Efecto de época del año (verano vs. invierno) en variables fisiológicas, producción de leche y capacidad antioxidante de vacas Holstein en una zona árida del noroeste de México Effect of season (summer vs. winter) on physiological variables, milk production and antioxidant capacity of Holstein cows in an arid zone of northwestern Mexico

SUMMARY Heat stress (HS) induces the production of oxygen-derived free radicals associated with deleterious effects on the physiology, pathology and productivity of dairy cows. In order to evaluate physiological and productive responses of Holstein cows during summer and winter in an arid area, 40 lactating cows (100 to 150 d in milk) were randomly selected from a representative month per season of the year (n = 20/season). Data were analysed by ANOVA under a completely random experimental design, with season of the year as factor of interest (summer vs. winter), and a level of significance of 5%. During winter time, cows were under their thermoneutral zone, whereas in summer they were under severe HS conditions. The skin temperatures of right side, buttock, udder and head, as well as raw milk were about 10 oC higher in summer (P < 0.05). Respiration rates were twice higher 106.87 ± 0.05 vs. 47.74 ± 0.06 (P < 0.05) in summer when compared with winter, while the rectal temperature increased 1.6 oC in summer. Cholesterol and TAS concentrations, ghecose, TOS and IEO were lower (P < 0.05) in summer, while glucose TOS and IEO were higher (P < 0.05) in summer than in winter. Milk production was reduced by more than half (P < 0.05) in summer, and significant reductions of 42.6% in energy production in milk and 48.4% in the milk production adjusted to 3.5% fat were observed. The percentages of fat and protein in milk were higher (P < 0.05) in summer. In conclusion, the effect of heat stress is evident in summer and has detrimental effects on physiological responses and productivity of lactating Holstein cows. Environmental innovations are needed to reduce the negative impact of high temperatures in this arid region of Mexico.