2016 Volume 7 Issue 1 Pages 14-19
The aim of this study was to investigate the relationship between calving season and postpartum reproductive performance of seasonally bred Japanese beef cows fed in the Aso grasslands in Kumamoto, Japan. Thirty-one Japanese beef cattle were kept under free grazing with a dry matter grass intake of about 10 kg/head/day from April to October and then given restricted feeding with dry matter grass silage intake of about 8 kg/head/day from November to March. Artificial insemination was performed from April to July. The cattle were divided into three groups according to their calving season: early calving （E group: January 1st to February 15th; n=6）, middle calving （M group: February 16th to March 31st; n=17） and late calving （L group: April 1st to May 15th; n=8）.
The first ovulation after calving in the L group （mean ± SD, 44.4 ± 15.1 days） was earlier than in the E group （72.8 ± 15.5 days）（p<0.05）. The mean ± SD postpartum body condition score （BCS） on Day 7 was 2.75 ± 0.40, 2.42 ± 0.33 and 2.40 ± 0.34 in the E, M, and L groups, respectively. Postpartum days of nadir BCS in the L group （18.2 ± 6.3 days） was shorter than that in the E group （44.8 ± 6.3 days）（p<0.05） . Moreover, BCS recovery （to the postpartum level） in the L group （40.6 ± 16.0 days）was faster than that in the E group （82.6 ± 16.0 days）（p<0.05） . Among the three groups, there was no difference in conception rate. These results indicate that the calving season can affect the postpartum reproductive performance of seasonally bred Japanese beef cattle that were given restricted daily feeding. Since the period of artificial insemination in the L group is shorter, it is very important to check the first ovulation in order to ensure conception at the next estrus. It is also important to monitor the ovarian activity, such as a dominant follicle and functional corpus luteum.
