Effect of Summer Weather on Performance of Holstein Cows in Three Stages of Lactation

Abstract

Energy intake, milk yield, milk fat and rectal temperature of 36 Holstein cows representing three stages of lactation, were examined in relation to air temperature, relative humidity, indices combining these two variables (THI and THC) and wind velocity occurring June to August. Averages for maximum daily temperature, mean daily, THI, and relative humidity were 31 C, 26 C, 75 and 63%. Values of THI and maximum temperature showed closest relation to cow response. For all stages of lactation, 9% of the variation in milk yield, 13% in milk fat, 5% in feed intake and 65% in rectal temperature were attributable to weather conditions. Rectal temperature and energy intake had the highest correlation with weather on the same day while conditions 2 to 3 days previous were most closely associated with milk yield and milk composition. Changes in feed intake and milk yield depended largely on the stage of lactation. Midlactation cows (100 to 180 days) were most adversely affected; late (180 to 260 days), intermediate, and early (< 100 days) least. Early stage cows consumed the least feed energy but were highest in production indicating they utilized body reserves rapidly to offset the impact of thermal stress. Although weather conditions for the 52 days of study were significantly correlated with milk yield, milk fat percent and energy intake, effects were generally less than reported from studies in heat chambers. © 1972, American Dairy Science Association. All rights reserved.