Energy balance, first ovulation and the effects of naloxone on LH secretion in early postpartum dairy cows
1991
Relationships among energy balance (EB), first ovulation, circulating metabolites and opioid involvement in peripheral LH concentrations were assessed in 40 multiparous Holstein cows assigned randomly to an experiment with a 2 X 2 factorial arrangement of treatments. Cows were either milked normally (2x/d) and expected to be in negative EB (LAC) or were milked for only 1 d postpartum (PP) to ensure a rapid return to positive EB (NONLAC). Blood samples were collected from NONLAC cows every 12 min for 16 h on d 7, 10, and 14 PP and from LAC cows on d 7, 14, 21 and 28 PP. Within each group, half the cows received naloxone (50 mg/h) and half received saline during h 9 to 16 of each frequent sampling series. The mean number of LH pulses/8 h for NONLAC cows was 4.3, 4.7 and 5.3 at 7, 10 and 14 d PP and for LAC cows 4.3, 5.7, 6.4 and 7.0 at 7, 14, 21 and 28 d PP, respectively. The LH pulse frequency was not different between NONLAC and LAC cows at 7 and 14 d PP, yet NONLAC cows had fewer days to first ovulation (P < .01). Naloxone did not affect any parameters of LH secretion in either group. Following parturition, NONLAC cows averaged 4.0 d to negative EB nadir and 14.3 d to first ovulation. The LAC cows averaged 13.6 d to negative EB nadir and 27.0 d to first ovulation. Days PP to first ovulation were highly correlated (r = .85) with days PP to negative EB nadir. In LAC cows, nonesterified fatty acids (NEFA) were correlated (P < .05) with EB in 15 of 20 animals. Glucose, insulin and EB were higher (P < .01) and NEFA were lower (P < .05) in NONLAC cows during the first 3 wk PP. The data suggest that EB and the interval to first ovulation are directly related, as are EB and NEFA, making NEFA a potential cue of EB status on initiation of first ovulation. Second, endogenous opioid peptides do not seem to be involved in modulating the effects of negative EB on pulsatile LH secretion during the early PP period. Finally, LH pulse frequencies did not differ between LAC and NONLAC cows at 7 and 14 d PP, but ovulation occurred earlier in NONLAC cows. This suggests that the ovaries of LAC cows may be less responsive to a similar LH signal, reflecting the metabolic state of the cow during negative EB.
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