Fourteen dairy cows were maintained on high and standard (low) levels of nutrition. Plasma progesterone concentration was determined by a protein-binding method every 4 days from 10 days post partum until pregnancy. Cows that conceived after one insemination had significantly higher progesterone levels during the oestrous cycle preceding insemination than did cows that did not conceive. Correlation coefficients between the occurrence of conception after the first insemination and plasma progesterone concentration during the preceding luteal phase were statistically significant. Cows maintained on a high level of nutrition required fewer inseminations per conception, conceived earlier and had a high plasma progesterone level 23 days earlier than cows maintained on a standard level of nutrition. In cows that conceived after one insemination, level of nutrition had no effect on progesterone concentration but it had a profound effect in cows that needed more inseminations for conception. During the luteal phase preceding insemination, cows that conceived after the first insemination gained weight whereas cows that did not conceive lost weight; the difference approached significance. The correlation coefficient between body weight changes and progesterone concentration 8 to 15 days before the first insemination approached significance (r = 0·492). It is suggested that plasma progesterone concentration during the oestrous cycle preceding insemination is closely related to the occurrence of conception.
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