David L. Morris: Vet Column 5-16-11 | TheFencePost.com

David L. Morris: Vet Column 5-16-11

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

When beef cows don’t become pregnant as expected, nutrition is often explored as a potential cause. If problem cows do not appear to be thin, the nutritional exploration frequently jumps to micromineral status. Beef cattle microminerals include zinc, copper, manganese, cobalt, iodine, molybdenum, selenium, and sometimes chromium, tin, nickel and fluorine. Trying to troubleshoot nutrition problems after the fact, though, is challenging.

Previous studies have shown that in many regions of North America, less than adequate blood concentrations of copper, selenium, manganese, and vitamin E in beef cows does exist. Rather than have to assess micromineral status after the fact, wouldn’t it be useful if one could assess micromineral status prior to the breeding season? Investigators from the University of Saskatchewan recently published a report examining the association between blood concentrations of copper, molybdenum and selenium in addition to vitamin A and vitamin E in beef cows before breeding and pregnancy outcomes under field conditions to answer that question.

For this study, 771 beef cows from 39 herds were used. All cattle were grazed on five pastures managed by the Canadian federal government in southern Saskatchewan with histories of poor herd-level pregnancy rates. Blood samples were taken from cows on arrival at each of the pastures. Samples were analyzed for copper, selenium, molybdenum, vitamin A and vitamin E levels. Dental examination for age estimation of the cows was conducted. Pregnancy examination occurred more than 40 days following bull removal.

After adjusting for pre-breeding body condition score, age, and calving-to-breeding interval, serum (blood minus the red blood cells and other constituents) concentrations of selenium, molybdenum, vitamin A, and vitamin E were not associated with pregnancy status in beef cows in this study. In cows less than 10-years-of-age, low serum copper concentrations (less than 0.4 ppm) were associated with increased odds of not being pregnant.

Seasonal fluctuations in serum copper concentrations have been described previously, and have been reported to be at their lowest values in February and March in western Canada. Forage copper concentrations vary depending on plant species, soil type and growing conditions. Western Canadian forages, cereal hay and cereal grains have frequently been reported to have copper levels below the suggested National Research Council requirements for beef cattle.

The liver is the primary storage organ for copper and maintains copper balance. Inadequate copper intake is not reflected in low serum copper concentrations until liver copper concentrations are less than 40 ppm. Serum copper concentrations of 0.45 ppm have been correlated with low liver copper concentrations. In this study, the strongest association with non-pregnancy was in cows with serum copper concentrations less than 0.40 ppm.

Although not significant in the final model, vitamin E concentrations cannot be overlooked. Stored hay and silage typically contain less than 20 percent of the vitamin E concentrations found in fresh forages. Cattle do not store vitamin E for any extended period and without supplementation, vitamin E levels will decrease during winter feeding periods. When growing conditions are good, a very rapid increase in circulating vitamin E concentrations can be expected after cows are placed on pasture with green forage.

Based upon this study, low copper concentrations in pre-breeding beef cows can impact subsequent pregnancy rates. Taking blood samples to determine copper concentrations from a subset of cows pre-breeding may be useful if open cows are a herd-level problem. Establishing a cost-effective trace mineral program for beef cow breeding herds is an important management priority.

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