Fall-grown oat forage (OF) has potential to offer a second crop to augment forage supplies. Our objective was to evaluate lactating dairy cow performance when OF was substituted for corn silage (CS). Lactating Holstein cows (47) were randomly assigned to diets in a randomized complete block design with a 2-wk covariate period in which cows consumed a common diet, followed by an 8-wk period in which 3 experimental diets were fed. Two cows were removed for stealing of feed. Measurements were made in the last week of each period. To evaluate total-tract neutral detergent fiber (NDF) digestibility (TTNDFD), 6 fecal samples were collected from each cow over 3 d to represent every 4 h in a 24-h period; indigestible NDF was used as the internal marker. Experimental diets contained 20% alfalfa silage, 35, 27, or 19% brown midrib CS, and 0, 8, or 16% OF on a dry matter (DM) basis; dietary canola meal was reduced and high moisture corn increased with increasing OF. Diets were formulated to have similar concentrations of crude protein (CP) and NDF. OF and CS respectively contained 12.5 and 7.1% CP, 0.2 and 34.1% starch, 48.7 and 36.4% NDF, and 10.3 and 1.6% water-soluble carbohydrates on a DM basis. Significance was declared at P < 0.05. DM intake and eating time did not differ among diets. Fat- and protein-corrected milk (−0.27 kg/1% diet OF) and milk urea nitrogen declined linearly with increasing OF. TTNDFD and rumination time showed quadratic responses. Oat forage can support high production, however lactation performance was less than with corn silage. Alternate diet formulations with OF should be explored. Table 1.

Measure1	0% OF2	8% OF	16% OF	SED3	P-value
DMI, kg	29.7	29.4	29.0	0.54	0.44
3.5% FPCM, kg	50.5	48.5	46.2	1.44	0.02
Milk urea N, mg/dl	12.6	12.5	11.9	0.26	0.02
TTNDFD, % of NDF	56.3	54.9	58.0	0.81	<0.01
Eating, min/d	202	204	207	16.0	0.95
Rumination, min/d	441	485	467	18.2	0.06

¹FPCM = fat- and protein-corrected milk; TTNDFD = total-tract NDF digestibility. ²OF = oat forage. ³SED = standard error of the difference.