Corpus luteum function in the bovine: in vivo and in vitro evidence for both a seasonal and breedtype effect.

Breedtype and season appear to modify the reproductive capacity of cattle. This study was designed to elucidate the effect of these factors on the bovine corpus luteum. Six Brahman (B) and six Hereford • Holstein (H) heifers were bled via coccygeal vessel puncture on d 2, 4, 6, 8, 10, 12 and 13 postestrus within 20 d of both the summer and winter solstices. On d 13 postestrus, the corpus luteum (CL) was surgically removed from each heifer following blood sampling. Serum progesterone, total progesterone content of each CL and in vitro progesterone ( P 4 ) p r o duction of dispersed luteal cells was determined. Serum progesterone on d 2 through 13 did not vary with breedtype or season. However, the CL weights differed (P<.05) between breeds (H heavier than B), and there was a tendency for CL weight to be greater during the summer than winter among both B and H heifers. CL concentrations of P4 (/~g/g) also differed between B and H. Concentrations of P4 in B were greater (P<.10) during winter than during summer, but H concentrations of P4 did not differ between seasons. Total P4 content was greater (P<.01) in the winter than in summer and in H than in B. Luteal cell viability did not vary with breedtype, season or LH dosage. Breedtype (P<.001) and season (P<.001) influenced the capacity of incubated luteal tissue to release progesterone in response to luteinizing hormone