The effect of aspirin administration and parity on plasma salicylate concentrations and postpartum reproductive parameters in Brahman cows.

Forty pluriparous (M) and 20 primiparous (P) suckled Brahman cows were used to evaluate the effect of aspirin and parity on plasma 13,14-dihydro-15-keto-prostaglandin F2alpha (PGFM) and progesterone (P4) concentrations and some reproductive parameters. On Day 7 after calving (PP), the cows were allocated within parity into 2 groups: the aspirin group received concentrate containing aspirin at a rate of 100 mg/kg of body weight every 12 h until Day 13 PP; and the control received concentrate every 12 h for the same interval. Blood samples were collected after first and last aspirin feeding and daily from Day 1 PP to Day 6 PP and from Day 14 PP to Day 21 PP, twice daily from Day 7 PP to Day 13 PP, and weekly until first non-return to estrus. Plasma salicylate concentrations in the aspirin group cows were affected by parity (P < 0.01) and time after feeding (P < 0.0001). P cows showed higher plasma salicylate concentrations with a later peak and slower decrease than M cows. Aspirin-treated P cows had longer PP intervals than either control P, control M, or aspirin-treated M cows. Cows receiving aspirin had a lower pregnancy rate, an increased incidence of abnormal estrous cycles, and a decline in the presence of corpora lutea after estrus. Cows that formed a corpora lutea and had received aspirin had higher P4 release between Day 6 and 14 after estrus. Aspirin-treated cows that did not form corpora lutea had lower P4 release between Days 9 and 14 after estrus. A treatment by parity interaction affected mean PGFM proportions (P < 0.01) during the treatment period. Aspirin-fed P cows increased PGFM release as measured by mean proportion of Day 6 PP values. Aspirin-fed M cows showed a decrease in mean PGFM proportions. Aspirin feeding during the early PP showed different effects on some reproductive parameters in P and M Brahman cows, indicating differences in PP physiology between parities.