Management practices and dietary physically effective fiber are related to bulk tank milk de novo fatty acid concentration on Holstein dairy farms

This study investigated the relationship of management and diet with de novo fatty acid (FA) concentration in bulk tank milk from commercial Holstein dairy farms. De novo FA are synthesized primarily from rumen fermentation products acetate and butyrate. It was hypothesized that farms with higher de novo FA concentrations would prioritize management that optimizes rumen conditions and cow comfort. Farms (n = 39) located in Vermont and northern New York were selected based on high (HDN; 24.5 ± 0.8 g/100 g FA; mean ± SD) or low (LDN; 22.9 ± 0.9 g/100 g FA) bulk tank de novo FA from the previous 6 mo. Milk FA were analyzed using mid infrared spectroscopy chemometric prediction models. Management was assessed during one visit per farm (February to April 2015). Total mixed ration samples were collected and analyzed for chemical composition using near infrared spectroscopy and for particle size distribution using a Penn State Particle Separator modified to include a 4-mm screen. Data were analyzed using the GLIMMIX procedure of SAS with de novo group as the fixed effect and farm as the random effect. In addition, data were categorized as above or below a defined threshold and odds ratios (OR) were calculated using a binary distribution with GLIMMIX. Milk fat (4.0 vs. 3.8%, SE < 0.1, P < 0.01), true protein (3.2 vs. 3.1%, SE < 0.1, P < 0.01), de novo FA concentration (26.0 vs. 23.8 g/100 g FA, SE = 0.2, P < 0.01) and de novo FA yield (315.6 vs. 276.2 g/d, SE = 9.5, P < 0.01) were greater for HDN than LDN farms. Milk (31.9 ± 4.1 kg/d; mean ± SD), fat (1.2 ± 0.2 kg/d), and true protein (1.0 ± 0.1 kg/d) yields and days in milk (173 ± 30 d) were not different (P > 0.25). Bunkspace (50.0 vs. 39.8 cm/cow; SE = 3.7, P = 0.06) tended to be greater for HDN than LDN freestalls. High de novo freestalls tended to be more likely to feed twice per day (OR = 5.0, 95% CL = 0.9 to 28.0, P = 0.07), have a stocking density = 1.1 cows/stall (OR = 4.7, 95% CL = 0.8 to 27.2), P = 0.08) and = 46 cm bunkspace per cow (OR = 10.1, 95% CL = 0.9 to 112.4, P = 0.06). Dry matter (42.6 ± 4.8%), crude protein (15.8 ± 1.5%), neutral detergent fiber (36.4 ± 4.0%), and starch (23.4 ± 4.5%) were not different (P > 0.20) between groups. Ether extract was lower (3.7 vs. 4.0%, SE = 0.1, P < 0.01) and physically effective neutral detergent fiber was higher (26.8 vs. 21.4%, SE = 1.1, P < 0.01) for HDN diets. Overcrowded freestalls, reduced feeding frequency, greater dietary ether extract and lower physi