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; Comfort).

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 heat-stressed Hanwoo steers and have positive effects on performance recovery from post-heat stress.