Rations for high-producing cows must be carefully balanced to provide enough non-fiber carbohydrates (NFC) which contain readily available energy sources for rumen microbes. However, fibrous carbohydrates are also important for rumen health. Because rate of passage through the rumen is faster for higher producing cows, there is less opportunity to degrade feedstuffs in the rumen. In addition, as cows are bred to produce higher levels of milk, providing highly digestible forages becomes extremely important.
    Forage quality and digestibility are affected by several factors which impact animal performance:

• NDF content and quality--Higher percentage neutral detergent fiber (NDF) in the ration can adversely affect milk production by decreasing intake and digestibility.
   

• Lignin content--Lignin is indigestible and may be a good predictor of forage digestibility.
   

• Rumen fermentation--The ration must provide adequate nutrients available in the rumen for microbes to degrade fiber effectively.
   

• Forage fermentation--Haylage and silage fermented under less than ideal conditions (improper moisture content or temperature) can cause depressed intakes and digestibility.
   

• Forage genetics--Physical characteristics, such as kernel texture of corn and lignin content, will affect forage digestibility.

NDF Digestibility
Neutral detergent fiber defines the fiber content of a feedstuff and is directly related to rumen fill and inversely related to intake. In addition, NDF and NFC are inversely related to each other. NFC represents total dry matter minus the slowly degraded carbohydrates and the non-carbohydrates (protein, ash, and fat). When ruminants ingest NDF, they slightly chew it before swallowing. The fiber then enters the rumen for more degradation. From the rumen, it is regurgitated and re-chewed, which furthers the digestive process. The amount of chewing is dependent on particle size and effective fiber.
 
    Forage quality has a major impact on milk production regardless of the grain mix that is fed with the forage. Lower NDF levels in forages indicate dry matter intake will be higher and digestibility will be improved. NDF consists of cellulose, hemicellulose, and lignin plus a portion of pectin, which is a non-structural carbohydrate. Lignin is indigestible and as its content in forages increases, the forage becomes less digestible. Less digestible forages provide less energy in the ration and, therefore, less energy available for ruminal protein synthesis. Less digestible forages will increase rumen fill. Excessive NDF ration levels will cause a decrease in dry matter intake with a consequential drop in milk production.
    Intake of a low NDF ration will cause inadequate rumen fermentation resulting in a drop in fat test. Also, cow health can be adversely affected. Recommended levels of NDF for high-producing (>90 lb) cows should ideally fall between 25% and 28% of the ration dry matter. Cows producing less milk will require an NDF level of 27% to 33% of ration dry matter. In high-producing cows, as percent forage NDF increases, less of the forage can be formulated into the ration which enables intake and energy to be maximized. Table 1 shows expected milk production based on different forage NDF levels.

    Grass hay and haylage contain higher levels of fiber compared to alfalfa hay and haylage. Therefore, in areas where less alfalfa is available for forage, high ration NDF levels are common. Grasses tend to be digested at a slower rate than legumes and tend to have a longer lag time before digestion begins. Table 2 shows that, in general, legumes and corn silage are more completely digested compared to grasses.

    The most important effect of harvest on digestibility is the stage of maturity when the forage is cut. Table 3 shows rate of NDF digestion and indigestible NDF for alfalfa, bromegrass, and orchardgrass at different maturities. Lignin proportion of NDF increases, leading to a decrease in digestible NDF. It has been suggested that one unit increase in NDF digestibility is equivalent to an increase of 0.38 lb/day of dry matter intake and an additional 0.56 lb/day of 3.5% FCM.

    As corn silage increases in maturity, it becomes drier. Fermentation is adversely affected when corn silage is dry (>40% DM), which consequently lowers digestibility. Starch digestibility ranges between 50% and 90%, and decreases as kernel dry matter increases. In addition, kernel texture, which is affected by genetics, also has an effect on digestibility.

Microbial Fermentation
Digestibility of forages also depends to some extent on the grain portion of the ration. Since forages are digested by microbes in the rumen, digestibility is improved when nitrogen and carbon sources are optimum for rumen microbes. Therefore, degradable and soluble protein, as well as fermentable carbohydrates, fed to the cow will affect the extent of forage digestibility. Soluble protein from several sources, which could include urea, or haylage provide a source of nitrogen for microbes while fermentable carbohydrates provide energy for microbes. In addition, various sources of protein that are degraded at different rates available to the cow throughout the day will provide nitrogen to microbes almost continuously.

    Several studies have shown the benefit of feeding cobalt glucoheptanate to improve forage digestibility. Cobalt glucoheptanate is available to microbes in the rumen which can lead to improved microbial efficiency. In addition, silage and haylage treated with a silage inoculant containingLactobacillus plantarum MTD/1 yield higher quality ensiled forage. Better forage value can lead to better nutritional value which impacts dry matter intake and milk production.

Genetics
The physical characteristics of forage have an affect on microbial attachment and degradation of forage. Research on the effects of genetics on forage digestion has increased recently. Brown mid-rib corn (BMR) is one new variety corn silage that has recently received considerable attention. Research has shown this variety of corn silage corn is highly digestible. In fact, the digestibility of BMR corn silage was 10% higher and NDF digestibility was 20% higher compared to traditional varieties of corn silage. Actual fiber content from lab analyzes may be the same with BMR corn silage and traditional corn silage, but digestibility of BMR has been shown to be higher. In research studies, when rations were balanced to compensate for the higher NDF digestibility, lactating cow performance improved.

Predicting Forage Digestibility
The indigestibility of lignin makes it an important indicator of NDF digestibility for specific forages. However, several factors, including cutting and environmental conditions, affect the association between lignin and NDF digestibility. In general, high-quality alfalfa and corn silage contain less than 16% and 6% lignin, respectively, as a percentage of NDF. To help predict forage digestibility, lignin and NDF must be analyzed by chemical methods.
 
    Relative feed value (RFV) is a calculation based on ADF (acid detergent fiber) and NDF (see Table 4). Because ADF and NDF indicate the amount of fiber in the forage (but not the amount of digestible or indigestible fiber), it can help to evaluate forage digestibility. Relative feed value may be an indicator of forage digestibility that is easy to obtain and evaluate. A value of 130% to 150% is ideal for high-producing dairy cows and would be equivalent to early or pre-bloom alfalfa haylage.

Conclusion
High-producing dairy cows in early lactation could greatly benefit from forages that are more digestible because these cows require more energy and fermentable carbohydrates, especially when fats are already at maximum levels in the ration. As plant genetic research continues to improve NDF digestibility of forages, nutritionists can more closely predict the digestibility of forages using lignin and total NDF.