BEEF CATTLE REPRODUCTION IN RELATION TO THE ENVIRONMENT

Abstract

Results are reported from a 10-yr study of lifetime reproductive performance of beef cows maintained under two contrasting environments. The data comprised records for 8160 cow years. The breeding herds were maintained for 6 yr in accordance with the management practices conventional to each location with winter management changed (to drylot feeding) for the final 4 yr. Location differences in cow performance favored the herd maintained under semi-intensive management (Brandon, Manitoba). Conception rates were higher, calf mortality lower, and calf gains preweaning greater than recorded for the contemporary herd maintained under extensive range management (Manyberries, Alberta). These differences in performance gave Brandon a 16% advantage in weight of calf weaned per mating exposure. Cows at Manyberries, in the fall of 1977, the seventh year of study, were lighter (15%) and had less fat (60%) than their Brandon contemporaries. These differences increased to 19 and 80% during the first winter in feedlot confinement even though the daily feed energy allowance was 20% above requirements (National Research Council-National Academy of Sciences, NRC-NAS). Conception rates at Manyberries were seriously affected by mortality, both cows and calves, inversely related to cow fat at calving. An increase in feed inputs at Manyberries (42% above NRC-NAS) the following winter, coupled with the opportunity for tissue restoration provided by a barren year, reversed the downward trends in weight and fat and weight equivalence of the two herds was achieved by 1980. Milk yields from pastured cows during the last 4 yr were generally higher for the Brandon herd, but calf growth rate from birth to 2 mo of age was greater at Manyberries. Key words: Beef cows, reproductive performance, lifespan, environment