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Circulating concentrations of LH and FSH and pituitary responsiveness to GnRH in intact and ovariectomized suckled beef cows in two levels of body condition

Published online by Cambridge University Press:  02 September 2010

I. A. Wright ,
S. M. Rhind ,
T. K. Whyte ,
A. J. Smith ,
S. R. McMillen  and
R. Prado
I. A. Wright
Affiliation:
Macaulay Land Use Research Institute, Pentlandfield, Roslin, Midlothian EH25 9RF
S. M. Rhind
Affiliation:
Macaulay Land Use Research Institute, Pentlandfield, Roslin, Midlothian EH25 9RF
T. K. Whyte
Affiliation:
Macaulay Land Use Research Institute, Pentlandfield, Roslin, Midlothian EH25 9RF
A. J. Smith
Affiliation:
Macaulay Land Use Research Institute, Pentlandfield, Roslin, Midlothian EH25 9RF
S. R. McMillen
Affiliation:
Macaulay Land Use Research Institute, Pentlandfield, Roslin, Midlothian EH25 9RF
R. Prado
Affiliation:
Macaulay Land Use Research Institute, Pentlandfield, Roslin, Midlothian EH25 9RF
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Abstract

The effect of body condition on hypothalamic and pituitary function in post-partum suckled beef cows was studied. Thirty-eight Blue-Grey cows were calved at a body condition score of either 2·35 (L) or 2·80 (H), fed to maintain live weight and body condition thereafter and ovariectomized at either 5 or 9 weeks post partum. The H cows had higher LH pulse frequencies before ovariectomy (0·36 v. 0·28 pulses per h; P < 0·05) and 15 days after ovariectomy (0·86 v. 0·72 pulses per h; P < 0·01), indicating a direct effect of body condition on GnRH release from the hypothalamus. Body condition had no effect on the ability of the pituitary to release LH in response to a physiological dose (2·5 u.g) of GnRH in entire or ovariectomized cows at 5 or 9 weeks post partum. The magnitude of the increase in circulating levels of LH following GnRH injection was greater at 9 weeks compared with 5 weeks post partum (2·23 v. 1·73 u.g/1; P < 0·001) in entire cows, but there was no difference between 5 and 9 weeks in ovariectomized cows. Following ovariectomy there was a 2·5-fold increase in LH pulse frequency within 15 days and LH pulse frequency approached 1 pulse per h in all treatments groups. It is concluded that both the hypothalamus and the pituitary are capable of functioning normally by 5 weeks post partum in suckled beef cows, that the pulsatile release of GnRH from the hypothalamus is inhibited, possibly due to a very high sensitivity to oestradiol, and that the sensitivity of the pituitary to the negative feedback effect of oestradiol may decrease with time post partum.

Keywords

body conditioncowsgonadotropinspostpartum interval
Type
Research Article
Information
Animal Production , Volume 51 , Issue 1 , August 1990 , pp. 93 - 101
Copyright
Copyright © British Society of Animal Science 1990

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