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Not all NDF is the same!

Jim Paulson

One of the highlights for me of attending the World Dairy Expo in Madison, WI each year is to look over the forages that are entered into the Forage Super Bowl. I look at every entry of hay, haylage, baleage and corn silage. I love to smell some of the hay entries that are as sweet as the day they were baled. I look at the milk per ton values and wonder how they were able to harvest such incredibly high-quality forage! I look at the differences between the BMR corn silages as compared to the regular varieties. I compare grass versus alfalfa, dry hay versus haylage and see what varieties were used for each. And, I get a copy of all the forage analyses to study further. To me, forage is very interesting.

Forage, on a dry matter basis, is composed of protein, fat, non-fiber carbohydrates (NFC), fiber (also carbohydrates) and ash. Neutral detergent fiber, or NDF, is part of the carbohydrates of forage and a significant factor in the amount of energy that forages can provide to a ruminant. But NDF is different in different forages. It is different in the amount in the forage and it is different in composition. In addition, we know that even in the same kind of forage, whether it is alfalfa, grass or corn, it will differ with the stage of maturity, the year and season it is grown, as well as variety within a species.

We measure NDF by using either NIR methods (light spectrum readings) or using what we call wet chemistry. In wet chemistry, a small ground sample is boiled in neutral detergent solution to extract soluble material and leave the structural fiber. Think of a forage plant as a tall building such as a tall hotel or office building full of rooms. The rooms are full of soluble plant material. The neutral detergent solution opens all of the doors and lets everything in the rooms out. What is left is the structural part, which we call neutral detergent fiber. Like a building which is made of bricks, mortar, steel and other materials, NDF is made up of building materials like cellulose, hemicellulose, lignin and minerals. Acid detergent fiber is measured by boiling a forage sample in an acidic solution. Hemicellulose is the difference of NDF minus ADF and is very digestible in the rumen. ADF measures cellulose, lignin and ash. Cellulose can be digested with time if it is not too bound with lignin. Lignin is an indigestible compound in NDF and, as it increases, it also interferes with microbial access to cellulose further limiting digestibility. This limits rate and extent of digestion, intake of forage and, thus, energy consumed.

We measure the digestibility of forage at different time points. The 30-hour time point (NDFD30) is used most often by nutritionists because it is an average time spent by forages in the rumen. A higher number for digestibility is better, indicating the forage is more digestible. The more digestible the forage, the faster it disappears from the rumen and the cow can then eat more. Remember, we use NDF as an indicator of intake. But we now also use NDFD30 to help us understand rate and extent of digestion which will influence intake.

Ash is also indigestible since it is composed of minerals in the forage. Alfalfa is usually higher in ash because it is higher in calcium and potassium as compared to corn or grass. But soil from raking or mowing can also contaminate forage and increase ash. Since ash is part of NDF, it leads us to measure more NDF than is actually there. This can lead to an elevated energy level. This is why many labs are reporting NDF less the ash content. This is listed as NDFom for NDF organic matter basis.

Table 1. Forage fiber and starch concentration for different forages
ID DM NDF ADF Lignin Starch NDFD30 TDN
Silage 1, BMR 38.0 32.4 19.9 2.38 42.3 59.9 77.3
Silage 2, conventional 39.9 41.3 26.2 3.45 33.4 49.9 71.9
Grass hay 85 60 30 4.5   55 60
Alfalfa hay 85 40 30 7.5   46 65

Table 1 shows forage fiber concentrations of four common forages: two corn silages and two dry hay samples. Notice the difference in NDF concentration across the forage sources. Even within corn silage, they differ significantly. There are differences between grass and alfalfa because they are completely different forages. The differences between the corn silages can be due to one being a BMR hybrid, different populations and other factors.

Why are these differences important? Part of the answer is shown in the last column: TDN or total digestible nutrients. TDN gives us an estimate of the energy potential of forages. It is an estimate based on the digestibility of the forage and other nutrients like the starch content. But all of the numbers are important because they help us understand how forage will fit into a diet. High-quality forage is the base for an affordable diet for our dairy animals. As human population competes for grain in the future, forage needs to be a bigger part of the diet.

October 2016

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