Studies on post-partum anoestrus in Alentejano beef cows

Post-partum ovarian activity in Alentejano beef cows maintained on natural pastures was monitored by measuring plasma progesterone levels using the radioimmunoassay method. Post-partum anoestrus was compared in cows calving in two different seasons (winter and summer). Differences between primiparous and multiparous cows, the effect of short term nursing interruption (72 hours by the 20th day post-partum) and the dam's body weight at calving (BWC) were also considered with respect to their influence on the onset of post-partum ovarian activity. Fertility after two breeding seasons of natural mating (April-May) and November-December) was also studied. Post-partum anoestrus was significantly longer in winter calving cows than in those calving in summer, in both multiparous and primiparous cows (73.9 versus 33.1 and 111.1 versus 35.6 days, respectively; P<0.001). Primiparous cows took a significantly longer period to resume cyclicity after calving than multiparous cows, during the winter season (P<0.001). BWC was significantly lower during the winter season, in both multiparous and primiparous cows (565.8 versus 642.8 and 435.8 versus 577.0 kg, respectively; P<0.001). Short term calf removal accounted for a shorter post-partum anoestrus only during the summer season (30.01 versus 39 days; P<0.05). BWC and the interval between calving and 21 June (C-21J) were significantly correlated with the duration of the post-partum anoestrus (y=162.17-0.222(BWC)+0.316(C-21J); r2=0.41; P<0.03). Because of a longer post-partum anoestrus, fertility was reduced in cows calving during the winter season compared with those calving in summer in all three years of the study (63.9 versus 76.9%, 57.8 versus 62.9% and 36.5 versus 60.6%, for 1984, 1985 and 1986, respectively). The results presented show that the winter period in Portugal impairs the return to ovarian activity post-partum in this breed. 19 refs, 2 figs, 4 tabs