LAUSR

Abstract This study aimed to evaluate the effects of replacing corn silage (CS) with whole-plant soybean silage (SS) in dairy cow diets on nutrient intake and apparent total-tract digestibility, rumen fermentation, milk yield, composition and fatty acid profile of milk, serum metabolites, and feeding behavior. Twenty-four Holstein cows were enrolled to a replicated 4 × 4 Latin square experiment (21 d-periods), with samples and data collected during the last 7 d of each period. Treatments consisted of a control diet (CON) with 480 g/kg DM CS, and SS (ensiled at R5.5 phenological stage) replacing CS at 167, 333, and 500 g/kg DM. Soybean silage linearly decreased DM intake and digestibility, and intake of feed particles shorter than 4 mm. Cows fed SS had a linear increase in ruminal pH and ruminal concentrations of butyrate, branched-chain fatty acids, and NH3-N. Soybean silage linearly decreased milk and solids yield. Dietary inclusion of SS at expense of CS linearly increased milk fat concentration of unsaturated (C18:1 and C18:2) and long chain fatty acids (>C16). Microbial protein synthesis was linearly decreased and blood urea nitrogen concentration was linearly increased by SS treatments. Soybean silage had a positive quadratic effect on periods of rumination and chewing (min/d). Replacing CS with SS ensiled at R5.5 phenological stage decreases feed intake and performance of cows, without affecting milk fat content. Dietary SS, however, can be used to modulate ruminal fermentation and increase long-chain fatty acids (>C16) and unsaturated fatty acids proportion on milk fat.