Articles

Digestible NDF – Important for Dairy Cows!
Mark G. Cameron, PhD, Dairy Nutritionist, Cargill Animal Nutrition

With the high feed/ingredient costs in Western Canada and the lower quality alfalfa hay/haylage this year, I have noticed a trend towards higher starch lactating cow diets.

Producers’ are trying to lower their lactating diet costs/cow/day by feeding lactating diets containing more local grain sources such as barley, wheat and/or oats. From an economic perspective this strategy makes sense but we need to remember that any nutritional change still needs to focus on optimizing rumen health and microbial efficiency – providing the correct level of digestible NDF is one component of a properly balanced lactating cow diet.

Dairy cows require fiber for two reasons. There is a physical and a chemical requirement for fiber in the rumen. Slowly degradable fiber particles are retained in the rumen long enough to form a mat that traps other feed particles and acts as a physical buffer. Fiber is also needed for its slow release of rumen-available energy; this is the chemical requirement. The rumen is most efficient if rumen microbes have a steady source of energy. If energy sources to rumen microbes are not steadily supplied, the rumen becomes inefficient because microbes need time to get started (lag time). Having a slowly available source of rumen energy to fill in the gaps where fast sources of energy may be missing during the day keeps rumen microorganisms in a steady supply of energy. Digestible Neutral Detergent Fiber (NDF) is an important source of slowly available energy for rumen microbes.

 Ruminants have the ability to digest significant amounts of plant cell wall. The digestive enzymes of the cow do not break down plant cell walls. Plant cell walls can only be broken apart by enzymes provided by microorganisms residing inside the cow. Rumen microbes do the majority of plant cell wall digestion. The Neutral Detergent Fiber (NDF) fraction of feed is a good estimate of the plant cell wall portion. The portion of the plant cell wall, or NDF, that can be digested by rumen microbes, is the Digestible NDF.

Neutral Detergent Fiber is made up of three chemical parts: lignin, cellulose and hemicellulose. Lignin is unable to be broken apart, even by rumen microbes. Lignin can interfere with NDF digestion by physically decreasing microbial access to the cellulose and hemicellulose. The cellulose and hemicellulose portions of NDF can be broken down further by rumen microbes into sugars. The sugars are then available to rumen microbes as energy sources.

Rumen microbes utilize the energy they get from cellulose and hemicellulose to grow and multiply. The growing rumen microbial mass becomes an important source of high quality protein as the microbes are washed out of the rumen into the lower gut of the cow. So, the estimate of NDF digested in the rumen is critical in determining the amino acids provided by the diet.

Another important reason to place a value on the NDF digested in the rumen is to estimate intake. As rumen microbes break down NDF it becomes denser and takes up less space in the rumen. The smaller, denser NDF particles leave the rumen faster than larger, lighter particles. Diets with high levels of digestible NDF will not reach the fill limit of the cow as soon as diets that have low levels of digestible fiber. Diet intake will be greater if digestible NDF is higher.

Understanding the difference between Digestible NDF (dNDF) and NDF digestibility (NDFD) is important (Table 1). Digestible NDF is used to calculate NDFD, where NDFD = dNDF ÷ NDF. So, dNDF is an actual nutrient that can be analyzed in a lab, whereas NDFD is calculated as the percentage of NDF that is dNDF. This can be confusing when comparing ingredients when you only focus on evaluating the NDF level (Table 2) – it is essential that you understand the potential NDF digestibility of each forage that you grow or purchase. If you look at barley silage and corn silage, it would appear that their NDF percentages are very similar at 42.9 and 44.7% but when you factor in the actual NDFD the barley silage can provide more kilograms of dNDF. So, 5 kg (DM basis) of barley silage and corn silage will 1.43 and 1.01 kilograms of dNDF, respectively.

Table 1. Explanation of terms.

Term

Abbreviated Term

Components measured

Units

Neutral Detergent Fiber

       NDF

Lignin+Cellulose+ Hemicellulose

  Percentage of  total feed dry matter.

Digestible Neutral Detergent Fiber

      dNDF

Portion of Cellulose
and
Hemicellulose digested.

  Percentage of total feed dry matter.

Neutral
Detergent Fiber Digestibility

     NDFD

Portion of Cellulose
and
Hemicellulose digested

  Percentage of   total NDF in feed.

 

Table 2. dNDF, NDF and NDFD in some common ingredients.

  dNDF NDF NDFD
Soybean Hulls 50.1 62.7 79.9
Alfalfa Hay 17.3 43.9 39.4
Alfalfa Haylage 14.8 36.0 41.1
Beet Pulp 26.3 39.3 66.9
Distillers Grains 25.3 33.7 75.1
Barley Silage 28.6 42.9 66.7
Corn Silage 20.1 44.7 45.0
Corn Silage-BMR 24.7 40 61.8

 

Three methods are commonly used to estimate or measure dNDF: the Weiss equation (calculation using lignin and NDF), the dNDF lab assay (24, 30 or 48 hours), and in vitro gas production assay.   Consistency is important when determining dNDF, and calculating NDFD. Because of the variety of methods used by commercial labs, results from different labs cannot be compared. It is even possible that runs within a lab can differ in their estimate of digestible NDF. At Cargill’s Innovaction Campus Lab we use an index based on several lab methods that are applied consistently. Furthermore, we use Near Infrared Radio Spectroscopy (NIRS) to minimize lab sampling errors. Ultimately, cow performance is the best measure of digestible NDF. Figure 1 shows the correlation of CAN Elk River’s estimate of diet NDFD to milk production on 19 dairy farms compared to a commercial lab. The correlation is much stronger for the Innovation Campusr lab estimate of dNDF because of our consistent use of lab methods.

Digestible NDF (dNDF) and NDFD can be used to help us better determine forage quality. Both dNDF and NDFD must be considered. Forage with a very high NDFD, but low NDF will not supply much dNDF and other sources of dNDF may be needed in the diet. Conversely, forage that is low in NDFD, but high in NDF will supply a lot of dNDF, but intake limits may prevent the cow from having enough kilograms of dNDF in the diet. In forages the primary factor that influences dNDF is plant maturity. In the early stages of growth, NDFD decreases slowly. At later stages, lignification increases, sharply decreasing NDFD. Other things that can influence NDFD are forage source and variety. For instance, grasses at early stages of maturity have higher NDFD than corn sources. Legumes are more highly lignified than grass, which negatively impacts the NDFD, but in general the NDF is lower. Specially modified varieties such as BMR corn will have increased NDFD when compared with typical corn silage varieties because of the lower lignin concentrations.

In summary, when evaluating your on-farm or purchased feed ingredients it is important that you understand the digestibility of all the nutrients – protein, starch, fat and NDF. With NDF-based feeds, you should not only be analyzing for ADF, NDF and lignin but also get the dNDF fraction measured by the forage lab – this will help you to decide if this forage is worth the purchase price.

 

 


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