Effect of diets differing in ratio of alfalfa hay and corn silage on lactational performance and rumination of Holstein dairy cows.

Our objective was to measure the effects of different ratios of alfalfa hay and corn silage on amount and efficiency of milk true protein and fat production in Holstein cows. One hundred and 5 cows (45 primiparous, 60 multiparous), in 2 enrollments, were used in a randomized complete block design study with 1-wk covariate (COV) and 4-wk experimental period. Following the COV period, cows were blocked by parity, milk yield, and DIM and assigned randomly to 1 of 5 diets comprised of 62% forage (DM basis) with the forage consisting of different DM ratios of chopped alfalfa hay and corn silage: 10:90 (10ALF); 30:70 (30 ALF); 50:50 (50ALF); 70:30 (70 ALF); and 90:10 (90ALF). The 50ALF diet was also fed during the COV period. Diets were formulated to provide similar metabolizable energy and protein although MP supply increased by 5 g/kg of DMI between 10ALF and 90ALF when dietary ingredient samples were analyzed following the study. Data were analyzed as a RCBD with cows blocked based on parity, milk yield, and DIM. Data from the fourth wk were subjected to analysis of covariance using MIXED procedure of SAS (v 9.4; SAS Institute Inc., Cary, NC). Model included COV and fixed effects of treatment and enrollment with block as a random effect. Linear, quadratic, and cubic orthogonal contrasts were tested. Dry matter intake, ECM yield, and ECM/DMI were not affected by diet. However, increases in milk true protein yield and de novo fatty acid concentration were greatest, whereas MUN concentration was least, for cows fed the 30ALF diet. Daily rumination time decreased linearly as alfalfa replaced corn silage. An optimal combination was approximately 30:70 alfalfa-to-corn silage in 62% forage diets although the appropriate mix of alfalfa and corn silage for a specific farm will be based on nutritional and non-nutritional considerations.