Effects of sorghum grain processing and forage fiber source on milk production, digestibility and kinetics of passage in Holstein cows.

Experiments were conducted to determine effects of source of forage and starch degradability on milk production, digestibility and passage in Holstein cows. A preliminary experiment showed that sampling site, dosing time and passage model had little influence on passage parameter estimates for grain in lactating cows. Mean retention times were 17 and 25 h for duodenal and fecal sampling, respectively, and 16 and 18 h at the duodenal site and 24 and 26 h at the fecal site for doses given before rather than after feeding, respectively. In an 8 wk trial cows in early lactation were fed diets with 30% NDF with forage NDF from wheat straw or alfalfa hay in proportions of 0:3, 1:2, 2:1 or 3:0. Intake and milk yield were not influenced, but milk fat percentage, acetate to propionate ratio (C2:C3) and persistence were decreased with increasing straw, and yield of FCM decreased on all straw. Ratio of forage NDF to ruminally degradable starch (FNDF:RDS) was 1.10 1.01,.92 and.84 in 0:3, 1:2, 2:1 and 3:0 diets, respectively, and it was concluded that a ratio of 1:1. Forage source did not influence any performance parameter nor did forage source interact with grain processing. Steam-flaking grain increased milk yield 12%, milk protein yield 14% and improved efficiency and persistence. Although milk fat percentage was decreased from 3.6 to 3.2, yield of milk fat was not influenced. It was concluded that increasing starch degradability may improve performance if the ratio of FNDF:RDS is maintained > 1:1. In the last experiment, duodenally cannulated cows were fed diets similar to those in the second lactation trial, except straw was substituted for 2/3 of the alfalfa hay on an NDF basis. Substituting straw for alfalfa did not influence flow of OM, starch or any CP fraction, but resulted in decreased (54 vs 47%) ruminal cellulose digestibility. Steam-flaking sorghum grain increased ruminal digestion of starch (74 vs 48%), increased flow of non-ammonia CP (120 vs 110% of intake) and bacterial CP (2.8 vs 2.2 kg/d), and decreased ruminal digestibility of cellulose (47 vs 53%), compared to dry-rolling. Response to steam-flaking in the lactation trial was probably due to both increased energy availability and improved duodenal flow of CP