A field study of the behavioral and physiological effects of varying amounts of shade for lactating cows at pasture.

Shade reduces the negative effects of heat load, but little is known about how much is required for efficient cooling in commercial settings. The effect of the amount of shade on 8 Holstein-Friesian herds was studied for 2 consecutive summers (mean temperature: 23 °C) on 6 commercial, pasture-based dairy farms. Farms varied in the amount of natural shade provided (range: 0 to 15.6m(2) shade/cow). Time spent in shade, near water, eating, ruminating, lying, and standing were recorded between 1000 and 1530 h in 31 shaded and 11 unshaded paddocks using 20-min instantaneous scan observations of 15 focal cows/herd. Respiration rate and panting score (0 to 4.5) was recorded for focal animals once per hour. The total numbers of cows in shade, near water, and with panting scores ≥ 2 were recorded every 30 min. Cows without shade spent 4% more time lying than cows with shade (standard error of the difference, SED = 1.9%). A larger proportion of the herd had panting scores ≥ 2 when no shade was available (6 vs. 2% of the herd, SED = 1.2%), and respiration rates were higher by 8 breaths/min in cows without shade (SED = 4.7 breaths/min). Under the conditions tested, the maximum proportion of the herd that was observed using the shade increased by 3.1% for every 1-m(2) increase in shade size [standard error (SE) = 1.51%], and all cows were first seen simultaneously using shade when 2m(2)/cow was provided. For every 1-m(2) increase in shade, 0.3% fewer cows had panting score ≥ 2 (SE = 0.12%). We observed no significant relationships between the amount of shade available and any other variables. Although additional work is required to make specific recommendations, these results indicate that providing more shade allowed a higher proportion of animals to use this resource and reduced respiratory signs of heat load.