Plasma luteinizing hormone (LH) and progesterone levels in heifers on restricted energy intakes
REVERSAL type experiment was con- ducted in which measurements were made of plasma levels of luteinizing hormone (LH) and progesterone in six Holstein heifers dur- ing three estrous cycles when they were main- tained 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 in- take. 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 progester- one 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 circulat- ing levels of LH, as was previously hypoth- esized, and further suggest that the first effect is a 'reduced ability of the ovarian tissue to respond to LH.