Abstract
The objective of this study was to evaluate the effects of feeding graded levels of hybrid rye silage (HRS) as a replacement for barley silage (BARS) on DMI, milk yield and composition, ruminal fermentation characteristics, omasal nutrient flow, total-tract nutrient digestibility, and nitrogen (N) utilization in dairy cows. Twelve lactating Holstein cows (4 primiparous and 8 multiparous; 670 ± 68 kg BW and 104 ± 14 DIM at the beginning of the study) were used in a replicated 4 × 4 Latin square design with 28-d periods consisting of 18 d for dietary adaptation and 10 d for data and sample collection. Four multiparous cows in one Latin square were ruminally cannulated to facilitate ruminal and omasal sampling. Experimental diets consisted of a control diet containing 46.4% (DM basis; designated CON) BARS as the major forage source, and diets containing 15.3% (LOW), 31.1% (MED), and 46.4% (HIGH) HRS as a partial or complete replacement for BARS. Increasing the dietary inclusion of HRS linearly decreased DMI, and milk and ECM yields. Milk fat and lactose concentrations were unaffected by diet; however, milk protein concentration decreased linearly as the dietary inclusion level of HRS increased. Milk yields of fat, protein, and lactose decreased linearly as the dietary inclusion of HRS increased. Increasing dietary inclusion of HRS linearly decreased ruminal acetate concentration, but ruminal concentrations of propionate and valerate, and the acetate:propionate ratio were not affected by diet. The ruminal concentrations of butyrate, isovalerate, isobutyrate, and total branched short-chain fatty acids changed quadratically as the dietary inclusion of HRS increased. There was a tendency for a linear decrease in ruminal total short-chain fatty acid concentrations with increasing HRS; however, mean ruminal pH was not affected by diet. Ruminal ammonia-N concentration increased linearly with increasing dietary inclusion of HRS. When expressed as absolute amounts, apparent ruminal digestion of DM and OM and true ruminal digestion of OM responded quadratically as the dietary inclusion of HRS increased, but ruminal digestion of NDF was unaffected by diet. Intake, apparent ruminal digestion (expressed as absolute amounts), and omasal flow of starch all decreased linearly as the dietary inclusion of HRS increased. Replacing BARS with HRS did not affect apparent total-tract digestibilities of DM, CP, OM, and ether extract; however, apparent total-tract digestibilities of NDF and starch linearly increased with increasing dietary levels of HRS. Increasing the dietary inclusion of HRS linearly decreased N intake and RDP supply and tended to linearly decrease RUP supply. Omasal flow of total microbial NAN linearly decreased as dietary inclusion level of HRS increased. Urinary N excretion tended to decrease, whereas fecal N excretion (g/d) and milk N secretion (g/d) linearly decreased as dietary inclusion of HRS increased. In conclusion, the partial or complete replacement of BARS with HRS had negative effects on DMI, and milk and milk component yields, but improved total-tract digestibilities of NDF and starch.