Natural shade access can attenuate heat stress: physiological responses in the dry period and carry-over effects on subsequent lactation

Abstract

Introduction. Heat stress (HS) during the dry period (DP) challenges thermoregulatory capacity and triggers physiological responses to minimize internal heat load. This condition has been associated with negative carryover effects on subsequent lactation performance. Natural shade (NS) has been proposed as a strategy to attenuate HS. The aim of this study was to evaluate NS effect during the DP on physiological indicators and subsequent milk yield (MY). Materials and Methods. The study was done at the Estación Experimental Facultad de Agronomía Salto (EEFAS) during February and March 2024 on rangelands (approved ethical protocol No.1389).  Thirty-eight pregnant Holstein dairy cows (body weight: 566 ± 55 kg; body condition score: 2.9 ± 0.27) were dried off 72 ± 17.1 days before their expected calving date. Animals were randomly allocated to two treatments during the DP: access to natural shade (ANS; n=19) or no access (NNS; n=19). Three heat waves were occurred: February 23 - March 2, March 11 – 14 and 18 - 20. Rectal temperature (RT) and respiration rate (RR) were recorded weekly at 07:00 and 16:00 h (RT: digital thermometer; RR: flank counts). MY was recorded throughout the subsequent lactation: fortnightly from calving until 100 days in milk, and monthly thereafter until drying-off. Results. At both times, ANS had lower RT and RR compared to NNS cows. In the morning, RT was 38.2 vs. 38.6 ± 0.08°C and RR 36.3 vs. 45.1 ± 1 breaths/min; in the afternoon, RT was 39.1 vs. 39.5 ± 0.08°C and RR 46.4 vs. 62.0 ± 1,1 breaths/min (p<0,001). No significant differences were observed between treatments for MY (19.8 ± 0.60 l/day/animal). Conclusions. Providing NS during the DP markedly attenuated heat‑stress indicators (lower RT and RR) across both morning and afternoon measurements, however, no differences in MY were detected between treatments during the subsequent lactation.