Within-day Alteration of Ration Starch Fermentability had no Effect on Feed Intake, Total-tract Neutral Detergent Fiber Digestibility, and Milk Fat Concentration of Cows in Late Lactation
The objective of this experiment was to evaluate the effects of feeding lactating cows diets differing in starch fermentability twice per day on dry matter intake (DMI), total-tract neutral detergent fiber (NDF) digestibility, and milk fat concentration. Feeding highly fermentable starch sources to ruminants is expected to increase short-chain fatty acid production by rumen microbes, increase the flux of propionate to the liver during meals, and potentially decrease ruminal pH, NDF digestibility, and milk fat concentration. Diurnal variation in feeding results in lower digesta mass and rumen buffering, and increased plasma NEFA and hepatic acetyl CoA concentrations before the morning feeding. We previously reported that propionic acid is more hypophagic when hepatic acetyl CoA concentration is elevated. Therefore, we hypothesized that the negative effects of greater starch fermentability would be more pronounced following the morning feeding compared with the evening feeding. Sixteen multiparous cows (291 ± 68 DIM; mean ± SD) were used in a crossover design experiment with 14 d periods including 10 d for diet adjustment and 4 d for sample collection. Cows were offered diets containing either dry corn grain (DC, less fermentable) or high (33%) moisture corn grain (HM, more fermentable) at 0900h and 1700h each day in opposite sequences 1) HM at 0900h, DC at 1700h and 2) DC at 0900h, HM at 1700h. Sequence DC:HM tended to increase the amount of DMI following the morning feeding (10.6 vs. 9.7 kg, P = 0.07) and decrease DMI following the afternoon feeding (14.4 vs. 15.4, P = 0.08) compared with HM:DC, resulting in no overall effect of treatment (P > 0.15). Treatment did not affect digestibility of NDF or DM, or yields of milk, fat, protein, lactose, or milk composition. Sequence DC:HM tended to decrease BW (776 vs 771 kg, P < 0.10), but not BCS compared with HM:DC. Lack of treatment effects on digestibility of NDF and concentration of milk fat indicate that the buffering capacity of rumen contents was likely adequate to maintain ruminal pH during the morning when digesta mass is normally lowest. Opposite effects of treatment on DMI following the morning and afternoon feedings suggest that HM tended to decrease DMI compared with DC similarly at each feeding. These results indicate that potential advantages to altering ruminal starch fermentability within a day are minimal for late lactation cows.
Keywords: starch fermentability, feeding management, diurnal variation