Effect of Supplemental Fish Meal on Milk Yield and Milk Composition of Holstein Cows during Early Lactation

Abstract

Two studies, the first in the USA and the second in Australia were undertaken to investigate respiration rate (RR) responses of growing grain fed cattle exposed to hot climatic conditions. In the first study (Exp 1) eight Hereford x Angus x Simmental steers were exposed to 24 h cyclic hot conditions (24 o C to 39 o C). In the second study (Exp 2) six Murray Grey x Hereford steers were used. In this study ambient temperature (Ta) ranged from approximately 24 o C to 45 o C. Two cooling periods were used in Exp 2: day cooled (DC) or night cooled (NC). In each study RR was measured over three 24 h periods and generally increased as Ta increased. However, the rate of change was not constant either between studies or over time. In Exp 2, DC cattle typically showed an increase in RR at night when Ta was decreasing. In both studies RR lagged behind Ta by approximately 2 h. RR can be used as an indicator of heat stress in cattle, provided animal condition, prior exposure, ambient conditions (increasing or decreasing Ta) and previous cooling strategies are considered.