HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Williams, G. L. Articles by Johnson, L. J. Search for Related Content PubMed PubMed Citation Articles by Williams, G. L. Articles by Johnson, L. J.

Effect of Suckling on Pituitary Responsiveness to Gonadotropin-Releasing Hormone throughout the Early Postpartum Period of Beef Cows^1,2,

North Dakota State University³, Fargo 58105

Abstract

The objective of this study was to compare pituitary responsiveness to gonadotropin releasing hormone (GnRH) throughout the early postpartum period (PP) in beef cows suckling a single calf vs cows which had calves removed at birth. The experiment utilized 48 cows in a 2 x 4 factorial. Main effects studied were suckling status (suckled or nonsuckled) and days PP (d 3, 10, 20 or 30). Covariates studied were pre-treatment concentrations of progesterone (P), estradiol-17β (E), luteinizing hormone (LH) and follicle stimulating hormone (FSH). Suckled (S) cows nursed one calf, while nonsuckled (NS) cows had calves removed at birth. Treatment was a single im injection of 100 µg GnRH dissolved in saline. Pre GnRH plasma concentrations of E, P, LH and FSH and post treatment plasma concentrations of LH and FSH were determined at 15 min intervals (–30 to 360 min) in blood samples collected via jugular cannulae. Post treatment plasma concentrations of P were determined at 6 (n = 23) or 8 (n =23) days after treatment to monitor ovarian response. Concentrations of E, LH and FSH before GnRH injection did not differ (P>.1) between S and NS cows at any PP stage and data were pooled for statistical analyses. Pre-treatment LH, but not FSH, increased (r = .43; P<.002) with days PP and concentrations of LH were greater (P<.05) at 30 d than at 3 d PP. Pre-treatment P was greater (P<.05) in NS than in S cows on d 30 PP and P was correlated (r = .51; P<.01) with days PP in NS cows. However, the frequency of cows exhibiting plasma P1 ng/ml prior to GnRH was low on all days studied and was similar for both groups. LH release after GnRH was correlated with increasing days PP in both S (r = .73; P<.001) and NS (r = .61; P<.002) cows. Releasable FSH was not related to days PP in either group. Mean peak LH and total LH released were greater (P<.025 and P<.05, respectively) in S than in NS cows at 30 d PP. Mean peak FSH and total FSH released were greater (P<.05 and P<.07, respectively) at 10 and 30 d PP in S cows than in NS cows. Pre-treatment E, but not P, LH or FSH was positively related (P<.01) to GnRH mediated LH release in both groups. When pre-treatment E was partitioned into separate regressions for S (E_S) and NS (E_NS) cows, increased pituitary LH response in S cows on d 30 appeared to be due in part to a greater sensitivity of S cows to E modulation. We found no evidence to suggest that pituitary responsiveness per se is decreased in beef cows suckling a single calf.

¹ Published with the approval of the Director, North Dakota Agr. Exp. Sta.

² Appreciation is expressed to Dr. K. W. Cheng for FSH-HS-2 and bFSH antiserum, Dr. D. J. Bolt for NIH-FSH-B1, Dr. Jerry Reeves for bLH antiserum, Dr. R. H. Rippel, Abbott Laboratories, for GnRH and the NIAMDD for pituitary hormones. We also gratefully acknowledge the expert technical assistance of Mark Paul, Lori Marthaller and Melodie Wiger.

³ Dept. of Anim. Sci.