Animal Science Department


Date of this Version

December 1993


Published for Proceedings, The Range Beef Cow Symposium XIII December 6, 7, & 8, 1993, Cheyenne, WY.


Profitability of calf production can be greatly increased by improvements in reproductive efficiency. Losses in income are associated with nonpregnant cows, calves that are born late in the calving season and death of calves at birth. Too many heifers and cows exposed to bulls during a breeding season do not become pregnant and wean a calf the next year. A major reason for the inefficiency is that cows are not pregnant at the end of breeding because many have not been in estrus and bred by the bulls. In addition, some cows do not start estrous cycles until late in the breeding season, so if they become pregnant calves will be born late the next year. Calves lost at birth are usually associated with difficult or delayed parturition.

Two major factors that regulate reproductive performance of beef cows are nutrition and suckling. Nutrient intake influences body energy stores. Body energy stores can be mobilized when a cow receives less than the required amounts of nutrients during pregnancy or lactation. Body fat stores regulate the secretion of hypothalamic (brain) and pituitary hormones to control the functions of the ovary. If cows are too thin at calving, the hormonal signals necessary to stimulate the ovary and start estrous cycles are not released and cows don't exhibit heat until late in the breeding season, or not at all. The suckling stimulus also delays the release of hormones necessary for the reinitiation of estrous cycles after calving. Cows that lose their calves at birth usually come into heat sooner than cows with suckling calves. Under range conditions we need to utilize cows to convert forage to milk for growth of calves. There are few management options that can be used to increase reproductive performance by altering suckling. The practical approach to increase pregnancy rate, and time during the breeding season that cows become pregnant, is by strategic use of supplemental feeding during pregnancy.