The impacts of weaning timing on bison calf performance

Bison behave and use landscapes differently than cattle and may require adjustments to the typical livestock management system. Forcible weaning is a practice commonly employed on cattle ranches that may not translate directly to bison. Observation suggests that bison calves weaned forcibly are lighter at age compared to naturally weaned bison calves.  The process of weaning involves the cessation of suckling and parental care. In some cases, weaning also refers to the breaking of the maternal-filial bond. Weaning is typically accomplished in commercial cattle operations by physically separating the calf and dam. This typically takes place at around 6 months of age to improve the cow’s body condition and prepare her for the upcoming lactation demands of a new calf. Forcible weaning also allows for changes in pasture management and marketing of calves. Natural weaning in bison is a process the involves the steady reduction in milk production and decreased effort by the dam to care for the calf. Natural weaning in bison typically occurs between 9 and 12 months of age.  Suckling can continue for significantly longer with mothers that are barren the following year. While cattle are weaned to improve the condition of the mothers, forced weaning and separation over the winter does not significantly impact mass gain or reproductive rate in mothers. These results suggest that forced weaning is not necessary in bison herds to increase reproduction; however, forced weaning may be justified by management or marketing needs, or by increases in commercial production of weaned animals.  Forced weaning may impact the performance of bison for reasons beyond the removal of the supply of milk. Forced weaning typically entails a simultaneous change in the environment, social setting, and food supply, and a loss of the social contact with the mother. High rates of vocalization, more time walking, running, and pacing, and less time resting are typically observed in beef calves following separation from their mothers. This usually results in lower feed intake for a period of time that can result in decreased growth rate or even weight loss. During their second and third years, having been forcibly weaned may impact the amount of aggression calves receive from other herd members. It was reported that naturally weaned bison heifers on Antelope Island, Utah experienced loss of resources due to other herd members less often than those that had been forcibly weaned. Forced weaning may also curtail the period of adaptation to solid feed and learning of what to eat from the dam. Forced weaning (at six months) in beef cattle occurs without completion of this period of learning and adaptation. Natural weaning in bison occurs between 9 and 12 months, so weaning before this point may limit learning from the mother and have later impacts on diet selection. Our study seeks to understand the implications of weaning and weaning time on bison calf performance. Groups consisting of 50 males and 50 females will be subjected to 3 different weaning treatments: fall wean (FW) (November), spring wean (SW) (February), and natural wean (NW) (remain with mothers until finishing). All animals will be followed until ~20 months of age.