Biological and immunological luteinizing hormone activity and blood metabolites in postpartum Brahman cows.

Multiparous Brahman cows were assigned by order of calving and sex of calf to groups to be fed to maintain body condition score (BCS) of 6 or greater (M; n = 10) or to lose BCS (L; n = 10). Blood samples were collected weekly for progesterone analysis and at 15, 30 and 45 d after parturition at 15-min intervals for 6 h for determination of immunological (ILH) and biological (BLH) luteinizing hormone. Serum concentrations of ILH were determined using a double antibody RIA procedure, whereas BLH was determined using a rat interstitial cell-testosterone bioassay (RICT). By 45 d after parturition 7 of 10 M cows had returned to estrus. Therefore, comparisons between groups were made on d 15 and 30 postpartum. Cows in the M group had a shorter (P less than .001) interval to first estrus (46.7 d) than did L cows (91.2 d). The concentrations of bioactive and immunoactive LH were parallel between d 15 and 30 postcalving. However, a day x treatment interaction (P less than .05) showed that episodic BLH concentrations (ng/ml) decreased with day postpartum in L, but increased in M cows from d 15 to 30 postcalving. Likewise, relative biological activity, as measured by B:I ratios, decreased between d 15 and 30 in L cows, whereas it increased in M cows during the same period (B:I x day interaction; P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)