Document Type

Dissertation - Open Access

Award Date

1974

Degree Name

Doctor of Philosophy (PhD)

Department / School

Animal Science

First Advisor

Christian A. Dinkel

Abstract

Five heifers from each of four breed groups (Angus x Angus, Charolais x Charolais and the two reciprocal crosses) were used in a study of gonadal hormone levels during the interval from 16 weeks before puberty to puberty (trial I). Puberty was defined as the first standing estrus. Two heifer calves from dams from each of the above breed groups and sired by a single Polled Hereford bull were used in a study of gonadal hormone levels from birth to weaning (trial II). Blood samples were collected at weekly intervals from trial I heifers before puberty and three times weekly during the first post puberal estrous cycle. Heifer calves in trial II were sampled once weekly. Total progestins were assayed by a competitive protein birding assay. The binding protein was 2.5%, dog plasma containing 16 ng corticosterone-1,2-3H (CBG-B3H). Duplicate plasma samples were extracted two times each with hexane for the removal of progestins. Florisil was used for the removal of unbound steroids from the CBG-B3H. The recovery of progesterone added to plasma blanks was acceptable. Extraction of different quantities of plasma yielded comparable values. Total estrogens were assayed by a radioimmunoassay utilizing an antibody kindly supplied by Dr. R. D. Randel, U. S. Range Livestock Experiment Station, Miles City, Montana. The antibody was prepared in sheep by using an estradiol-17ᵦ, 11 succinyl-bovine serum albumin conjugate. Cross reactivities with other steroids were estrone 100%, estradiol-17a 80%, estriol 60% and none with cortisol, testosterone or progesterone. Estrogens were extracted from plasma with diethyl ether. Recovery of tritiated estrogens added to plasma was 93.9 ± 2.9 percent. A correlation coefficient between duplicate samples of 0.96 (n = 22 pairs) was obtained. Subsequently, singular assays were conducted on each plasma sample. Separation of bound from unbound estrogens was accomplished with charcoal and dextran T70. The average blank from the method was 10.0 ± 6.0 picograms. Recovery of estradiol-17ᵦ added to plasma blanks was acceptable. Extraction of varying amounts of plasma yielded comparable values. The correlation between values for samples assayed in this laboratory and in the laboratory of Dr. M. L. Hopwood, Colorado State University, was 0.80 (54 pairs). No differences for hormone levels between breed groups were detected in either trial. Therefore, for further analyses breed groups were pooled. Breed groups did not differ significantly for date of birth, actual and adjusted weaning weights, date of puberty and age at puberty (trial I). Straightbred Charolais heifers were significantly heavier at birth (P<.01) and at puberty (P<.05) than straightbred Angus heifers. The two reciprocal crosses did not differ significantly from each other or from straightbreds for these traits. Average total progestin levels decreased from approximately 1. 5 ng/ml at 112 days before puberty to less than 1. 0 ng/ml at 70 days before puberty and then gradually rose to approximately 3.0 ng/ml the last 2 weeks before puberty. Average plasma total estrogen levels in the prepuberal heifer decreased from approximately 1000 pg/ml at 16 weeks before puberty to approximately 200 pg/ml during the last 3 weeks before puberty. The regression of age and number of days before puberty on estrogen levels accounted for 31.7 and 29.6% of the variability in estrogen levels, respectively. In a multiple regression analysis these two variables accounted for 34.2% of the variation. Average plasma total progestin levels during the first post puberal estrous cycle rose from 1 ng/ml on day 1 to a peak of 6 ng/ml on day 13 and declined from day 14 to approximately 2 ng/ml on day 19. Total estrogen levels during the first post puberal estrous cycle ranged from 100 to 220 pg/ml. The variation within samples taken on the same day was large. Average estrogen levels fluctuated between 100 and 220 pg/ml from day 1 through day 11. Levels rose steadily from 114 pg/ml on day 12 to 220 pg/ml on day 18 and then dropped to 119 pg/ml on day 19. Samples obtained on the day of estrus were not included. Average progestin levels in heifer calves from birth to 28 weeks of age remained below 1 ng/ml. Estrogen levels were high during the first 2 days of life (300 pg/ml) but decreased to less than 32 pg/ml on day 3. Elevated estrogen levels (approximately 40 pg/ml) were observed during the sixth to tenth weeks of life. During the 12th to 23rd weeks levels remained below 20 pg/ml and rose gradually from the 24th week to 58 pg/ml during the 27th week of life. The discrepancy between estrogen levels in heifers nearing puberty in trial I and the heifer calves in trial II is unexplained.

Library of Congress Subject Headings

Beef cattle -- Breeding

Format

application/pdf

Publisher

South Dakota State University

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Beef Science Commons

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