Lactational performance of dairy cows fed diets based on corn silages varying in ruminal degradability of starch and fiber

Corn silage (CS) is a cornerstone component in dairy cattle diets, contributing up to 50% of the total dietary NEL intake. Selecting high-quality CS hybrids is critical for improving production efficiency and reducing the carbon footprint of dairy operations. The rumen fermentable organic matter index (RFOMI) assesses CS nutritional quality, with a focus on rumen fermentable NDF (RFNDF) and starch (RFST), which together account for the majority of digestible silage OM. The objective was to evaluate the effects of feeding CS with similar RFOMI but varying RFST and RFNDF contents on lactational performance, enteric gas emissions, nutrient digestibility, and nitrogen utilization in dairy cows. Rumen fermentable NDF and RFST were determined using established in vitro and in situ methods, incorporating digestibility coefficients and nutrient fraction analyses. A total of 48 mid-lactation Holstein cows (16 primiparous, 32 multiparous) were used in this study. Post-ensiling RFST to RFNDF ratios (RFST:RFNDF) in CS were used for treatment designation. The study was a randomized complete block design where cows within a block were randomly assigned to treatment diets. The diets were fed as TMR and contained 50% CS (dietary DM basis) with RFST:RFNDF of 1.62, 1.85, 1.92, or 2.08. Animal performance and enteric gas emissions were evaluated using polynomial linear or quadratic effects of postfermentation CS RFST:RFNDF ratios. A quadratic effect of treatment on DMI was observed, where cows fed CS with the lowest RFST:RFNDF ratio resulted in the greatest DMI. Dairy cows fed CS with an increasing RFST:RFNDF ratio tended to have increased milk yield, but no effects were observed on ECM yield. Increasing RFST:RFNDF of CS in the diet increased quadratically feed and ECM feed efficiencies, CH₄ emission yield (as g/kg DMI), and total-tract DM and OM digestibilities. These results indicated that feeding CS with RFST:RFNDF ratios within 1.85 to 1.92, when compared with CS hybrids outside that range of RFST: RFNDF, at 50% of dietary DM, may promote a reduction in DMI but increase feed and ECM feed efficiencies andCH₄ yield by enhancing apparent total-tract DM digestibility in lactating dairy cows.