The objective of this study was to compare responses of 59 lactating Holstein cows to molasses (M) vs. dry corn grain (CG) at 3 levels of M (0, 5.25, 10.5% of dry matter; DM) and 2 levels of ruminally degradable protein (+RDP or -RDP) in a randomized complete block design with a 3 × 2 factorial arrangement of treatments. Cows were individually fed a common diet during a 2 wk covariate period followed by 8 wk on experimental diets. Diets were formulated to be isonitrogenous and provide similar amounts of starch+water-soluble carbohydrates (ST+WSC). All experimental diets contained 35% corn silage, 20% alfalfa silage and 16.6% crude protein (DM basis); the 0, 5.25, and 10.5% M diets respectively contained, 19, 14.5, and 10% CG, 28, 25, and 22% ST, and 5.5, 8.5, and 11.5% WSC. Linear M effects were evaluated with contrasts. Significance was declared at P < 0.05 and tendencies at 0.05 < P < 0.10. Table 1 contains least squares means and standard errors of the difference (SED). DM intake, and production of milk, milk protein, and 3.5% fat- and protein-corrected milk (FPCM) declined linearly with increasing M, and there was a tendency for N efficiency (milk N/intake N; MN/IN) to do so. Differences among diets were not detected for milk fat production and FPCM/DM intake. No RDP or interaction effects were detected. On a DM basis, M and CG respectively contain 17.0 and 1.6% ash, approximately 60.4 and 80.4% ST+WSC as free monosaccharides, and 0 and 10% NDF. That milk production efficiency did not differ between M and CG suggests that DM intake was a primary driver of performance. The similar efficiencies observed would not have been predicted based on M and CG composition but may relate to differences in fermentation rates, digestion products, or sites of digestion. Table 1.

Diet	+RDP				−RDP			SED
0M	5.25M	10.5M	0M	5.25M	10.5M
DMI, kg	29.0	27.9	26.7		28.8	27.8	28.0	1.09
Milk, kg	47.7	44.8	42.3		48.3	44.7	44.2	1.81
Fat, kg	1.59	1.68	1.65		1.69	1.57	1.57	0.107
Protein, kg	1.51	1.38	1.36		1.49	1.37	1.32	0.054
FPCM, kg	46.5	45.8	44.7		47.9	44.3	43.8	2.07
FPCM/DMI	1.62	1.65	1.68		1.67	1.61	1.59	0.080
MN/IN	0.310	0.292	0.302		0.308	0.294	0.283	0.013