We conducted this experiment because published studies demonstrated a greater ovulation
181 rate for Cystorelin compared with Factrel when the GnRH products were administered on d 7
182 [6]. Souza et al. [6] reported that ovulation in response to Cystorelin (76.7%) was 21.4
183 percentage points greater than ovulation in response to Factrel (55.3%). Giordano et al. [10]
184 demonstrated that cows that ovulate in response to the first GnRH of an Ovsynch program
(typically given at approximately d 7) have a 13.7 percentage point advantage for P/AI compared
186 with cows that do not ovulate. If one accepts the literature values as correct and ignores possible
187 differences at second GnRH then multiplying 21.4% (ovulation rate advantage for Cystorelin
188 [6]) by 13.7% (P/AI advantage for cows that ovulate [10]) approximates the expected difference
189 in P/AI for cows treated with the different gonadorelin products. The calculated product (21.4 x
190 13.7 = 2.9%) is nearly identical to the non-significant numeral difference that we observed
191 (2.7%) in this trial.
192 Although treatment differences were small and not significant, there were large effects of
193 month, milk quality, and milk composition on P/AI. The study was conducted at a large dairy in
194 Northeast Missouri. The daytime temperature humidity index (THI) for July and August in
195 Northeast Missouri is typically 80 to 85 and well above the threshold of 73 reported to cause
196 summertime infertility in cattle [15]. We clearly observed reduced fertility during the summer
197 months (Figure 1). We did not observe interactions of treatment and month indicating no
198 advantage of one product over the other when fertility is suppressed in the summer.