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Impact of providing shade on the growth performance of grazing dairy heifers
Shade is recommended and often considered important for the wellbeing and productivity of animals. However, limited information is available on the growth performance of grazing dairy heifers when they are provided shade. The objective of this study was to determine the effects of shade on the growth performance of grazing Holstein dairy heifers throughout the summer months. Thirty-two heifers were randomly assigned to 1 of 4 groups according to BW (163.8 ± 7.27 kg, 150.6 ± 8.5 d of age). The groups were assigned to 1 of 2 treatments: no shade (NOSHADE) or 2.3 m2 shade per heifer (SHADE). Body weight, hip height (HH), withers height (WH), hip width (HW), body condition score (BCS), heart girth (HG), and rectal temperature were collected every 4 wk from May until September 2013. Blood samples were collected for plasma urea nitrogen (PUN) analysis. Temperature and relative humidity were recorded hourly both in the pasture and under the shade structures using HOBO data loggers, and temperature-humidity indices (THI) were calculated. Data were analyzed using PROC MIXED in SAS. Growth data were analyzed by heifer within paddock as repeated records and environmental data were analyzed by location (pasture or under shade). Daily high temperatures averaged 26.4°C and daytime temperatures (1200 to 1500 h) ranged from 13.5°C to 38.2°C during the study. There were no significant differences (P = 0.60) in ADG (0.84 and 0.89 kg/d for NOSHADE and SHADE, respectively) or BW at the end of the study with NOSHADE heifers averaging 220.6 kg and SHADE heifers averaging 224.3 kg (P = 0.35). Hip height (118.0 and 118.4 cm for NOSHADE and SHADE, respectively) were similar between treatments (P = 0.32), but WH (116.7 and 118.7 cm, respectively, for NOSHADE and SHADE) were greater (P = 0.001) for SHADE at the end of the study. Heart girth (P = 0.80), HW (P = 0.52), and BCS (P = 0.53) were similar between treatments. Also, PUN values were similar (P = 0.66) for heifers with SHADE compared to NOSHADE (11.9 and 10.8 mg/dl, respectively). Rectal temperatures during sampling were 39.6°C for both treatments (P = 0.64). Providing shade did not improve the growth performance of Holstein dairy heifers in this study.
Keywords: dairy heifer, grazing, shade