Nutritional anestrus in beef cows: body weight change, body condition, luteinizing hormone in serum and ovarian activity

Multiparous Hereford cows (n = 22) with moderate to good body condition scores (BCS) were randomly allotted to maintenance (M) or restricted (R) diets under drylot conditions. Cows on M diets received adequate feed to maintain initial BW, whereas R cows were fed to lose 1% of their initial BW weekly until luteal activity ceased. When most of the R cows became anestrous, their diet was increased to allow BW gain and resumption of ovarian cyclicity. Body weights and BCS were recorded weekly and luteal activity was assessed by weekly determination of progesterone in plasma. Concentrations of LH in serum were quantified in weekly samples and in samples obtained frequently at four selected times. Restricted cows had reduced BW (P less than .01) by 5 wk and reduced BCS (P less than .01) by 15 wk compared with M cows. Luteal activity ceased after 26 +/- 1 wk of reduced nutrient intake in 91% of the R cows; R cows had lost 24.0 +/- .9% of their initial BW and had a BCS of 3.5 +/- .3. Lack of luteal activity was associated with absence of behavioral estrus. Estrous cycles resumed 9 +/- 2 wk after the diet of R cows was increased, at which time R cows weighed 12 +/- 3% less than at the start of the experiment and had a BCS of 4.6 +/- .2. Concentrations of LH in serum samples obtained weekly were reduced (P less than .01) in R cows compared with M cows. In addition, LH pulse frequency was reduced (P less than .05) when R cows were initiating anestrus.