Heat tolerance or extensive ability to acclimate

Abstract

Heat thermal stress is a major concern environmental stress for dairy cattle, it limits animal growth, metabolism, and productivity. Taken this, the joint selection for productivity and adaptability should be considered in the actual dairy farms programs. This study aimed to evaluate the seasonal acclimatization process of cows with different milk yield potential. From a dairy farm located in Alentejo, Portugal, 13 Holstein-Friesian cows were chosen, 7 with high milk yield potential (HMP), ≥9,000 kg of milk at 305 days of lactation, and 6 with low milk yield potential (LMP), <9,000 kg. The trail was separated in 3 periods: (P1) Summer: acclimated cows in heat stress; (P2) Summer: acclimated cows in thermoneutrality; (P3) Winter: acclimated cows in thermoneutrality. Respiratory frequency (RF), rectal temperature (RT) milk composition and plasma triiodothyronine levels (T3) were collected. No differences were found in RF and RT between HMP and LMP cows in any of the periods. RF and RT values were significantly higher in P1 (64.13±12.78 mov./min. and 38.82±0.68 °C) than in P3 (36.13±7.67 mov./min. and 38.06±0.52 °C). Although, in P1, some HMP cows had RT values that indicated heat stress. We found no differences between groups in the lactose, protein, fat, β-Hydroxybutyric acid and somatic cell count. Urea was significantly higher in P1 in the HMP (293.62±35.97 mg/kg) than in LMP (253.69±33.81 mg/kg). T3, in both groups, gradually increased from P1 to P3 (P1-142.00±13.77; P2 – 157.36±10.72; P3 – 170.69±17.78 ng/dl). During summer, HMP had T3 values significantly lower than the LMP cows (P1: HMP-133.33±8.14, LMP-152.40±11.97; P2: HMP-146.50±7.64; LMP-170.40±12.29 ng/dl). Despite these results had revealed that HMP and LMP cows did not show significant differences in RF and RT variables, ongoing the acclimatization process, the HMP presented a lower metabolic activity as well a change in the nitrogen metabolic pathways.