Reduced-fat dried distillers grains with solubles reduces the risk for milk fat depression and supports milk production and ruminal fermentation in dairy cows

Twenty Holstein cows, 12 primiparous and 8 multiparous, with (mean ± SD) 91 ± 19 d in milk and 595 ± 81 kg were used in replicated 4 × 4 Latin squares to compare the effects of feeding conventional dried distillers grains with solubles (DDGS) and reduced-fat DDGS (RFDDGS) in combination with rumen-inert fat (RIF) on milk production and rumen fermentation; one square contained rumen cannulated animals for rumen measurements. In each 21-d period, cows were randomly assigned to 1 of 4 dietary treatments (values on a dry matter basis): (1) control (CON) that contained 0% DDGS; (2) DG contained 30% DDGS; (3) RFDG contained 30% RFDDGS in substitution of DDGS; and (4) RFDG+RIF was similar to RFDG with the addition of 1.9% RIF. Unlike most practical diets in the dairy field, our diets had <22% forage neutral detergent fiber and >18.0% crude protein. Dry matter intake was similar across treatments with any form of DDGS averaging 26.0 ± 0.6 kg/d, whereas the CON diet resulted in less dry matter intake, 21.6 ± 0.6 kg/d. Milk yield tended to be 1.7 kg/d greater for diets with either type of DDGS. Concentration of milk protein was greatest for the DG and RFDG diets, intermediate for the RFDG+RIF diet, and least for the CON diet, namely 3.22, 3.21, 3.12, and 3.07 ± 0.05%. Reduced milk fat percentage and yield were observed when cows consumed the DG diet, 3.27 ± 0.10% and 1.11 ± 0.04 kg/d, respectively, whereas these responses were similar among CON, RFDG, and RFDG+RIF, which averaged 3.68 ± 0.10% and 1.22 ± 0.04 kg/d. The presence of trans-10,cis-12 conjugated linoleic acid was only detected in milk from cows consuming the DG diet; similarly, concentration and yield of trans-10 18:1 were greater for cows consuming this diet. Rumen ammonia was similar across treatments averaging 27.0 ± 2.1 mg/dL. The CON and RFDG+RIF diets had similar mean pH, 6.1 ± 0.11, whereas DG and RFDG resulted in lower pH averaging 5.79 ± 0.11. No effect on total concentration of volatile fatty acids was observed; the overall mean was 121 ± 4.11 mM; molar proportion of acetate was affected by treatment resulting in 67.3, 63.2, 61.4, and 60.9 ± 0.93 mol/100 mol for CON, RFDG+RIF, RFDG, and DG, respectively. Results from DNA sequencing showed that rumen bacterial community structure was relatively stable with minor changes at the family and genus levels; these changes may be associated with low starch diets, and hence reduced amylolytic bacteria populations. Feeding high proportions of RFDDGS resulted in greater dry matter intake with low risk for milk fat depression while supporting ruminal fermentation.