Effect of shade on body temperature and performance of feedlot steers1

A 120-d feedlot study using 164 Angus steers (BW = 396.7 ± 7.0 kg) was undertaken in Queensland Australia (24°84' S, 149°78' N) to determine the effect of shade on body temperature (T(B)) and performance. Cattle were allocated to 20 pens: 16 with an area of 144 m(2) (8 steers/pen) and 4 with an area of 168 m(2) (9 steers/pen). Treatments (10 pens/treatment) were unshaded (NS) vs. shaded (SH). Shade (3.3 m(2)/steer) was provided by 80% solar block shade cloth. Before the study (d -31), 63 steers were implanted (between the internal abdominal muscle and the peritoneum at the right side flank) with a T(B) transmitter. Within each pen, 3 steers had a T(B) transmitter. Individual T(B) was obtained every 30 min. The cattle were fed a feedlot diet and had ad libitum access to water. Water usage and DMI were recorded daily on a pen basis. Average daily gain and G:F were calculated on a pen basis. Climatic variables were obtained from an on-site weather station every 30 min. Individual panting scores (PS) were obtained daily at 0600, 1200, and 1600 h. From these, mean PS (MPS) were calculated for each pen. At slaughter (d 121), individual HCW, loin muscle area (LMA), rump fat depth (P8), 12th-rib fat depth, and marbling score were obtained. Mean T(B) was not affected (P > 0.05) by treatment (SH = 39.58°C; NS = 39.60°C). However, during a 21-d heat wave when cattle were exposed to a mean ambient temperature (T(AM)) > 30°C for 8 h each d (T(AM) between 0800 and 1800 h = 29.7°C, and 23.4°C between 1830 and 0730 h), the T(B) of SH steers (40.41 ± 0.10°C) was less (P < 0.01) than the T(B) of NS steers (41.14 ± 0.10°C). During this period, pen-MPS were greater (P < 0.05) for the NS cattle at all observation times. Over the first 6 d of the heat wave, MPS of NS steers at 1200 h was 2.47 (P < 0.01) vs. 1.39 for SH steers. Hip height, DMI, ADG, and G:F were greater (P < 0.05) for SH cattle. Exit BW (final BW) of SH steers (596.1 kg) was greater (P < 0.05) when compared with NS steers (578.6 kg). During the heat wave, DMI was 51% less for NS steers and 39% less for SH steers when compared with the pre-heat wave period (P < 0.01). The HCW of SH steers (315.4 ± 0.8 kg) was greater (P < 0.05) than for NS steers (321.4 ± 0.8 kg). No treatment differences (P > 0.05) were found for LMA, P8, or marbling score. Access to shade improved (P < 0.05) ADG and G:F, increased HCW, and decreased MPS; however, shade did not completely eliminate the impact of high heat load.