Including soybean hulls (SH) in ruminant diets increase the carbon circularity of livestock systems. We evaluated whether SH could partially replace corn grain (CG) in the diet of lactating cows (≈7.5% of the total diet). Six Holstein multiparous cows were fed a 50:50 (dry matter basis) corn-silage to concentrate diet for two 25-day periods in three replicated 2 × 2 Latin Squares using linear mixed models. Feed intake, milk yield and composition, N excretion, CH₄ production, and ruminal microbial profiles were measured. Enteric CH₄ was measured in respiration chambers. The DMI of cows fed SH (28 kg DM/d) was greater (p = 0.034) than those fed CG (26 kg DM/d), but no differences were observed in milk production (p = 0.557) or enteric CH₄ emission intensity (g/kg milk, p = 0.387). Enteric CH₄ emissions tended to be greater in SH-fed cows (484 g/d) than CG-fed cows (456 g/d; p = 0.075), and N excretion was significantly greater in SH than CG cows (p < 0.001). No mayor shifts in ruminal microbiota or overall ruminal environment were observed. In conclusion, SH can partially replace CG in diets of lactating dairy cows without affecting production, but potential negative environmental impacts should be considered.

Dietary intake of omega-3 polyunsaturated fatty acids (n-3 PUFA) and conjugated linoleic acid (CLA) has been recognized as having potential positive effects on human health (Lock and Bauman, 2004). Furthermore, alteration of the fatty acid (FA) composition of milk through targeted manipulation of dairy cow diets can be achieved (Dewhurst et al., 2006). Although breed effects on the FA composition of milk have previously been reported (Kelsey et al., 2003), most work to-date has been conducted under high input TMR based systems. Furthermore, little is known on the effects of heterosis on concentrations of FA in milk. The objective of this study was to examine whether differences in milk FA concentration exist between two contrasting breeds of dairy cow and their F₁hybrid managed under a grazing based production system.