A ewe's nutritional needs are not static; they vary largely with her stage of production. For 16 to 20 weeks of the year, the ewe's energy needs are very critical (such as during breeding, immediately before lambing, and while lactating). Feed levels can be lowered to reduce the feed cost during the early stages of gestation and when ewes are dry.

Maintenance of the ewe is generally thought of in terms of her nutritional requirements when dry, because at that time her requirements are the lowest of the year. However, wool production is a continuous process that must be considered as part of the nutrient requirements throughout the year.

One of the most reliable sources of information regarding sheep nutrition is The Nutrient Requirements of Sheep (sixth edition, 1985), which was produced by the National Research Council (NRC). Table 3 illustrates the requirements of sheep of different biological types and in different physiological stages of production. Use the data only as guidelines, not as rigid standards. In any flock, sheep are of different sizes and in different stages of production, and it is not always possible to know at each feeding the exact nutrient composition of the feed. However, if the producers follow the NRC guidelines, the flock's nutritional requirements will be met as closely as scientifically possible at this time.

The energy requirements are a function of the animal's basic metabolic rate. However, several factors affect maintenance requirements.

Age. Yearlings tend to have about a 20 percent higher energy requirement than adult sheep. This is probably due to the yearling's additional requirements to support growth. This is of particular importance to producers who breed ewes to lamb first at 12 to 18 months of age.

Exercise. Grazing sheep may use from 10 to 100 percent more energy than do sheep in drylot conditions. However, the magnitude of increase depends on the distance sheep must travel to feed and water, and on the topography of the range.

Climate. Temperature, wind velocity, and humidity can jointly affect energy requirements. The length and density of the fleece also affects energy requirements. Wool plays an important role in protecting sheep from both heat and cold. The insulating properties of wool help to cool the sheep in the heat of summer and keep body temperatures warmer in winter. Without wool, a sheep's energy requirements would be higher.

Body Condition. It takes more feed to maintain a fat sheep at a constant weight than it does a thin sheep. Keeping the sheep excessively fat is not only expensive because of the feed, but also it is detrimental to the ewe's reproductive capabilities and overall production efficiency. A ewe should lose 5 to 7 percent of her body weight during lactation and recover this weight loss during the dry period. Additionally, the ewe should gain body weight during gestation in proportion to the weight of the fetus and accompanying fluids.

In most sheep production situations, it is most economical to increase body condition of the ewes during the nonlactation period and "milk it off" in lactation, especially when low-cost pasture is available from early to mid-gestation.

Reproduction Requirements. Reproductive efficiency depends largely upon proper nutrition before and during the breeding season. Large-bodied ewes tend to produce more lambs per ewe. Do not confuse ewes of large size and scale with ewes that look large because they are fat. Usually, excessively fat ewes have lower conception rates and higher embryonic mortality. Furthermore, extremely poor body condition is not conducive to efficient fertility and reproductive performance. Ewes that have not had a properly balanced diet, including adequate phosphorus and vitamin A, may have a poor lamb crop percentage.

Flushing. Flushing can improve the ewe's body condition just before and during the breeding season. Generally, the practice is thought to increase ovulation rate. Flushing has more effect early in the breeding season. It is also beneficial late in the season, as it tends to increase the opportunity for all ewes to become pregnant. Flushing may be achieved by moving the ewes to a better pasture shortly before breeding. The provision of a supplemental energy source (that is, 3 / 4 to 1 pound of whole corn per head per day) and(or) the introduction of ewes to fresh pasture also can enhance the potential for ewes to respond to flushing. The length of the flushing period can vary, but it probably should begin 21 days before the breeding season and continue through one estrous cycle (17 days) into the breeding season if possible.