The Thai Journal of Veterinary Medicine


The objectives of this study were to analyze the potential impact of heat stress on reproduction in differentregions of Thailand, to determine the monthly distribution of calving throughout the year and to investigateenvironmental sources of variation of days-open (DO) in first lactation dairy cows. Data including 13,548 lactationrecords were collected during 2004 to 2006. Climate data were obtained from the provincial meteorological stationsand the corresponding temperature-humidity indexes were calculated. The difference between regions in THI wasdetermined. The geographical regions studied were: Central, Eastern, Northeastern, and Northern. The distributionof calving by month was determined in the 1st to 5th lactation. The effect of month of calving (MOC) on DO wasdetermined only in first lactation dairy cows. The fixed effects in the model included MOC, region and MOC x region.The lowest mean THI was observed in December (72) and the highest mean THI in April (80). THI differedsignificantly between regions (p<0.0001), and months (p<0.0001). Significant interactions between region and month(p<0.0001) were found for THI. In all regions, minimum THI values were observed in December and January and thiseffect was more pronounced in the Northeastern and Northern regions. The highest frequency of calving for the firstlactation was observed in June (9.96%) and the lowest in February (6.63%). The highest frequencies of calving for the2nd (13.1%), 3rd (14.1%), and 4th (14.66%) lactation cows were observed in September while for the 5th lactation cowswas in October (14.91%). The lowest proportion of calving for 2nd (5.02%) and 4th (4.14%) lactation cows was inFebruary, and in March for the 3rd (4.35%) and 5th (4.85%) lactation cows. The average DO in first lactation cows was152 days. Significant effects of MOC (p<0.0001) and region (p<0.0001) were found on DO. February calving cows hadthe longest DO (219±11 days) while cows calving in October and November had a significantly shorter mean DO(133±7 days). The study indicates that the high proportion of cows calving in October and November corresponds tobreeding success in the previous months of December and January, which are the coolest months of the year. Cowswhich calved during hot months had a prolonged DO period of several months.



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