Dietary Urea for Dairy Cattle. I. Relationship to Luteal function

Abstract

Isonitrogenous complete mixed rations containing concentrates, corn silage, and ground corn cobs, differing only in source of supplemental nitrogen (Group I—soybean meal and Group II—urea), were fed to Holstein-Friesian heifers for a minimum of 60 days. The purpose was to study the effect of dietarv urea on luteal function. Jugular blood and corpora lutea were collected on Day 12 after estrus. Average concentrations of progesterone, luteinizing hormone, corticosterone, and cortisol in blood plasma were approximately the same for both groups. Ten corpora lutea from Group I were heavier (P<.005) than the 12 from Group I1 (6.4±.6g versus 4.3±.3g). The corpora lutea from Group II were softer and more fragile than those from Group I, but there were no discernible histological differences. The contents and concentrations of progesterone and 20β-hydroxy-pregnene-3-one in unincubated corpora lutea were not significantly different between treatments. However, corpus luteum tissue from Group I as compared to Group I1 synthesized more progesterone (P<.05) during in vitro incubation in blood plasma (2 hr at 37C) and contained more total progesterone (P<.05) after incubation. Source of jugular plasma substrate, either from Group I or Group II, had no significant effect on synthesis of progesterone. The relationship between differences in corpus luteum function due to ration and reproductive efficiency was not determined.