Results and Discussion
There were no significant effects of location or location by treatment interactions for any of the variables studied. The timing of estrus in the Control and GnRH groups for Herd A and B is shown in Figure 1. For both groups, the greatest numbers of heifers were detected in heat at 60 hours after PGF2a. Proportion of heifers exhibiting estrus from days 0 to 7 after PGF2a (83.4 and 86.5%), and the average day of insemination during the synchrony period (day 3.0 ± 0.09 and 2.8 ± 0.08) did not differ between the Control and GnRH treatments, respectively (Table 1). In the 2XGnRH group, 18/196 (9.2%) heifers were detected in heat before day 1.5 and were bred 12 hours after detection of estrus. Conception rate for this sub-group of heifers was 72.2%. Removal of these 18 heifers from the timed insemination group resulted in 178 heifers being time-inseminated on day 2, in conjunction with an injection of GnRH (196-18 = 178). In Herds A, C, and D, heifers in the 2XGnRH group were observed for estrus after timed AI. In Herd A, 23/107 (21.5%) heifers exhibited heat after the timed breeding between days 5 to 8. These heifers were bred 12 hours after detection of estrus and the assumption was made that they did not conceive to the timed AI. Conception rate for the 23 2XGnRH heifers that were bred between days 5 and 8 was 91.3%. In Herds C and D, no heifers were observed in heat during this time period.
Conception rate during the synchrony period for Control (160/211; 75.8%) and GnRH (160/225; 71.1%) treatments did not differ and was greater (P < 0.05) than the conception rate following timed AI in the 2XGnRH treatment (83/178; 46.6%). Similarly, pregnancy rate during the synchrony period did not differ for Control and GnRH treatments (63.2 and 61.5%, respectively) but was greater (P < 0.05) than pregnancy rate in the 2XGnRH treatment when only pregnancies resulting from timed AI were considered (83/196; 42.3%). If pregnancies in the 2XGnRH group that resulted from AI 12 hours after estrus, either before (13/18) the timed AI period, or to a second AI after timed AI (21/23) were considered, the pregnancy rate during the synchrony period in this treatment was 59.7%. Thus a combination of timed AI and some heat detection resulted in a similar pregnancy rate in the 2XGnRH as compared to the Control and GnRH groups. While time was still spent on heat detection in this case, time and sorting stress on the heifers was reduced. This labor and stress savings would be most relevant to producers synchronizing larger groups of heifers and would need to be weighed against the cost of two injections of GnRH per animal. Pregnancy rate for the entire breeding season did not differ among treatments.