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Fertilization and Blood Levels of Progesterone and LH in Beef Heifers on a Restricted Energy Diet¹

Colorado State University^,4, Fort Collins 80523

Abstract

Two groups of yearling beef heifers were fed different levels of nutrition to study the effect of reduced energy intake on fertilization rate and blood levels of progesterone and luteinizing hormone (LH). Control heifers received a ration meeting N.R.C. recommendations for all nutrients, while heifers on the restricted diet received only one-third of the recommended energy.

Ova recovery rates were 62 and 82%, respectively, for heifers fed low or high levels of energy. Eighty-eight percent of ova recovered from heifers on the low level of energy were fertilized compared to a 79% fertilization rate for ova recovered from heifers on the high level of energy. Neither of these differences were significant. No significant differences were found in blood levels of progesterone or LH either between nutrition group or among successive estrous cycles within nutrition group. No relationship could be established between actual weight change and blood levels of progesterone. Diet had no effect on number of follicles nor follicular or luteal volume. However, the ovary containing the corpus luteum was 57% larger in heifers fed adequate energy than in those restricted in energy intake (P<.05).

These results suggest that reduced pregnancy rate in heifers restricted in energy intake is not the result of fertilization failure, but some later causative factor. Determinations of levels of progesterone and LH in daily blood samples did not provide evidence to implicate any changes in these hormones that might indicate causes for embryonic loss.

¹ Published with the approval of the Director of the Colorado State University Agricultural Experiment Station as Scientific Series Paper No. 2230. Supported, in part, by Regional Project W-112, Reproductive Performance in Beef Cattle and Sheep.

² Present address: Texas Agricultural Extension Service, The Texas A&M University System, Bryan.

³ Present address: Texas Agricultural Experiment Station, The Texas A&M University System, Beeville.

⁴ Departments of Physiology and Biophysics and Animal Science.