In their natural state, sheep are seasonal breeders; offspring are born at the time most favorable for their survival. In some domestic sheep, the breeding season has been altered both naturally and through the use of hormones.

Normal Breeding Habits of Sheep

Age of Puberty. Ewes typically reach puberty at 5 to 12 months, depending on breed, nutrition, and date of birth.

Anestrous Period (reproductive inactivity). This is the period when ewes normally do not demonstrate estrus (heat). Three types of anestrous are observed in ewes: seasonal (influenced by length of day), lactation (influenced by the sucking stimulus of lambs), and postpartum.

Length between estruses, or heat periods. The normal cycle for ewes is approximately 17 days between heat periods. However, it can vary from 14 to 19 days.

Duration of estrus, or heat period. The heat period usually lasts 30 to 35 hours, with a range of 20 to 42 hours. Ovulation occurs late in the period.

Gestation Period. The normal gestation period of ewes is approximately 147 days, ranging from 144 to 152 days. The medium-wool breeds and meat-type breeds ordinarily have a shorter gestation period than do the fine-wool breeds. High temperatures and high nutrition levels may shorten the gestation period two or three days. Ewes bred to white-faced, wool-breed rams may have a slightly longer gestation period than those bred to black-faced, meat-type rams.

Breeding Ewe Lambs. Ewe lambs that breed and lamb as yearlings generally have a greater lifetime production than ewes that have their that first lamb as 2 year olds. Since the onset of puberty depends largely upon body weight, ewe lambs should be provided adequate levels of nutrition to reach at least two-thirds of mature weight before breeding. Also, lambs born in winter or early spring are more likely to exhibit heat the first year than are lambs born later. Separate ewes that lamb as yearlings from mature ewes, and manage and feed them so that the yearling ewes can grow to their maximum potential size.

Ewe lambs and yearlings are normally rather shy breeders. For best results, breed them separate from older ewes. In some cases, it may be better to use rams of smaller breeds on young ewes to minimize the chance of lambing difficulties.

Effects of Environment

Sexual activity in sheep is primarily controlled by the ratio of daylight to dark. Estrus becomes more frequent as the days become shorter. In general, fertility is highest and most efficient when ewes are bred in September, October, or November; ewes bred at this time generally produce the highest percentage of multiple births.

High temperatures are detrimental to fertility, embryo survival, and fetal development. This is the biggest objection to fall lamb production. High temperatures at breeding can reduce conception rate. Heat stress during gestation impairs fetal development and can cause lambs to be significantly smaller at birth.

Psychological Stimulation

The introduction of a ram near the end of the anestrous period appears to psychologically stimulate ewes. It brings about earlier ovulation and estrual activity. The ram can be either fertile or surgically sterilized. Rams should be kept with the ewes for about 10 to 14 days and removed from the flock before breeding begins. Then, at the beginning of the breeding season, rested fertile rams that are intended to sire the lamb crop can be introduced. The stimulation does not occur when rams are placed with ewes earlier, or when rams are simply left with the ewes continuously.

Effect of Nutrition

Nutrition has a direct bearing upon reproductive performance. Ewes kept in acceptable condition before breeding normally produce more lambs if they are flushed, or given the chance to gain weight before and during the breeding season. They can be flushed with rested pastures or by supplementation. Begin flushing three weeks before breeding and, if possible, continue through the first cycle (approximately 17 days).

Flushing ewes is most effective when they are mated early in the breeding season. Since ovulation rate is near a maximum during the middle of the season, flushing at this time is not as beneficial. The results of flushing are quite variable. Sometimes, when farm flock ewes are already on a high nutrition level before the breeding season, flushing may not affect ovulation or lambing percentage.

Nutrition affects total lifetime productivity of sheep 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 grazed on legume pastures, such as alfalfa and clover, may at times be less fertile. Under some conditions, the estrogen content of these legumes is related to reproductive disorders. Breeding dates may be delayed and conception rate reduced when ewes are on pastures that have a high estrogen content. However, the estrogen content of legumes declines during the later stages of maturity.

Effect of Lambing and Lactation

Both lambing and lactation suppress estrous cyclicity in ewes. Generally, the postpartum anestrous phase lasts through lactation, even though the uterus typically returns to normal two to three weeks after lambing. Most ewes that lamb in late winter or spring do not exhibit estrus until the following breeding season. However, ewes that lamb in the fall usually exhibit a fertile heat four to eight weeks after lambing, or approximately two weeks after weaning.

Effect of Disease and Parasites

Heavy infestation of internal parasites can reduce the body condition of breeding ewes and may reduce reproductive performance. To minimize negative effects, follow a regular parasite control program and vaccination schedule. A local veterinarian should be able to provide sufficient information to develop a flock health program.

