Effects of cold temperature and fat supplementation on growth performance and rumen and blood parameters in early fattening stage of Korean cattle steers

•Cold stress (CS) and fat supplementation may affect growth and rumen fermentation.•Mild or moderate CS does not affect growth performance of Korean cattle steers.•CS changes ruminal concentrations of total volatile fatty acid but RPF does not affect.•Rumen-protected fat (RPF) supplementation does not decrease nutrient intakes.•RPF supplementation does not improve growth performance. We investigated the effects of cold ambient temperatures and the supplementation of rumen-protected fat (RPF) on the growth performance, rumen fermentation parameters, and blood metabolic parameters of Korean cattle steers during the early fattening stage. Twenty Korean cattle steers (body weight = 356.5 ± 3.72 kg, age = 16.8 ± 0.34 months) were divided into two groups: 10 steers were fed a conventional concentrate (control), and 10 steers were fed a concentrate supplemented with 8 g/kg RPF (treatment). Steers were fed a concentrate at 1.6% body weight and 3 kg tall fescue hay daily for 12 weeks in 2016: January 8 to February 2 (P1), February 3 to March 3 (P2), and March 4 to April 1 (P3). Body weight was measured, and blood and rumen fluid were collected four times from each steer: at the start of the experiment, and then at 4-week intervals. The mean indoor temperatures during P1 (−7.37 ℃) and P2 (–2.96℃) were lower (P < 0.001) than the mean P3 temperature (6.22 ℃). These lower temperatures were within the range that causes mild to moderate cold stress (CS) in other cattle breeds. By contrast, the mean indoor temperature during P3 is considered thermoneutral. The average daily gain (P = 0.39 and P = 0.16) and gain-to-feed ratio (P = 0.26 and P = 0.57) were not affected by RPF supplementation or time-period. Total dry matter, crude protein, ether extract, neutral detergent fiber, and acid detergent fiber intakes per body weight were higher (all P < 0.001) in colder P1 or P2 compared to other time periods, but these variables were not affected (P ≥ 0.31) by RPF supplementation, except ether extract intake. Ruminal total VFA concentrations were higher (P = 0.001) in a colder January than in other months, but these were not affected (P = 0.26) by RPF supplementation. The ruminal propionate proportion was higher (P < 0.001) in a colder January than in other months, whereas the butyrate proportion was lower (P = 0.01). Serum triglyceride (P = 0.007), total cholesterol (P = 0.001), and high-density lipoprotein (P = 0.03) concentrations were lower during colder months. RPF supplemen