Estrus detection programs practiced on most U.S. dairy farms are not intense
enough to provide the information needed to accurately time insemination, thus
preventing AI from obtaining its full conception rate potential. Herds (n = 17)
participated in a trial designed to evaluate percent pregnant relative to various
characteristics of estrus. Herds utilized HeatWatch® electronic estrus detection
system to detect and record mounting activity for cows in estrus. Inseminations
were performed daily during a three hour interval for all cows identified in estrus
the previous 24 h. Model characterizing percent pregnant for cows (services =
2661) included effects of interval from first mount to AI (P < 0.01), mounts per
estrus (P < 0.01), DIM at insemination (P < 0.01), herd (P <0.05), and season of
AI (P < 0.05). As mounts per estrus and days in milk increased, percent diagnosed
pregnant increased. Interval affected probability of pregnancy with highest odds
ratios for percent pregnant occurring >4 to 16 h following onset of estrus. Model
for heifers (n = 306) included linear effects of interval (P < 0.01), season
(P < 0.05), and herd (P < 0.01). In dairy heifers, as interval from first mount to AI
increased, percent pregnant decreased. Timing of insemination in dairy cows can
now be performed relative to first mount of estrus, with highest probability of
pregnancy occurring between >4 to 16 h after onset. If onset of estrus is not
known, insemination should be performed at the next most convenient time
within 3 h.
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