Diets containing large quantities of forages can have an effect on feed DMI, because of the amount of fiber present, the digestibility of the fiber, and the passage rate of undigested residues from the digestive tract. There are differences in the digestibility and rate of digestion for different forage species. However intake is considered more important than digestibility for influencing digestible DMI from forages. When both grasses and legumes were considered, relative contributions to intake of digestible DM were 70% for DMI and 30% for digestibility (4).

Mertens (8) suggests that the maximum NDF intake (NDFI) that will not reduce milk yield below the cow's potential is 1.2% of body weight (BW). However, the potential for distension of the rumen may allow for greater content of bulky feeds. Ruminal pool size of NDF for Coastal bermudagrass was projected to be 1.38% of BW compared with 1.1% of BW for alfalfa (9). If this is true, then one would expect that cows may be able to consume a greater percentage of NDF with this grass and maintain intake. Work in Georgia supports this, for Holsteins fed diets containing 15 and 30% Tifton 85 bermudagrass (NDF percentage of TMR of 39.5 and 46.6%) consumed as much feed DM as those fed diets containing similar amounts of alfalfa but with lower TMR NDF content (NDF percentage of TMR of 33 to 35%) (18).

Florida workers (12) compared four ensiled forages (dwarf elephant grass, forage sorghum, bermudagrass, and corn silage) in diets formulated for 31, 35, and 39% NDF. Not surprisingly, DMI declined with increasing dietary NDF content, while NDFI increased from 1.15 to 1.32% of BW with increasing NDF. Intake of dwarf elephantgrass and corn silage was greater than intake for bermudagrass and forage sorghum, which was consistent with the greater extent of in situ digestion of DM, NDF, and cellulose for the elephantgrass and corn silage. Milk yield was least for the forage sorghum, which had the lowest fiber quality by analysis. The authors concluded that a measure of fiber digestibility would help to explain differences in fiber quality among dietary sources, that a measure of undigested NDF intake could describe the indigestible and slowly digestible portion of the diet that occupies space in the digestive tract, and a measure of digestibility by analysis could help to describe the potential energy content of the forage, and ultimately, the diet. Differences among species in the rate of digestion and passage of fiber suggest that analyses beyond the traditional chemical analyses are necessary to describe the forage and its energy content and intake potential.

Voluntary intake of cell-wall (fiber) components and cellulose has a highly negative relationship for several grasses, while there was no relationship for these components in alfalfa (14). So despite rations balanced for similar NDF content, those based on grasses usually have lower DMI than those based on legumes.

As plants mature, crude protein decreases, fiber increases, and digestibility and energy content decline. This is true for alfalfa hay (Table 4) and sorghum (Table 5). Sorghum does not demonstrate fiber changes as large as those of alfalfa, but the digestibility of sorghum declines dramatically (Table 5).

 As forages mature, there is generally a point at which the accumulation of digestible DM declines despite increasing forage DM yield. At this point the general effect is both a decline in energy content of the forage as well as reduced intake. Intake of DM declines with increasing ADF and NDF content of the forage, and digestibility declines with increasing lignin content of the forage (15). These responses are relatively well known, and the obvious means to minimize the effects of maturity is to harvest at optimum maturity. However another common approach is to reduce the content of low quality forages and substitute another feedstuff (such as grain) of greater energy density; the rationale being that high energy feeds offset the lower energy content of the forage. However when alfalfa was harvested at pre, early, mid, and full bloom and fed at 80, 63, 46, and 29% of diet DM, with the remainder from concentrates, the most milk was produced for diets containing the pre-bloom alfalfa (Table 6, 6). Cows fed diets containing 80% of pre-bloom alfalfa actually produced more milk than cows fed diets of only 29% of full bloom alfalfa and 71% concentrates. High energy concentrates could not overcome the intake depressing effects of mature alfalfa coupled with the low energy content.

Adapted from Kawas et al. (6).

Physical form of forages affects digestion because of effects of particle size on outflow from the rumen. A rapid outflow of solids from the rumen has beneficial and negative effects. Rapid removal from the rumen means more space is available for additional feed intake, but beneficial rumination and chewing (which buffers the rumen) is reduced. Simulations revealed that pelleting alfalfa or bermudagrass reduced ruminal NDF digestion and total DM digestion in steers (9). However ruminal turnover time was more rapid so that intakes of DM and digestible DM were sharply increased over long hay.

Despite results which show greater DMI with chopping or pelleting of forages, one must also consider that these studies were often conducted with the forages in question as the sole feed source. Recent studies with dairy cows showed little increase in DMI when forages were chopped, perhaps because of their use in TMR and because of the large amount of concentrates fed. Chopped alfalfa hay ranging from .26 to .9 cm did not affect DMI but milk fat depression was prevented when mean particle length exceeded .25 inches in length (20). In a Canadian study, cows were offered diets with low (35%) or adequate (65%) forage content with two chop lengths of alfalfa silage, .5 and 1.0 cm (2). In the low forage diets the increase in chop length did not affect DMI but did increase FCM, while in the adequate forage diets longer chop length tended to decrease DMI. Interestingly, addition of hay to these diets increased DMI, but the addition of hay to the low forage diets was less effective than increasing particle length of the silage. This suggests that greater particle length of all forage sources is more effective toward maintaining DMI, but the addition of some hay helps to offset the negative effects of low fiber diets. It has been recommended that 15 to 20% of forage particles should be greater than 3.3 cm long to maintain normal rumen function (13). For dairy rations, attention to adequate particle length is of greater concern than fine particle size to stimulate intake. In addition, much of the particle length work has been done with alfalfa. Less is known about the effects of particle length from grasses and the effectiveness in dairy rations.

The use of NDF within forages is an indicator of forage quality and quality is closely linked to cow performance. However great variation exists between forage types and must be considered. Generally grasses are more fibrous than legumes and result in lower DMI.

The minimum NRC (10) recommendations for ADF and NDF are probably too conservative for many diets. However, this is very dependent on fiber source and physical form, which must be considered.

Forage particle size greatly influences the effectiveness of fiber. Allen (1) recommends no adjustment from 30% dietary NDF for silage with 5-10% of particles more than 3.8 cm; a decrease in NDF of 2 units when more than 15% of particles exceed 3.8 cm, and an increase in NDF of 2 units when few long particles exist.

There is a wide variation in the chemical composition, physical form, digestion rate, and rate of passage of NDF among feeds, making it very unlikely that any single value for NDF will apply in all feeding situations. In addition the interrelationships of fiber with other carbohydrate components (especially nonstructural carbohydrates and starch) are critical to the performance of the animal, and influence effectiveness of the fiber present in the diet. The overall balance of the diet is the determining factor for animal performance. Forage continues to provide the base for dairy rations, and forage quality, based on fiber and energy content, maturity, digestibility, and species differences which affect utilization, must be considered for optimal use of the forage.