Can We Build the Cowherd by Increasing Longevity of Females?

Abstract Text: Increasing longevity of beef cows by decreasing proportion culled due to reproductive failure reduces number of replacements needed to sustain a constant herd size.  Rate of reproductive failure varies with cow age, where failure in cows younger than 4 yr of age can be 2-fold greater than in cows 4 yr and older.    In addition, BW of cow and calf at weaning also increase as cows advance from 2 to 5 yr of age.   Cumulative effect of improving retention in young cows is greater production efficiency through decreased replacement rate and a consequent change in age structure of the herd towards a greater proportion of cows at their maximal production potential for calf BW at weaning and cow BW at time of culling.    Calculations from cow age specific culling and BW data from commercial and research herds indicates that reducing replacement rate from 20 to 15% can result in annual  increases of 20% of total calf crop weight and  10%  in cull cow BW.   Although improving longevity can foster increases in efficiency, genetic advancement in longevity is challenging as it is the sequential culmination of the annual repetition of numerous discrete physiological processes, each  ending in a qualitative response; including puberty, ovulation, transport of male and female gametes, fertilization, implantation, pregnancy maintenance, parturition, and calf survival.  Successful completion of one process is prerequisite to evaluate subsequent processes.  Comparisons among different biological types of cattle maintained under varying levels of nutritional inputs provide evidence for genetic variation in prioritization of nutritional partitioning among production traits (i.e., milk, growth, and reproduction) and the apparent nutritional threshold required for initiation of reproductive processes indicating genetic by nutrition interactions.   This is in contrast to traits for which EPDs exist, where genetic by environmental interactions are not substantial.   The impact of nutrition on reproduction has been extensively studied.  Results for this research led to recommendations that heifers and cows be fed to a threshold BW or BCS to ensure reproductive success; a process that basically overrides nutritional interactions resulting in reproductive failure thereby minimizing selection of animals better suited for sustained reproductive function under limited nutrition.    Rearing and managing cows under nutritionally limited environments can result in adaptation leading to relatively high levels of reproduction with lower levels of input.   These management strategies may result in fetal programing that improve chances for longer retention in their offspring.

Keywords: Beef cow, longevity, efficiency