Lack of effect of climatic heat challenge of dairy cows in Canterbury on their milk production : A dissertation submitted in partial fulfilment of the requirements of the degree of Bachelor of Agricultural Science with Honours At Lincoln University

Abstract

The occurrence of some hot weather in Canterbury during the 2008/2009 summer has provided an opportunity to investigate the effects of climatic heat challenge on milk production by cows on commercial dairy farms in that region. An analysis of daily milksolids production and the peak temperature-humidity index (THI) recorded on each previous day was conducted for 13 farms in mid-Canterbury using data from 1 December 2008 to 28 February 2009. There were 19 days (out of 90) when dry bulb air temperature exceeded 27 °C, corresponding to a THI of 72 or higher, at which temperature stress has been suggested to occur in dairy cows. However, there was no inverse relationship between THI and milksolids production (P ranged from 0.193 to 0.897) on any of the farms whether using all the data or just confining the analysis to when THI exceeded 72. This finding is largely explained by the dryness that is associated with high air temperature in Canterbury - humidity being strongly inversely related to dry bulb temperature (slope= -2.75, r² = 0.759 and slope = -2.60, r² = 0.663, at Riverlands and Broadfields recording stations respectively). As a result THI values rarely exceed 75 in this region. Because there were no discernible effects on productivity of these dairy cows during the episodes of hot weather, it is also unlikely that there would be any animal welfare concerns that could be attributable to climatic heat challenge in these conditions.