Effect of Ram

Infertile, diseased, or disinterested 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 of the 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 upon season, temperature, sex drive, and body condition. Rams six years and older that are 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 the ram's semen quality. Rams may be completely sterile or show lower fertility during late summer as a result of the heat. If the temperature exceeds 90°F for an extended period, especially if the 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 desire to mate. Common diseases, such as those affecting the feet or any of the external breeding organs, can make it impossible for a ram to breed ewes.

The formation and development of sperm requires six to seven weeks. Therefore, after recovery from sickness 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 a lower lambing rate.

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, the use of a marking harness can be beneficial. If several of the ewes return to heat, it may be necessary to substitute another ram.

Using Hormones to Control Reproduction

Reproduction in sheep can be controlled by artificially inducing estrus, ovulation, and fertilization. The use of hormones is effective if management, genetic selection of breeds, and strains of breeds allow for out-of-season breeding. For accelerated lamb production or out-of-season breeding, use sheep that most normally fit the desired reproductive pattern. To further alter the reproductive process, regulate conditions such as light, temperature, nutrition, association with the ram, and other environmental factors that affect reproduction.

Hormones, along with practical selection and management practices, are useful to:

• Synchronize estrus during the breeding season.
• Increase the ovulation rate and incidence of multiple births.
• Induce fertile mating during anestrus.
• Induce early puberty.

In general, three types of hormones are used alone or in combination to achieve these objectives.

Progestogens. These are female sex hormones. They include those produced naturally as well as artificially. Progesterone is produced after ovulation by the corpus luteum, which forms on the ovary. Exogenous progestogens are used during the breeding season to synchronize estrus and ovulation. They also may be used during the anestrous period to help prepare the uterus for pregnancy and to sensitize the animal to be more responsive to hormones that cause estrus and ovulation. They can be administered by ear implant, daily injection, daily feeding, or by insertion of an impregnated sponge (pessary) placed in the vagina.

During the normal breeding season, progestogens can be used to synchronize estrus when used for a 10- to 12-day period. Estrus and ovulation usually occur between the second and fifth day following the end of treatment. However, fertility is usually suboptimal on the first cycle after progestogens are administered. Higher fertility is obtained from breeding at the second estrus. When ewes have been synchronized, they generally remain well synchronized through at least the first three post-treatment estrous periods.

Estrogens. Estrogens also are female sex hormones. They are produced naturally by the ovary or they can be produced synthetically. The estrogen concentration in the blood is highest just before and during estrus. The follicles on the ovary from which eggs are developed and released are the main source of estrogens in the female. The estrogen level, therefore, drops rapidly near the end of estrus, when ovulation occurs. Estrogens are responsible for behavioral estrus (or heat). In combination with progesterone, they sensitize the animal to respond to ovulating hormones. They also influence uterine development and the preparation of the uterus for pregnancy.

Gonadotropins. Gonadotropins are hormones that cause ovulation. They are produced by the pituitary gland as well as by certain other tissues. The gonadotropin that is used most successfully in controlling reproduction in sheep is follicle stimulating hormone. Additionally, human chorionic gonadotropin (HCG) has been used to induce ovulation.

Synchronizing Estrus

Some farm-flock producers find it advantageous to plan their breeding season so that all ewes lamb at approximately the same time. This can be largely achieved by treating ewes with progesterone for 10 to 12 days to synchronize estrus. When the progesterone is removed, the ewes exhibit estrus and can be bred at this time. However, for the largest lamb crop, breed the ewes following the second estrus after progesterone treatment. An injection of gonadotropin can contribute to multiple ovulation. Therefore, pregnant mare serum gonadotropin (PMSG) can be given as the progesterone treatment is terminated, and again 16 to 18 days later.

Accelerated Lambing or Out-of-Season Lambing

Accelerated lambing means lambing more often than the conventional once-a-year approach. Since ewes are pregnant for five months and nurse lambs for only about three months, they can be considered idle four months of the year. It is possible to lamb ewes every eight months (in some cases, every six months). On the surface, this seems like a logical approach to efficient sheep production, but that is typically not the case. Even with use of hormones, the success of most accelerated lambing programs depends entirely upon the competence of management. Because of the increase in disease, stress, and death loss associated with lambing, accelerated lambing is likely to reduce the length of the ewe's productive life and increase feed, labor, and managerial expenses.

An accelerated lambing program necessitates that lambs be weaned early. The recommended hormone treatment is the same as for estrous synchronization in that progesterone should be administered for 10 to 12 days and followed immediately with an injection of 500 to 750 international units of gonadotropin, and again 16 days later. If the ewes are in an anestrous period, it is sometimes helpful to administer 2 milligrams of estradiol two days before the start of progesterone treatment.

Producers who lamb outside the natural season can expect some loss in reproductive efficiency. Only outstanding managers and those who can afford to experiment should try accelerated lambing.

Artificial Insemination

The use of artificial insemination (AI) in sheep has been the subject of research for a number of years in the United States. Currently, frozen ram semen is available commercially. Additionally, transcervical techniques for AI have allowed some commercial producers to introduce AI into their breeding programs, but it is not commonly used by seedstock producers.