The relationships between plasma progesterone and lh and the numbers of corpora lutea and embryos in purebred and crossbred gilts.

Relationships between plasma progesterone and luteinizing hormone (LH) and the numbers of corpora lutea and embryos were determined in 133 8-month-old purebred Duroc, Hampshire and Yorkshire gilts and two-breed crosses among the three breeds. The numbers of corpora lutea and normal embryos were determined at slaughter on day 30.8 +/- .2 of pregnancy, and progesterone and LH were quantified in plasma obtained the previous day. Progesterone concentrations in plasma were influenced by sire (P less than .10), breed of dam (P less than .05) and breed of sire x breed of dam (P less than .05). Hampshire gilts had higher concentrations of progesterone in plasma than did Durocs (P less than .05) and Yorkshires (P less than .01). None of the effects in the model was significant for LH. Plasma progesterone and LH had heritability estimates of .48 +/- .41 and .29 +/- .40, respectively. Most genetic and phenotypic correlations of plasma progesterone and LH correlations of plasma progesterone an LH with other traits were small. These data indicate that progesterone concentrations in plasma at 30 days of pregnancy are not a satisfactory predictor of the numbers of corpora lutea and embryos. The differences in plasma progesterone between breeds illustrate the need to consider the breed of gilts when selecting animals for the study of luteal function or physiological factors that may influence progesterone concentrations in plasma.