Recent research with lactating cows suggest that NFFS not only reduce the amount of starch fermented in the rumen when substituted for grain but also reduce the ruminal digestibility of the remaining starch. Illinois researchers substituted soyhulls for dry ground corn at 0, 10, 20, 30, and 40% of dietary DM and reported that ruminal digestibility of non-structural carbohydrates decreased from nearly 30% to less than 5% without reducing ruminal or total tract digestibility of organic matter. We recently reported similar effects on ruminal starch digestibility when pelleted beet pulp was substituted for high moisture corn at 0, 6, 12 and 24% of dietary DM. The amount of starch truly digested in the rumen decreased dramatically from 8.4 to 1.5 lb/d, partly because of the expected reduction in the starch intake from lower dietary starch concentration but also from an unexpected reduction in true ruminal starch digestibility from 47% to 17% as beet pulp replaced high moisture corn in diets. There was no effect of treatment on total tract starch digestibility despite this large reduction in ruminal starch digestion.
In addition, the digestibility of organic matter in the rumen was not affected by treatment because of increased NDF digestibility; increased ruminal NDF digestibility and compensatory intestinal starch digestibility of organic matter as beet pulp replaced corn in the diet. The dramatic reduction in ruminal starch digestion decreased from 11.3 to 1.7%/h and rate of starch passage from the rumen increased from 15.9% to 23.5%/h. It is not known if other NFFS have the same effects on rate of digestion and passage from the rumen, but they also might either increase or decrease ruminal starch digestion.
It might not be desirable to reduce ruminal starch digestibility to the extent observed with the highest substitution rates of beet pulp or soyhulls, but the linear responses obtained in these two experiments suggest that NFFS can be used at lower substitution rates to manipulate ruminal starch fermentability and site of starch digestion. Therefore, substitution of NFFS for both forage and grain might be necessary to limit diet fermentability as the forage content of the diet decreases.
A limitation of NFFS as a forage alternative is that particle length is not adequate for formation of the rumen mat. In addition, passage rate is generally greater for NFFS NDF than for forage NDF, and ruminal digestibility can decrease if poor mat development reduces NDF retention time in the rumen. If adequate long particles are provided by coarse forage in the diet, NFFS can contribute to mat formation because they tend to be buoyant and become entrapped. Another limitation of NFFS is that, compared with cereal grains, NFFS provide fewer metabolic precursors of glucose, which can be limiting for lactose synthesis and therefore for milk production, particularly for high producing cows.