Reduced fertility associated with low progesterone postbreeding and increased milk urea nitrogen in lactating cows

The primary objectives of this study were to determine whether a delay in the onset of the luteal phase, or high milk urea nitrogen at breeding, or both were associated with failure of pregnancy early in gestation. Milk samples were collected twice daily from cows in a single herd during the week following breeding; single samples were collected on d 14 and 21 postbreeding. Progesterone was measured in all samples, and a total of 156 sample sets was used. The progesterone data combined with results from pregnancy examinations were used to distribute the cows into three groups: 1) pregnant, 2) nonpregnant with a low concentration (< 2 ng/ml) of progesterone on d 21, and 3) nonpregnant with a high concentration (> or = 2 ng/ml) of progesterone on d 21. The interestrous interval for cows in group 3 was longer than that for cows in group 2. Beginning 4.5 d after breeding, pregnant cows had higher concentrations of progesterone than did cows in group 3. Pregnant cows also had higher concentrations of progesterone than did all open cows on d 14 and 21. The onset of the luteal phase was earlier in pregnant cows than it was in cows in group 3. Milk urea nitrogen at breeding was similar in pregnant cows and in cows in group 3, but was higher in cows in group 2. Increased milk urea nitrogen was also statistically associated with decreased fertility. We propose that the cows in group 3 likely had embryos that initiated pregnancy recognition and prolonged luteal function, but these embryos were compromised by suboptimal exposure to progesterone early in development.