Ovarian Functions, Intervals between Estrus, and Conception Rates in Dairy Cattle

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Abstract
Dairy cows (400) were alternately assigned at random to an experimental and a control group. Regular ovarian examinations for the experimental group provided information on ovarian function, regularity of estrus, and influence of these on conception rates. When all 500 intervals of estrus were considered for the 200 cows in the experimental group, it was found that 60.4% of the intervals were between 18 and 25 days. A total of 37.4% of the intervals were over 25, and 2.2% were less than 18 days, when estrous cycles indicated from cystic ovaries were excluded. Irregular intervals were due to 18.6% silent estrus, 15.2% persistent corpus luteum, 3.6% smooth, nonfunctional ovaries, 0.8% of false estrus and 1.4% with a true estrus at an interval of less than 18 days. Low conception rates prevailed after a period of recovery from cystic ovaries and from services for intervals less than 18 days. A high rate of conception was obtained for the cows in the entire group, for cows bred during a silent estrus, during a normal estrus after a silent period, and during delayed estrus due to persistent corpus luteum and smooth, nonfunctional ovaries. Results on breeding efficiency indicated that the chances for conception in cows returning at irregular intervals are not decreased except for cows with short intervals and cows with cystic ovaries. It can be recommended to withhold service to cows with short intervals between estrus but breeding of cows with irregular long cycles should not be restricted, as is sometimes advocated.