By Glenn Selk, Oklahoma State University Extension Beef Cattle Specialist
Occasionally coffee shop wisdom can lead a cow-calf producer to a wrong conclusion. Such is the case with the situation of birth weights that can be expected of fall calving cows.
The spring calving cows that have lost condition throughout the winter are nearly always thinner in February and March, than their counterparts that calve in September and October. Many producers may think that because the fall calving cows are in excellent body condition, that they will automatically have larger birth weight calves than similar cows calving in the spring.
Oklahoma State University researchers used five years of data from the North Lake Carl Blackwell range to answer the question of birth weight differences by seasons. Records of 414 gestations and live births (242 spring and 172 fall) from cows of five crossbred cow groups were analyzed for differences in gestation length and birth weight. The cows ranged in age from 4 to 7 years old. Cows were bred artificially to either Salers or Limousin bulls. Birth weights were taken within 24 hours of birth. Fall calving cows delivered smaller birth weight calves (77.7 pounds) than did spring calving cows (82.2 pounds).
As producers select replacements, especially potential herd sires, they want to take into account the influence that season of the year may have on average birth weights. Fortunately, some breed associations adjust birth weights according to the season of the year that the calf was born. This should equalize the calves, so that when the EPD's for birth weight or calving ease are calculated, they will not be biased by the seasonal affect on birth weight.
The reason that fall calving cows have lighter birth weights is generally attributed to the fact that the cows are gestating in hot weather. Blood flow patterns of cattle during periods of high temperatures change in an effort to dissipate heat from the body. Blood (and the nutrients that it carries) is shunted to the outer extremities during hot weather to dissipate heat. Therefore less blood flow is sent to the inner core of the cow where the fetus is gestating. This subtle change in blood flow is commonly thought to be reason that lighter birth weights occur to cattle that are pregnant in June, July, and August. The small amount of difference noted in Oklahoma cattle has not caused a loss of viability of calves born in September and October. (Source: Selk and Buchanan, 1990 OSU Animal Science Research Report.)
Another data set that gives insight into lighter birth weights in fall-calving cows is provided by the 2004 OSU Animal Science Research Report. Researchers had cows bred to calve in August or November. The cows were bred artificially to the same bulls. The project was conducted on two consecutive years. The first year cows that gestated in hot weather (August calvers) had calves that were 3.74 pounds lighter than the cows that calved in November. The second year the difference was 9.68 pounds with the lighter calves again coming in August. The average difference for the two years was 6.7 pounds lighter birth weight in the cows that were in late pregnancy in hot weather. (Source: Kastner and co-workers. 2004 OSU Animal Science Research Report.)
Follow this link to watch video from Selk's "Cow-Calf Corner" program.