Genetic analysis of postpartum measures of luteal activity in dairy cows.

The objective of this study was to estimate genetic parameters for measures of luteal activity during the first 60 d postpartum. Analyses were made with different sampling intervals to investigate the possibility of combining progesterone measurement with routinely performed milk recording. Progesterone level in milk as an indicator of female fertility when selecting sires in a progeny-testing scheme was also examined. Data were collected from 1996 to 1999, and comprised 1,212 lactations from 1,080 British Holstein-Friesian cows at 8 commercial dairy farms in the United Kingdom. Milk samples for progesterone analysis were collected thrice weekly. Mixed linear animal models were used to analyze the data. Heritability for the percentage of samples with luteal activity during the first 60 d postpartum (PLA) was 0.30 and decreased with more infrequent sampling to 0.25, 0.20, and 0.14 for weekly, twice-monthly, and monthly sampling, respectively. Measures of PLA had a high negative genetic correlation with prolonged anovulation (-0.53 for monthly sampling, < -0.87 otherwise) and a moderate positive genetic correlation with persistent corpus luteum in the first estrus cycle (>0.65 if at least twice-monthly sampling). Genetic correlations with interval from calving to commencement of luteal activity were close to -1 for all PLA measurements and the selection index calculations showed that monthly progesterone sampling could be used with high accuracy (0.80 with 50 daughters per bull) to predict breeding values for commencement of luteal activity. Progesterone analysis at the time of regular milk recording could thereby be used to select for an early interval from calving to commencement of luteal activity and, at the same time, a decreased frequency of prolonged anovulation during the postpartum period.