Dietary effects of protected fat, soybean meal, and heat-treated soybean meal on performance, physiological parameters, and behavioral measurements of early-fattening Hanwoo steers under heat stress conditions

Objective: This study evaluated the effects of increased levels of dietary total digestible nutrient (TDN) and crude protein (CP) using protected fat (PF), soybean meal (SB), and heat-treated soybean meal (HSB) on performance, physiological parameters, and behavioral measurements of early-fattening Hanwoo steers under heat stress conditions. Methods: Thirty-six steers (480.9 +/- 58.6 kg, 15.9 +/- 1.4 months) were assigned to 4 treatments: control (TDN 75%, CP 15%, rumen degradable protein [RDP]:rumen undegradable protein [RUP] = 62:48); PF (TDN 82.5%, CP 15%, RDP:RUP = 62:48); PF+SB (TDN 82.5%, CP 16.5%, RDP:RUP = 62:48); and PF+SB+HSB (TDN 82.5%, CP 16.5%, RDP:RUP = 48:52) for a total of 16 weeks with division of 4 weeks. The average temperature-humidity index (THI) was 82.9 (1st; moderate), 76.9 (2nd; mild), 70.9 (3rd; comfort), and 65.8 (4th period; Results: Dry matter intake during whole period did not differ among treatments but decreased by 34% and 19%, respectively during 1st and 2nd compared to 4th. Average daily gain (ADG) of PF+SB+HSB was higher (p<0.05) than that of control during the 1st period, and those of both PF+SB and PF+SB+HSB were higher (p<0.05) than Control during the 2nd. The ADG during whole period was in the order of PF+SB+HSB (1.23), PF+SB (1.18), PF (1.11), control (0.98 kg/d) (p<0.05). As THI increased, rectal temperature and all blood parameters increased, while blood glucose levels decreased (p<0.05). Behavioral changes during 1st period compared to 3rd included decreases in lying (24%), walking (48%), and eating (40%), and increases in total standing (50%) and drinking (43%) (p<0.05). Rumination during standing was 38% higher, and rumination during lying was 32% lower (p<0.05). Conclusion: This study demonstrates 10% of increased levels of dietary TDN using PF and CP considering RUP can prevent performance reduction in early-fattening of heatstressed Hanwoo steers and have positive effects on performance recovery from post-heat stress.