Effect of heat stress on milk production in dairy cows

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 lt 0.05. A higher rate of milk fat was recorded in spring in relation to summer period the level of significance being P lt 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 lt 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).

Toplotni stres kod mlečnih krava uzrokovan je većim brojem spoljašnjih faktora - temperaturom, relativnom vlažnošću vazduha, sunčevom radijacijom i kretanjem vazduha. Genetska selekcija za veće proizvodne rezultate povećava osetljivost na toplotni stres. To je jedan od razloga što laktaciona kriva za vreme letnjeg perioda ima trend pada u poređenju sa prolećem kada laktaciona kriva dostiže visoke nivoe. Ogled je izveden u prolećnom (april-maj) i letnjem periodu (jun-jul) na ukupno 40 mlečnih krava holštajn-frizijske rase u ranom laktacionom periodu (prvih 60 dana laktacije). Mlečnost krava i hemijski sastojci mleka (mlečna mast i proteini ) su bili statistički značajno veći u prolećnom periodu u odnosu na letnji, dok veće vrednosti laktoze nisu bile i statistički značajne. Ukupna prosečna proizvodnja mleka po kravi je bila značajno veća u prolećnom periodu (42,74±4,98l) nego u letnjem (39,60±5, 09l) na nivou od P lt 0,05. Veći procenat mlečne masti zabeležen je u prolećnom nego u letnjem periodu na nivou značajnosti P lt 0,01. Sadržaj proteina u mleku u prolećnom periodu bio je 13% veći nego u letnjem. Ustanovljena razlika bila je visoko statistički značajna (P lt 0,001). Dobijene vrednosti za procenat laktoze malo su varirale (4,45±0,54% u prolećnom prema 4,03±0,24% u letnjem periodu; P>0,05).