Vet Column 5-3-10 |

Vet Column 5-3-10

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

For those beef producers who implement a fall-calving season, weaning generally takes place in mid-April around 210-days-of-age. As one attempts to generate as much profitability as possible, the issue of allowing beef calves born in the fall to nurse their dams longer to potentially increase weaning weights becomes worth considering. A recently published research article by investigators at Oklahoma State University offers some insight into this practice.

Data published more than 45 years ago document that body condition score at calving is an important factor affecting the length of the interval from calving to re-breeding, and subsequent pregnancy rates. For spring calving beef cows, as the body condition score at calving increases, the interval to first estrus shortens. What’s at stake for a fall calving season is the issue of grazing late summer grasses and the potential lack of body condition at fall calving which might compromise the post-calving breeding performance.

For four consecutive years beginning with weaning in 2004, 158 predominantly Angus fall-calving beef cows were used to determine the effects of weaning date and cow age class on cow and calf performance. Cows were divided into 3-year-old and younger and mature cows 4-years-old and greater. Weaning dates were considered normal weaning in mid-April at 210 days-of-age and late weaning in mid-July at 300 days-of-age.

Calving season began in late August and continued until late October. Breeding season began in late November and continued through late January. The study was conducted at the same location near Stillwater, Oklahoma for the four year period of the study.

Although body condition score was similar throughout the trials among both classes of cows, mature cows were heavier than young cows. For young and older cows weaned normally, the body condition score and body weights were similar. At the beginning of the calving season, however, the normal weaning cows were heavier and had greater body condition scores than the late weaning cows. Post-calving body weight losses and body condition score losses were greater for the normal weaning cows which resulted in similar body weight and body condition scores at the beginning of the breeding season and on through April.

Pregnancy rates for late weaning young cows were less than pregnancy rates for late weaning mature cows and normal weaning young cows. Pregnancy rates for normal weaning mature cows did not differ from that of late weaning mature cows or late weaning young cows. Normal weaning mature cows did have lower pregnancy rates than normal weaning young cows.

Calves born to normal weaning cows were heavier at birth by 5.3 pounds and weighed 17.6 pounds heavier at the time of normal weaning. This was partially explained by the normal weaning cows producing 1.3 pounds more milk as measured in February, a statistical finding in this study. Although normal weaning calves were heavier in April, the late weaning calves were heavier in July because of an increased average daily gain of 0.73 pounds more per day during the weaning interval.

Conclusions from this study indicate that when implementing a fall calving season for beef cows, it may be more advantageous to delay weaning of calves born to dams 4-years-of-age or older while maintaining a normal weaning of 210 days for dams 3-years-of-age or younger at the time of calving. Late weaning had no detrimental effects on the performance of mature cows.

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