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Estrus, Ovulation and Conception in Hormone Treated Prepuberal Hereford Heifers¹

University of Georgia, Experiment 30212 and Athens 30601

Abstract

INITIATION of estrus with subsequent fertility was studied over a 2-year period using a total of 87 prepuberal Hereford heifers averaging 327 days of age and 214 kg in weight. The heifers were divided into 13 lots. Although estrus was not observed prior to treatment, this observation was confirmed the second year by use of a marker bull which stayed continuously with a control lot. The other heifers were individually fed a progestin, either 120 mg 16, 17 dihydroxyprogesterone acetophenide (DHPA) daily for 9 days or 180 mg medroxyprogesterone acetate (MAP) daily for 18 days. On the second day of progestin feeding, each heifer received 5 mg estradiol valerate and near the end of progestin feeding, either PMS (500 IU to 700 IU) or FSH-P (3.1 mg to 6.4 mg) was injected. One day after the last DHPA feeding the second year, some lots received estradiol-17ß (0.7 mg or 1.0 mg) and at breeding, either PLH (50 mg) or HCG (2,500 IU) was injected.

Estrus occurred in 50% to 100% of the heifers in treated lots, usually within a 24-hr, period within lots. Those in estrus were hand mated to fertile bulls with one service per 10- to 14-hr, interval of standing estrus. The first year, ovaries were palpated 7 days post-breeding and a random portion of the heifers were laparotomized at that time. The second year, all treated heifers were laparotomized. Lot averages for the number of CL per heifer ranged from 0.0 to 6.3; for follicles greater than 10 mm in diameter from 0.5 to 4.7; for follicles less than 10 mm in diameter from 0.5 to 6.0; for percent pregnant 50 to 85 days post-breeding of those in estrus from 0.0% to 50%. The number of CL and large follicles at palpation were significantly (P<.01) greater for the lot which received 700 IU PMS than for lots which received 500 IU PMS, 3.2 mg FSH-P or 6.4 mg FSH-P. Heifers which received 600 IU PMS had significantly (P< .05) more follicles greater than 10 mm in diameter (2.1) than those which received 6.4 mg FSH-P (0.8). Other differences between lot averages within year were not statistically significant.

¹ Appreciation is expressed to E. R. Squibb & Sons, Inc., Three Bridges, NJ. 08887, for 16, 17 dihydroxyprogesterone acetophenide and estradiol valerate (Delestrogen); to Armour-Baldwin Laboratories, Omaha, Nebraska 68103, for FSH-P and PLH; to G. D. Searle Company, Chicago, Illinois 60680, for estradiol-17ß used in these investigations.

² Georgia Station, Experiment, Georgia 30212.

³ College of Veterinary Medicine, Athens, Georgia 30601.

⁴ The authors gratefully acknowledge the assistance of Dr. D. M. Witherspoon and students of the College of Veterinary Medicine for their part in the conduct of these experiments.