产犊月份、胎次和热应激对乳牛产奶量的影响

根据南京一个牧场乳牛群1977～1987年的生产记录和该地的气象资料,采用最小二乘法分析产犊月份和胎次对各泌乳阶段产奶量的影响;采用逐步回归法等统计方法分析热应激对产奶量的影响。结果表明:泌乳前期、中期、后期实际产奶量和305天校正产奶量最高的产犊月份分别是4,10～12,4和4月;产奶量最低的产犊月份分别是5～7,5,11～1和5日;第3胎的305天校正产奶量达最高水平;急性热应激对泌乳中期牛和高产牛的影响较大;平均气温和温湿度指标显著影响夏季日产奶量,而其影响程度与泌乳阶段和生产水平相关。

THE INFLUENCES OF CALVING MONTH, PARITY AND HEAT STRESS ON MILK YIELDS OF HOLSTEIN COWS

Based on the productive records of dairy herd in Experimental Farm of Nanjing Aricultural University and the meteorological data in Nanjing region from 1977 to 1987, least-square method was used to estimate the effects of calving month and parity on milk yields of various stags and whole period of lactation. The effects of heat stress on milk yield was analysed by the method of gradual regression et al. The results indicate that the calving month of the highest milk yield of eary-, mid-, late-lactation and the corrected milk yield of 305 days are April, October to December, April and April, respectively, and those of the lowest milk yields are May to July, May, November to January and May, respectively. Milk yield increase significantly from the first to second parity, and at third parity the corrected milk yield of 305 days reaches the highest level. Acute stress reduces slightly milk yield in the first and fourth (or over fourth)lactation periods, but does more milk yield at mid-lacatation and in high-yielding cows. The acclimation degree of cows to heat increases after heat stress. The milk yield is influenced in summer by air temperature and THI, and is associated with lactation stage and productivity of cows.