Plasma Luteinizing Hormone (LH) and Progesterone Levels in Heifers on Restricted Energy Intakes

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Abstract
A Reversal type experiment was conducted in which measurements were made of plasma levels of luteinizing hormone (LH) and progesterone in six Holstein heifers during three estrous cycles when they were maintained on either 100% or 62% of Morrison's TDN allowances. A progressive increase in plasma LH was noted from the first to third estrous cycle in heifers on the low energy intake. This increase was first seen in the estrous peaks, but by the third cycle the basal mean for the low TDN group (2.7 ng/ml) was also significantly higher than that of the control heifers (1.9 ng/ml, P<0.005). During the first cycle, plasma progesterone was slightly higher in the low calorie group, but became progressively lower in the subsequent cycles (P<0.05 and P<0.01). Both total progesterone and progesterone concentration in corpora lutea taken on the 10th day of the third cycle were lower in the low-TDN heifers than in their normal counterparts, P<0.01 and <0.05, respectivley. The results show that ovarian hypofunction under conditions of restricted energy intake is not due to reduced circulating levels of LH, as was previously hypothesized, and further suggest that the first effect is a reduced ability of the ovarian tissue to respond to LH. Copyright © 1973. American Society of Animal Science. Copyright 1973 by American Society of Animal Science.

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