Short communication: Forage particle size and fat intake affect rumen passage, the fatty acid profile of milk, and milk fat production in dairy cows consuming dried distillers grains with solubles

Abstract Four ruminally cannulated Holstein cows averaging (±SD) 116±18 d in milk and 686±52kg of body weight were used in a 4×4 Latin square design with a 2×2 factorial arrangement of treatments to test the effects of forage particle size and concentration of corn oil on milk fat depression. Cows were housed in individual stalls, milked daily at 0700 and 1800h, and individually fed daily at 0900h for ad libitum consumption allowing approximately 10% orts. Four 28-d periods, in which each cow was offered 1 of 4 total mixed rations, included reduced-fat dried distillers grains with solubles at 30% of dietary dry matter and differed in forage particle size by inclusion of chopped grass hay (LONGP) or grass hay pellets (SHORTP) and 0 or 2% corn oil (CO). Dietary treatments were 0% corn oil + short particle size (CO0+SHORTP), 0% corn oil + long particle size (CO0+LONGP), 2% corn oil + short particle size (CO2+SHORTP), and 2% corn oil + long particle size (CO2+LONGP). Dry matter intake and milk yield were not affected by treatment averaging 26.5±1.19kg/d and 32.8±3.34kg/d, respectively. A decrease was found in 3.5% fat-corrected milk with the inclusion of oil resulting in 34.6 and 26.6±2.6kg/d for 0 and 2% oil diets, respectively. An oil × size interaction was found for milk fat concentration resulting in 2.27, 3.02, 3.62, and 3.62±0.23% for CO2+SHORTP, CO2+LONGP, CO0+SHORTP, and CO0+LONGP, respectively. Fat yield was reduced from 1.22 to 0.81±0.09kg/d with 2% oil diets. Cows consuming diets with long particle size spent 29 more minutes eating compared with the cows consuming short particle size (198 and 169±15min/d). Rumination time decreased from 504 to 400±35min/d for cows consuming short particle size compared with long particle size. Total chewing was reduced from 702 to 570±4min/d when cows consumed short particle size. Feeding long particle size decreased rate of passage of dry matter from 3.38 to 2.89±0.42%/h; concomitantly mean retention time increased from 31.7 to 38.4±5.36h for diets containing long particle size. The results of this experiment show that effects of oil on milk fat depression were less severe when cows consumed long particle size, suggesting that dietary manipulations that modify rumen kinetics also affect milk fat production in dairy cows consuming reduced-fat dried distillers grains with solubles supplemented with corn oil.