This study evaluated the effect of exogenous PGF(2) alpha on circulati
ng LH concentrations in postpartum multiparous (n = 32) and primiparou
s (n = 46) Brahman cows. The cows were randomly allotted within parity
and calving date to receive 0, 1, 2 or 3 mg im PGF(2) alpha (alfapros
tol)/100 kg body weight (BW), with or without GnRH on Day 30 after cal
ving. Blood samples were collected at weekly intervals from calving th
rough treatment. Serum progesterone concentrations were determined usi
ng RIA procedures to assure that only anestrous cows were treated. Ste
rile marker bulls were maintained with cows on Coastal bermudagrass pa
stures until the first estrus was detected. Multiparous cows had a sho
rter (P < 0.05) interval from calving to estrus than did primiparous c
ows. Serum LH was affected by time (P < 0.0001), PGF(2) alpha dose (P
< 0.0002), GnRH (P < 0.0001), parity by PGF(2) alpha dose (P < 0.0003)
, PGF(2) alpha dose by GnRH (P < 0.0009), parity by GnRH (P < 0.0008),
and by parity by PGF(2) alpha dose by GnRH (P < 0.0005). Multiparous
cows not receiving GnRH had higher mean serum LH (P < 0.02), LH peak p
ulse height (P < 0.03), and area under the LH release curve (P < 0.03)
compared with primiparous cows. The number of LH pulses/6 h was great
er (P < 0.06) in multiparous than primiparous cows, and was greater (P
< 0.02) in multiparous cows receiving 3 mg/100 kg BW than in cows rec
eiving 2 mg/100 kg BW, but not in the controls or in cows receiving 1
mg/100 kg BW. Exogenous GnRH resulted in increased (P < 0.0001) serum
LH concentrations in all cows, and LH was enhanced (P < 0.0009) by sim
ultaneous treatment with PGF(2) alpha. Primiparous cows had a greater
response (P < 0.0005) to PGF(2) alpha and GnRH compared with multiparo
us cows. Pituitary release of LH in response to GnRH was enhanced by s
imultaneous exposure to PGF(2) alpha in Day 30 postpartum cows.