Management of age at puberty in beef heifers to optimize efficiency of beef production

A primary determinant of pregnancy success in the first breeding season of a heifer is the time at which puberty occurs relative to the start of her first breeding season. Subsequently, this affects her ability to rebreed in subsequent years and remain in the herd as well as her lifetime productivity. The critical nature of this physiological endpoint is influenced by factors related to management of the annual cycle of beef production as well as the physiology and genetics of the female. Beef production in almost all situations is seasonal in nature to coordinate feed resources with the nutrient requirements of the dam, and to some extent, the requirements of her calf. In the temperate regions of North America, most beef cows calve in late winter/early spring to match peak lactation (2 to 3 months post-calving) with peak growth of cool-season grasses and other grazed forages. In the sub-tropical regions of South America, the calving season begins shortly before the start of the rainy season, again to match forage availability with lactation requirements. The time at which female cattle, including heifers, must become pregnant each year is constrained by this seasonal cycle, with typical breeding seasons being two to four months in duration. In North America, a majority of Bos taurus heifers (breeds such as Angus, Hereford, Simmental, and Charolais) are expected to calve for the first time at 22 to 24 months of age (at which time they are referred to as primiparous cows) and at approximately 12-month intervals thereafter until 6 to 10+ years of age. To provide additional time for primiparous cows to recover from their first calving in preparation for their second breeding, they are often bred to calve 3 to 4 weeks before the multiparous cows in the same herd. This requires that heifers become pregnant for the first time at 12 to 15 months of age assuming a gestation period of slightly less than 9.5 months (280 to 285 days) and accounting for more than 2 months variation in the birth date of heifers selected to serve as replacements. Thus, consideration of seasonal constraints implies that puberty should occur within this time frame. However, there are other factors that impact recommendations as to when puberty should occur relative to the first breeding season in heifers. It has been demonstrated that conception rate of heifers increased by approximately 21% (Byerley et al., 1987; Perry et al., 1991) from the first ovulation to their third estrous cycle. It has also been shown that timing of conception in the first breeding season impacts lifetime productivity. Beef heifers that conceived early during their initial breeding season and calved as 2-year-old females had a greater probability of becoming pregnant as primiparous cows (Burris and Priode, 1958), had greater lifetime production reflected in greater weaning weights, and tended to calve earlier in subsequent years (Lesmeister et al., 1973) compared with females that conceived later in their first breeding season. Hence, age at which puberty occurs and timing of this event relative to the breeding season will impact the time of conception in the first breeding season, lifetime productivity, and economic efficiency of beef production.