Two experiments were designed to assess the importance of maternal nutrition in the later stages of gestation on reproductive performance of beef females and on the growth and survival of their calves. In experiment 1, 59 Hereford first-calf heifers were assigned to one of two levels of dietary energy (high [HI which was 100% of the recommended level of prepartum energy or low [L] which was 65% of the N.R.C. [1970] recommended level) 100 days prior to predicted calving. After calving both groups were fed N.R.C. (1970) recommended levels of energy and protein.Heifers fed the H ration gained 36.1 kg during the 100-day prepartum period while the heifers on the L ration lost 5.8 kilograms. The heifers restricted in energy prepartum had lighter calves at birth but the calves were born with the same degree of calving difficulty as calves of adequately fed dams. More calves from nutritionally deprived heifers died at or near birth and the surviving calves were lighter at weaning. There was a tendency for a higher percentage of adequately-fed heifers to show estrus by 40 days postpartum.In experiment 2, 43 second-calf cows were allotted, 100 days prepartum, to a restricted intake ration of approximately 50% of the N.R.C. (1970) recommended level of energy for cows during gestation. Thirty days prior to predicted calving, one group of cows (H) was t

A solid-phase radioimmunoassay for ovine luteinizing hormone (LH) has been developed, utilizing antibody-coated polystyrene tubes for incubation of the assay and counting of the bound tracer. Tubes were coated with equine antiserum to bovine LH (Snook), and purified ovine LH (Papkoff) was used for iodination with 126 I and assay standards. The procedure is simple and rapid, being completely performed in the assay tubes and giving results after a period of 48 hr. Basal levels of plasma LH were 2.9 ±0.9 ng/ml in the cycling ewe, 1.2 ±0.9 ng/ml in the ram, 12.6 ±5.5 ng/ml in the oophorectomized ewe, and 1.2 ±0.9 ng/ml in the pregnant ewe. These values obtained from jugular venous plasma are 25% higher than those measured in peripheral venous plasma. The sheep does not appear to produce a placental gonadotrophin comparable to human chorionic gonadotrophin in regard to immunological crossreaction with pituitary LH. Plasma levels of LH rose sharply 4-16 hr after the onset of estrus to levels of 80-200 ng/ml, over a total period of only 10 hr. Frequent blood sampling is necessary to delineate the estrous LH release in the sheep, and daily estimations carry a greater than 50% chance of completely missing the LH peak. Administration of estradiol-17/3 to anestrous sheep by intramuscular injection and intravenous infusion was regularly followed by a typical estrous peak of LH secretion. The latent period of the estrogen stimulus was approximately 9 hr, and the dose required to produce an ovulatory LH peak was 6-10 ng, an amount similar to that secreted by the ovary at estrus. It is likely that estrogen secretion by the ovary is a major factor in stimulating the LH release accompanying estrus in the sheep (Endocrinology 85: 133, 1969) T HE STUDY of pituitary gonadotrophin secretion in the sheep was previously dependent upon indirect observations and the use of bioassay techniques. The timing of luteinizing hormone (LH) release has been examined by estimation of pituitary LH content (1) and by measurement of LH in blood obtained from the cavernous sinus (2). These studies have relied upon ovarian ascorbic acid depletion assays (3, 4) for quantitation of LH, a procedure which is adequate for the measurement of pituitary LH content but not generally regarded as satisfactory for the assay of blood LH levels. A decrease in pituitary LH content may indicate release of LH into the circulation, though this is uncertain unless the rate of hormone synthesis is also known, while cavernous sinus

Twelve first-calf, 2-year-old, Hereford heifers were randomly assigned in equal numbers on two levels of energy (17.6 Meal DE/day and 11.4 Mcal DE/day) beginning 100 days prepartum. Jugular blood was collected three times weekly from the heifers starting 14 days before parturition, and continuing through the postpartum period until 20 days following the first estrus. There was no significant effect of nutrition on peripheral levels of progesterone or estradiol either prior to or following parturition. Progesterone levels declined from 2.0 to 2.5 ng]ml during late gestation to 0.7 to 0.9 ng/ml at parturition and remained low until just preceding the first postpartum estrus. Progesterone levels were higher immediately preceding the first postpartum estrus in those heifers that conceived. A short estrous cycle (6 to 8 days) was observed in three heifers following first estrus. In these, progesterone levels failed to exceed 0.6 ng/ml. Estradiol levels also declined sharply at parturition and remained in the range of 4 to 8 pg/ml during the postpartum period rising at the occurrence of first postpartum estrus.