Nutrition plays a role in the reproductive performance of heifers. In a Journal of Dairy Science article published earlier this year, Israeli researchers reported the effect of high dietary crude protein on characteristics of preovulatory follicles in Holstein heifers.
Treatments consisted of a low (6 percent), moderate (13 percent) or high (20 percent) crude-protein diet. Diets were formulated to contain approximately 66 percent wheat straw and various proportions of ground corn and soybean meal.
The researchers synchronized the estrous cycles of the heifers and gave prostaglandin (PGF2α) injections 14 days after behavioral estrus. On day 39 of each period, they collected follicular fluid from follicles with a diameter greater than 7 mm.
The high protein diet increased urea concentrations in preovulatory follicular fluid. Heifers fed the high protein diet had greater estradiol and progesterone concentrations in follicular fluid than heifers fed the moderate protein diet. However, there were no differences in estrogen to progesterone ratios between diets. There also were no differences in follicle diameter between the three treatments.
The researchers concluded that high levels of dietary crude protein caused increased concentrations of urea in preovulatory follicles, but this did not have a detrimental effect on the follicles. They contend that heifers possess "fewer nutritional and physiological constraints" than lactating cows, which may help offset a negative reproductive response when high protein levels are fed.
Although this study showed no harm in feeding high levels of crude protein, the Gold Standards II encourage a more moderate crude protein content of 13.5-14 percent for breeding-age and older heifers.