Vet Column 7-27-09 |

Vet Column 7-27-09

David L. Morris, DVM, Ph.D.
Fort Collins, Colo.

More than 45 years ago, pioneer research by Dr. James Wiltbank documented the relationship between nutrition and reproductive performance of beef cows. An important component of beef cow-calf production management revolves around balancing cost-effective, nutrient quality diets during late gestation when requirements increase most rapidly. Since many beef cows are lactating during mid-gestation, does it make sense to wean calves at six months-of-age rather than seven-plus months-of-age as a management tool to increase next calving season’s breed back?

Nutritionally, beef heifers nursing that first calf are affected most, partitioning nutrients for not only maintenance and lactation, but also for growth toward mature size. Data exist that documents that pregnancy rates in two-year-old and three-year-old cows are often the lowest in the herd. Investigators from West Virginia University recently published data evaluating the variation of time of calf weaning on cow body energy status throughout the production cycle and postpartum reproductive efficiency.

Mostly Angus and Hereford cows, 408 head, pregnant and lactating, were used in this study over a three year period of time. Calving season began in early February and lasted until early April. Breeding season lasted 60 days beginning in mid-May until mid-July. Cows were maintained in separate breeding age groups of 2-, 3- to 4-, and ≥5-yr-old. Bull to cow ratio was maintained at 1:20.

Profiles of energy status included measurements of body weight, body condition scores, and rib and rump fat. Reproductive performance was evaluated by calving intervals, days from initiation of breeding to calving, retention in the herd, and adjusted 205-day weaning body weight of the subsequent calf. Early weaned calves were weaned at 180 days-of-age. Control calves were weaned at 225 days-of-age.

The average calving interval in this study was 372.4 days. It varied with year and tended to be affected by cow age. It was not affected by weaning treatment. 2-year-old cows had the longest calving interval, whereas 3-year-old cows had the shortest. Data from this study indicate that once cows calve a second time, the effects of age on reproductive efficiency become minimal.

Retention in the herd was a function of pregnancy, calving, and culling rates. Early weaning increased the proportion of cows retained in the herd by 11 percent.

Calves from early weaned cows were heavier at weaning (18.9 lbs) than from traditionally weaned calves. Calves from ≥4 yr-old cows were heavier than from 2- and 3-year-old cows.

Conclusions from this study indicate that early weaning improved energy status and production efficiency in beef cows. Age of the cows affected body weight, body condition scores, rib fat, rump fat, calving interval, breeding to calving interval, and subsequent calf weaning weights. Based upon this study, calves from first and second parity cows should be weaned earlier than normal weaning of calves from older cows.

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