Effects of calving interval on milk production of dairy herd at Hiroshima University farm

Using the records of Hiroshima University Farm for the Holstein dairy herd from the year 2000 to 2017, we investigated the effects of the calving interval on the milk production, length of lactation and dry period, and body weight change of dairy cows in early lactation. The 232 records for periods from one calving to the next calving including the dry period were divided into three parity groups (first parity, second parity, and third or greater parity) and analyzed within each group. An automatic milking system (AMS) was introduced on the farm, and the feeding system was changed from separate feeding to partial mixed ration feeding in March 2011. The data sets for the periods during which the AMS was introduced were not included the data set used for the analyses. We compared the responses listed above between calving interval groups (CIG ; CIsub(S) < 404 d, CIsub(L) ≧ 404) and between periods before and after the AMS introduction. An increase in the calving interval significantly elongated lactation period and dry period. However, the ratio of the lactation period to the calving interval did not decrease with increasing calving interval. The decrease in the milk yield at 45 days before dry off (45_day_Milk yield) with increasing calving interval suggested decreases in the milk yield during the later period of lactation, and seems to have caused the decrease in the milk yield per lactation day and the milk yield per calving interval. Those results indicated that an increase in the calving interval decreased the efficiency of the milk production and might seriously diminish the management efficiency of dairy farmers. The energy balance in the early lactation stage estimated from the body weight change, and the milk yield in early lactation period did not significantly differ between the CIGs for all parity groups. After the introduction of the AMS, the increase in the milk yield in early lactation did not result in an increase in body weight loss, probably due to the introduction of the partial mixed feeding system. However, the dry period and calving interval for the second or greater parity were elongated. Further research is required to investigate the factors that contribute to the calving interval elongation of dairy cows. To improve the milk production efficiency under the condition of calving interval elongation, factors affecting the decrease rate of the milk yield in the later lactation stage should be investigated.