Fat and starch as additive risk factors for milk fat depression in dairy diets containing corn dried distillers grains with solubles

Two experiments were conducted to evaluate the additive effects of starch and fat as risk factors associated with milk fat depression in dairy diets containing corn dried distillers grains with solubles. In experiment 1, 4 multiparous ruminally cannulated Holstein cows, averaging 114 ± 14 d in milk and 662 ± 52 kg of body weight, were randomly assigned to 4 treatments in a 4 × 4 Latin square to determine the effect of these risk factors on rumen fermentation and milk fatty acid profile. In each 21-d period, cows were assigned to 1 of 4 dietary treatments: a control diet (CON; ether extract 5.2%, starch 19%); CON with added oil (OL; ether extract 6.4%, starch 18%); CON with added starch (STR; ether extract 5.5%, starch 22%); and CON with added oil and starch (COMBO; ether extract 6.5%, starch 23%). After completion of experiment 1, milk production response was evaluated in a second experiment with a similar approach to diet formulation. Twenty Holstein cows, 12 primiparous and 8 multiparous, averaging 117 ± 17 d in milk and 641 ± 82 kg, were used in replicated 4 × 4 Latin squares with 21-d periods. Results from experiment 1 showed that ruminal pH was not affected by treatment averaging 5.87 ± 0.08. Molar proportion of propionate in rumen fluid was greatest on the COMBO diet, followed by OL and STR, and lowest for CON. The concentration of trans-10,cis-12 conjugated linoleic acid in milk fat increased with the COMBO diet. Adding oil, starch, or a combination of both resulted in lower concentration and yield of fatty acids <16 carbons. Compared with the control, OL and STR resulted in 13% lower concentration, whereas the COMBO diet resulted in a 27% reduction; similarly yield was reduced by 24% with the OL and STR treatments and 54% with the COMBO diet. In experiment 2, milk yield, milk protein percentage, and milk protein yield were similar across treatments, averaging 26.6 ± 1.01 kg/d, 3.2 ± 0.05%, and 0.84 ± 0.03 kg/d, respectively. Fat-corrected milk was greatest for CON, 26.5 ± 1.12 kg/d; no differences were detected among the remaining treatments, which averaged 23.5 ± 1.12 kg/d. Milk fat percentage was greatest when cows consumed CON, 3.3 ± 0.15%; OL and STR averaged 3.0 ± 0.15% and COMBO resulted in the lowest milk fat percentage, 2.73 ± 0.15%. Milk fat yield was 0.25 ± 0.05 kg/d greater for the CON diet compared with the other 3 treatments, which were similar. These results suggest that fat and starch are additive risk factors that will likely induce milk fat depression in diets containing high inclusion of dried distillers grains with solubles.