Nutrition
By Dr. Jim White

For the soon-to-calve cow's diet, steady as she goes

You ought to start feeding cows an increasing plane of nutrition their last 2 months of pregnancy. Improved cow body condition score will improve cow reproductive efficiency. Some producers have concerns that feeding cows during this time will increase either calf birth weight or create calving difficulty—or both. Dystocia, or calving difficulty, is problematic. Given that the calf in the cow is mostly protein and water, there has been some thought that to reduce calf size, producers could cut back on feeding. But this is a bad idea; low-protein feeding (less than 2 pounds of crude protein per day) during gestation results in decreased calf vigor, delayed uterine involution, increased interval to estrus and decreased conception rates following calving. These problems get worse when energy is also short.

I concede that there are papers in the realms of scientific literature that support the notion that high-energy diets can increase calving difficulty. A 1967 paper from Christenson et al. showed the effects of feeding a high-energy diet compared to a low-energy diet to cows for the last half of pregnancy. Results showed increased dystocia for the cows fed high-energy diets. The high-energy cows also had increased milk and subsequent estrus activity.

Any number of earlier papers reported increased birth weight without increased dystocia (Laster 1973, 1974, Corah et al 1975, Bellows and Short 1981, Houghton et al 1981).

The 1996 Beef NRC summarized studies that looked at the effects of calf birth weight as influence by the body condition score of the cow. If the thought of “let’s calve heifers thin to hold down calf birth weight” holds, then you would expect to see birth weight increase as body condition score increases. The chart below summarizes the NRC data.

There is substantial variation at each body condition score, but we can use some elegant math to arrive at averages and expected values. The line on the chart is the line for the average, or expected, values. On average, what we see is that the birth weight of calves does increase in cows with BCS of 2 moving to a BCS of 3.5. From BCS 3.5 to 7.0, we see birth weights trend downward, and they sharply drop when cows are fat. The effect of restrict-feeding cows is most noticeable when it is done on cows in the extremes. We do not want to have cows in the extremes.

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