Short communication: Substituting dry distillers grains with solubles and rumen-protected amino acids for soybean meal in late-lactation cows' diets based on corn silage or ryegrass silage

Excess protein in dairy cattle diets increases production costs and contributes to environmental pollution. The objective of the present study was to evaluate the effect of feeding dry distillers grains with solubles (DDGS) supplemented with rumen-protected Lys and Met in place of solvent-extracted soybean meal on the performance of late-lactation cows. Two experiments were carried out, with each using 24 late-lactating dairy cows distributed among 4 pens. In trial 1, corn silage was the main forage source. Control (HP1) total mixed ration (TMR) contained 16.3% crude protein (CP) with soybean meal as the main protein source. Treatment TMR (LP1) had 13.7% CP when soybean meal was replaced with DDGS and rumen-protected Lys and Met. Forage in trial 2 was ryegrass silage; control TMR (HP2; 15.4% CP) contained soybean meal and rumen-protected Met, whereas treatment TMR (LP2; 13.8% CP) contained DDGS and rumen-protected Lys and Met. Trials were analyzed as crossover design using the MIXED procedure of SAS (SAS Institute Inc., Cary NC) with cow as sampling unit and pen as the experimental unit. Treatments were similar in dry matter intake (21.0 and 20.4 kg/cow per day for HP1 and LP1, respectively) and milk yield (20.7 and 20.5 kg/cow per day for HP1 and LP1, respectively) during trial 1. Milk composition was similar between treatments, averaging 4.22, 3.73, 4.54, and 9.15, respectively, for fat, protein, lactose, and solids nonfat. Milk urea nitrogen decreased from 17.2 mg/dL for HP1 to 9.93 mg/dL for LP1. In trial 2, no significant differences were observed for dry matter intake (21.4 and 20.9 kg/cow per day for HP2 and LP2, respectively), milk yield (28.1 and 26.6 kg/d for HP2 and LP2, respectively), fat yield (0.99 vs. 0.92 kg/d for HP2 and LP2, respectively), protein yield (0.94 vs. 0.86 kg/d for HP2 and LP2, respectively) and lactose yield (1.37 vs. 1.28 for HP2 and LP2, respectively). Milk urea nitrogen decreased from 9.88 mg/dL with HP2 to 6.39 mg/dL with the LP2 treatment. Milk N efficiency tended to be higher for LP treatments in trial 1, but not in trial 2. Low milk urea N suggested nitrogen losses to the environment may be lower when cows were fed diets based on DDGS in both trials. The studies indicated that DDGS with rumen-protected Lys and Met could substitute solvent-extracted soybean meal in low-protein corn silage- and ryegrass silage-based diets for late-lactation dairy cows averaging 20.6 or 27.4 kg of milk/d, respectively.