There are additional limits to the range in forage content of diets that are independent of the relative prices of ingredients. Cows consuming diets with inadequate forage content are more likely to have ruminal acidosis, which might decrease fiber digestibility, microbial protein production, and milk fat content, as well as increase incidence of health problems, such as laminitis and displaced abomasums. This is because forages are higher in fiber than most other feed ingredients and fiber helps prevent ruminal acidosis.

Fiber generally ferments less rapidly than starch and sugars, so high-fiber diets result in a less severe drop in ruminal pH after meals. In addition, long-forage fiber stimulates chewing and secretion of salivary buffers which neutralize fermentation acids in the rumen. Increasing the forage content of the diet provides a more consistent fermentation and increases stability of rumen pH. However, cows consuming high-forage diets are more likely to have dry matter intake limited by physical capacity of the rumen.

Fiber is more filling than other feed components because it is more slowly fermented and has a longer retention time in the rumen. High-forage diets are more likely to limit milk production, increase loss or decrease gain of body condition, increase incidence of ketosis, and decrease reproductive performance. When groups of cows are fed a total mixed ration (TMR), feed intake of some cows in the group will be limited by physical fill more than others. The most profitable feeding strategy is one that considers both costs of feed ingredients and animal performance.

Fiber digestibility of forages is variable and has a large effect on animal performance. More digestible fiber is less filling because it is retained in the rumen for a shorter period of time. Because it is less filling, diets containing highly digestible fiber allow greater dry matter intake for animals with intake limited by physical fill. The objective of this article is to answer some common questions about fiber digestibility of forages and strategies to maximize benefits of forages with high fiber digestibility.

and microbial activity. Neutral detergent fiber digestibility of forages is best compared using in vitro rumen fermentation. In vitro rumen fermentation is conducted by incubating forages that have been dried and ground with ruminal microbes for a specific period of time (~30 hours for dairy cows). In vitro fermentation eliminates many factors that affect NDF digestibility in experiments with animals and therefore allows a more fair comparison of feeds. In vitro NDF digestibility varies from 25 to 60% for both alfalfa and corn silage grown in different environments. Any way you look at it, NDF digestibility is extremely variable.

yield from forage with enhanced NDF digestibility was reported by Nebraska researchers who compared normal sorghum silage and a brown midrib (BMR) sorghum silage fed to mid-lactation dairy cows. The BMR sorghum silage had higher NDF digestibility in vitro and slightly higher NDF content than the normal sorghum silage. The silages were included in total mixed rations at 65% of DM (35% concentrates). The cows consuming the BMR sorghum silage with higher NDF digestibility consumed 24% more (~11 lb per day) feed and produced 46% more (~18 lb per day) milk. Although these were not high producing cows (average milk yield was 50 lb per day), physical fill probably limited feed intake because they were fed high-forage (65%), high-NDF (>40%) diets.

We recently compared a low lignin corn hybrid containing the brown midrib 3 (bm3) mutation to a normal hybrid using cows with high milk yield. The

corn silages were included in total mixed rations formulated to contain 56% forage with 80% of the forage as corn silage and 20% of the forage as alfalfa silage. In vitro NDF digestibility averaged 49% for the bm3 corn silage and 39% for the normal corn silage. When the cows were offered the bm3 corn silage, they ate 4.4 lb more feed per day, produced 5.5 lb more milk per day, and gained more body condition than when they were offered the normal corn silage. Milk yield before the experiment of the 32 cows in our study ranged from 65 to 120 lb per day. Cows producing 120 lb of milk per day had an average improvement of over 15 lb per day from the bm3 corn silage, but those producing less than 75 lb of milk per day had little or no improvement in milk yield. This implies that the highest producing cows were most limited by physical fill and benefited most from the lower filling effects of the highly digestible NDF in the bm3 corn silage. Furthermore, feed intake of the lower producing cows was not limited by physical fill with this diet.

In another experiment, we compared bm3 corn silage to normal corn silage in diets formulated to provide 29% or 38% NDF. The low fiber diets averaged 42% forage and the high-fiber diets averaged 66% forage. The bm3 corn silage increased feed intake and milk production at each level of forage in the diet and, as expected, the higher NDF digestibility corn silage was more beneficial when offered in the high-forage diet (see Table 1).

Cows offered bm3 corn silage in a high NDF diet produced as much milk as when they were offered the bm3 corn silage in a low NDF diet. Although energy intake was higher when consuming the low NDF diet, a substantial amount of energy was partitioned to body condition. However, when cows were offered the normal corn silage in a high NDF diet, they produced 4.6 lb less milk than when they were offered the same silage in a low NDF diet. This observation has important practical implications. When cows consuming TMRs are switched from a low forage diet to a higher forage diet to prevent excessive body condition gain, forages with high NDF digestibility might allow the cows to maintain milk yield. Because of this, forages with high NDF digestibility might also benefit cows in mid- to late-lactation when they are fed higher forage diets to prevent excessive body condition gain.

We also evaluated the effect of enhanced NDF digestibility across a wider range of forages using data reported in the literature and found that enhanced NDF digestibility of forage increased dry matter intake and milk yield. One unit increase of NDF digestibility was associated with 0.37 lb increase in dry matter intake and 0.51 lb increase in milk yield. This analysis demonstrates that enhanced NDF digestibility is an important factor affecting feed intake and milk yield over a wide range of conditions.