Dietary protein level and corn processing method: Intake, digestibility, and feeding behavior of lactating dairy cows✰

The effects of corn processing and dietary protein level and their interaction on intake, digestibility, and feeding behavior in dairy cow was examined using eight multiparous Holstein dairy cows in mid-lactation (105 ± 9 days in milk and 47.2 ± 3 kg/day milk production at the start of the experiment) that were placed in a replicated 4 × 4 Latin square with a 2 × 2 factorial arrangement of treatments. Experimental diets contained either finely ground corn (FGC) or steam-flaked corn (SFC) based on either low protein (LP, 14.8%) or high protein (HP, 16.2%) content. Diets contained 40% forage (corn silage and alfalfa hay) and 40% corn grain. Dry matter intake did not differ between HP and LP diets. In contrast, cows fed LP had greater total tract digestibility compared with that for HP-fed cows. Cows fed LP had greater chewing time and lower chewing rate (g of DM/min) compared to those for cows fed HP. Cows receiving SFC had a lower intake of nutrients and tended to have greater ADF and starch digestibility than cows fed FGC. There was an interaction between corn processing and level of protein with respect to the apparent total tract digestibility with cows fed FGC with HP resulted in the lowest digestibility. Steam-flaked corn had lower density and greater moisture and mean particle size than the FGC. In comparison with FGC, SFC did not pass through the 8-mm sieve and caused significant increase in physical effectiveness factor (pef) and physically effective fiber (peNDF) in SFC diets compared to that for FGC diets. Cows fed SFC sorted against longer particles (P = 0.04) to a greater extent than cows fed FGC. Dietary protein level had no detectable effect on sorting index. Chewing time did not differ for cows fed SFC or FGC; however, cows fed SFC had lower chewing and rumination rate (g of DM/min). Chewing time per kg of DM and CP were greater and per kg of peNDF were lower in cows fed SFC rather than cows fed FGC. Results indicated that corn processing and dietary protein level may interact to affect nutrient digestibility, but did not affect the feeding and chewing behavior of lactating cows under our experimental conditions. Results also showed that dietary peNDF content and chewing activity can be effectively manipulated by steam flaking of corn, and cows fed SFC had greater peNDF and energy intake despite lower DMI.