Fiber in the diet for cattle is a necessary component for rumen health and has been measured using methods such as crude fiber, neutral detergent fiber (NDF), and acid detergent fiber (ADF). One measure of fiber associated with rumen fill and rumination is undigested neutral detergent fiber at 240 hours (uNDF240om). There is limited information on the amount and source of uNDF240om in lactating dairy diets and its effect on performance and chewing behaviors. Last fall at Miner Institute we tried to answer those questions. We formulated three brown midrib (BMR) corn silage-based diets for similar predicted uNDF240om intake (5.11 lb/day and 0.33 % of BW) with varying lengths of straw and source of uNDF240om. The first two diets were identical except the length of the straw (0.82 vs 0.66 physical effectiveness factor; pef) and will be referred to as the long straw and short straw diets (Table 1). The third diet was comprised primarily of BMR corn silage with 1.2 lbs of long straw and will be referred to as the BMR diet (Table 1).

We used 56 Holstein cows averaging 102 (SD = 28) DIM and measured feed intake, milk yield and composition, and body weight weekly for 2-wk covariate and 4-wk treatment periods. Chewing behaviors were also measured the last week of the covariate and treatment periods with a 10 minute scan for 72 hours. Cows fed the long straw diet consumed more dry matter than those on the BMR diet, but DMI as percent of body weight was not different. The milk yield and solids corrected milk did not differ between treatments.

Cows fed the BMR and long straw diets had higher milk denovo fatty acids (g/100g FA) than the cows fed the short straw diet. The milk preformed fatty acids were higher in the cows fed the short straw than the cows fed the BMR diet. Milk de novo fatty acids are made from the end products of rumen fermentation in the cow’s udder and preformed fatty acids come from fat in the feed and body fat mobilization (Heather Dann and Rick Grant, January Farm Report). Even though feed intake and milk yield were not different, the milk fatty acid profile suggests the cows fed the long straw and the BMR diets were able to use more end products from the rumen than fat from the feed and body fat mobilization compared to cows fed the short straw diet.

The cows fed the long straw and BMR diets had longer eating (233 and 233 vs 216 min/d) and rumination (504 and 516 vs 495 min/d) times than the cows fed the short straw diet. The cows fed the BMR diet had shorter meals lengths but increased meal bouts per day compared to the cows fed the long and short straw diets. As expected, the eating and rumination time would be longer for the cows fed the long straw diet, but it was interesting that the cows fed the BMR diet had similar eating and rumination times most likely due to the physical effective NDF (peNDF) of 21.9 of the diet. Cows fed the BMR diet had increased meals with shorter duration than the cows fed the long or short straw diets. This should create a more stable rumen environment which was displayed with the milk fatty acid profiles, but need rumen parameters to verify this assumption.

These results suggest that when feeding a moderate level of uNDF240om the length of the straw is important for rumen health. The long and short straw diets had the same uNDF240om intake (0.33 vs 0.32 % of BW) and very similar peNDF of 18.5% and 18.4%, which are well below the standard of 21% for effective fiber in the diet. So the longer straw diet created a healthier rumen environment. The source of uNDF240om does seem to be important, but more research is needed to fully understand it. More trials are being planned and conducted at the Institute to comprehend the role of uNDF240om in lactating dairy cow diets.