During earlier studies we observed that ewes housed and sampled intensively to measure pulses of LH in plasma had a higher ovulation rate than similar ewes housed outside. In Expt 1, we pursued this observation by testing whether the increase was due to effects of housing or collection of blood samples. Ewes sampled at intervals of 4 h for 2 days before progestagen sponge removal and 2 days after sponge removal, and every 20 min for 12 h the day before sponge removal and every 10 min for 4 h on the day of sponge removal had a higher ovulation rate than ewes that were not sampled (1.72 versus 1.41; P < 0.05). The ovulation rate of the ewes housed indoors but not sampled was similar to that of ewes that remained in the paddock (1.43). In Expt 2, we studied the effects of blood sampling in three groups of 20 ewes sampled every 20 min for different periods of 24 h. Ewes from all three groups were sampled the day before sponge removal (day − 1) and, in addition, one group of ewes was sampled for the previous 48 h (i.e. days −3 to −1) and another group was sampled on day − 8. The frequency of LH pulses was lower (P < 0.05) in ewes sampled for the first time on day − 1 compared with the frequency of LH pulses in groups also sampled earlier in the cycle (day −8 or days −3 and − 2). In ewes sampled on days −3 to −1, the frequency of LH pulses was low for the first 24 h and then increased. Changes in the mean concentration of FSH followed a pattern similar to that of LH pulse frequency. The ovulation rate was increased in ewes sampled on day −8 (P < 0.01) compared with a control group of 40 unsampled ewes. We conclude that collection of blood samples from Merino ewes to measure LH pulses can change the frequency of those pulses and the mean concentration of FSH, and these changes can be accompanied by an altered ovulation rate.
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