Abstract
Heat stress in dairy cows is caused by a combination of environmental factors (temperature, relative humidity, solar radiation and air movement). Continual genetic selectin for greater performance results to increased sensitivity to heat stress. It was one of the reasons why lactation curve during summer has decreasing trend compared to spring in which lactation curve maintained within high levels. The trial was conducted in spring (April-May) and summer period (June-July) on total of 40 dairy cows of Holstein-Friesian breed in early lactation period (first 60 days of lactation). Milking capacity in cows and milk chemical ingredients (milk fat and proteins) were statistically significantly higher in the spring period compared to summer, while higher values of lactose were not statistically significant. Total average milk production per cow was significantly higher in spring period (42.74?4.98l) than in summer (39.60?5.09l) at the level of P<0.05. A higher rate of milk fat was recorded in spring in relation to summer period the level of significance being P<0.01. The content of proteins in milk in spring period was 13% higher than in summer. The established difference was highly statistically significant (P<0.001). Obtained values for percentage of lactose varied slightly (4.45?0.54% in spring versus 4.03?0.24% in summer period; P>0.05).
Highlights
A heat stress in dairy cows is provoked by a great number of external factors – temperature, relative air humidity, sunlight radiation, air circulation and precipitations
It is rather complicated to determine precisely the moment when the cow enters into the heat stress because the incidence of heat stress is not influenced by energy balance only and by a quantity of water, and metabolism of sodium, potassium and chlorine (Kadzere et al, 2002)
The aim of this study was to examine the influence of heat stress on the production and chemical composition of milk in Holstein-Friesian cows in early lactation period
Summary
A heat stress in dairy cows is provoked by a great number of external factors – temperature, relative air humidity, sunlight radiation, air circulation and precipitations. Selection in high-yielding dairy breeds of cattle directed towards improving genetic predisposition for higher production of milk and quantity of consumed food especially at the beginning of lactation has resulted in reduced production and reproductive potentials in the periods of increased outdoor temperature, because a due attention has not been payed to a thermoregulative capacity in animals. High-yielding dairy cows are the most sensitive to the influence of heat at the beginning of lactation and in cases when a body temperature is higher than 39oC a production of milk significantly falls (Ravagnolo and Misztal, 2000). At the outside temperature of 35oC a quantity of milk is decreased by 33%, and at the temperature of 40oC by 50% (West, 2003)
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