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
|
|
