for an extended period, especially if humidity is high, fertility of most rams is reduced. Rams must be in good physical condition for successful reproduction. Malnutrition, internal parasites, or disease can cause sterility or depress the ram’s mating desire. Common diseases, such as those affecting the feet or any external breeding organs, can make it impossible for a ram to breed ewes.
Sperm formation and development requires six to seven weeks. Therefore, after recovery from illness or heat stress, it takes six to seven weeks for a ram to produce sperm capable of fertilization. An infertile ram in a one-sire flock can cause complete lambing failure. Also, a single dominant infertile ram in a large flock incorporating several rams can prevent fertile rams from mating and result in lower lambing rates.
It is important to fertility test rams, particularly in one-sire flocks. Semen testing by qualified veterinarians is recommended to farm-flock producers, especially when only one or two rams are being used. If semen testing is not possible, using a marking harness can be beneficial. If several ewes return to heat, it may be necessary to substitute another ram.