In most regions of the world, there is an optimal period for females to calve, suckle, and rebreed. This period is mostly related to nutritional opportunity, although other environmental factors such as cold or heat stress and parasite populations may play a role. Producers have traditionally aimed for females to calve during this optimal period because they tend to breed back faster, and their calves are more likely to thrive, than females that calve at less opportune times. Benefits of restricted breeding seasons (65–80 days) include enhanced production potential, favorable environmental factors, a concentrated calving season and more homogeneous calf crop to sell, increased opportunities to perform prebreeding management procedures and monitor nutrition, improved female replacement and culling procedures, and the ability to detect problems early, using herd pregnancy diagnosis and breeding season evaluation. (Also see Management of Reproduction: Cattle.)
The longterm goal is to develop a beef herd of uniform, low-maintenance cows that thrive in their given environment. This goal is strengthened by having a restricted breeding season in which open cows are culled from the herd. Often, these cows are too big, produce too much milk, or lack inherent fertility compared with cows that are pregnant year after year. In herds that are years away from this goal, instead of simply calling cows pregnant or open, cows should be sorted by projected calving date and body condition score (BCS). The herd pregnancy diagnosis represents an important starting point for beef herd diagnostics and advice and is a pivotal component for informed decision making. It allows analysis of group patterns for problem solving (breeding soundness evaluation, see Breeding Soundness Evaluation), as well as sorting of animals into groups for specific purposes such as strategic feeding, calving supervision, culling, or rebreeding. Herd pregnancy diagnosis facilitates the selection of replacement females and culling for infertility.
Reproductive performance is influenced by many factors, including sire fertility, herd health aspects, and opportunities to mate. Breeding soundness evaluation is a technique to assess the reproductive performance of the cow herd. It includes obtaining relevant information, analysis and interpretation, and recommendations for improvement. One measure of reproductive performance is the number of bred females that actually raise a live calf. Other valuable information may be obtained from an analysis of the distribution of pregnancies (and calvings) that resulted from a particular breeding season. This distribution may be studied on the basis of timing (eg, 21-day estrus periods), breeding groups, female age or parity, nutritional opportunity (eg, BCS), etc. Such analyses provide the basis for evaluation of either the breeding or calving season. For evaluation of the breeding season, good records must be obtained at the time of pregnancy testing for the subsequent analysis to be valid.
Because reproductive performance is an important economic trait in the cow herd, the reproductive capabilities of breeding bulls assume great importance. The best assurance that a bull is likely to be fertile is a successful breeding soundness evaluation (see Breeding Soundness Examination of the Male). A rule of thumb for bulls is that they can breed ~1 cow/mo of age in a breeding season of 65–80 days. For example, a 38-mo-old bull that passes his breeding soundness examination should be able to breed 38 cows in 65–80 days. This rule can be applied to bulls approximately 14–50 mo old, with 50 cows per bull being the maximum.