Fig. 1. Glass filter will be used for separating fiber (NDF or ADF) from cell contents (mostly easily utilizable nutrients). The white disc in the glass cup is a filter mat which only allows liquid (cell content) to pass through.

Fig. 2. Fiber analysis using detergent analysis system. The brown solution in the glass filters is a slurry of forage sample boiled with detergent solution at 105 degrees C for 60 minutes. Fiber in a slurry will be recovered after the liquid part is vacuumed down.

Fig. 3. TDN and CP distribution of bermudagrass hay samples (LSU AgCenter, Forage Quality Laboratory, 1998 ~ 2008).

Our forage quality lab blog is focusing on sharing forage-related information including, but not limited to, forage quality analysis.

Knowing your forage quality

The goal of our first article from the Forage Quality Analysis Laboratory (FQAL) at the LSU AgCenter is to brush up on forage quality-related terminology.

After your forage sample submission, you will receive your analysis results of the following chemical component contents in a sample.     

DM stands for dry matter, and it is determined by drying forage sample at 55^oC for 48 hrs. Determination of forage DM is a simple procedure. However, it is important because nutritionists use analysis results on DM basis for ration balancing.   

NDF stands for Neutral Detergent Fiber, which is fibrous residue obtained after ground forage samples are boiled with a neutral detergent solution for an hour (Fig. 1 and 2).  This residue includes hemicellulose, cellulose, lignin,and silica.  High NDF content limits forage intake.

ADF is “Acid Detergent Fiber.” As the name indicates, fibrous component recovery is made with the same procedure as NDF except for using acid detergent instead of neutral detergent. The fibrous residue is mostly cellulose, lignin and silica. High ADF content decreases digestibility of the forage, i.e. high ADF generates low total digestible nutrient (TDN) value. 

CP (Crude Protein) has nitrogen as a main element. Unlike mono-gastric animals (eg. swine), microorganisms in a ruminant animal’s (cattle, goats, deer, sheep) stomach consume nitrogen (N) and energy in the feed and increase their populations. The rumen microbes that exceed the necessary rumen population pass into the ruminant digestion system and become a protein source. Therefore, all the N forms in forage will be a potential protein source. Because one unit of N can make 6.25 units of protein, forage N is analyzed and multiplied by 6.25 times to estimate CP.

Urea is another form of non-protein nitrogen available for cattle (not for horses). It contains 45% N. Therefore, CP value is calculated as 281% (45% × 6.25).  Adding 1% urea to a grain diet typically increase CP by 2.81%. 

TDN stands for “total digestible nutrients.” Unlike NDF, ADF and CP, reported TDN of your sample is a calculated value based on analyzed nutrient content.

Forage quality is the “potential of forage to produce the desired animal response” (Collins and Fritz, 2003). We need to know what is in our forage and available for our animals to actually utilize in their growth and maintenance so we can properly balance the diets with supplements.

Although forage agronomists distinguish forage nutritive value from forage quality, the two terms are frequently used interchangeably.

Measuring digestibility by using a live animal is the most accurate method of determining forage digestibility. However, it is a costly process, involving a great deal of time and labor. In addition to the time and labor involved, the results may be different because of the use of individual animals and generalization of the results is almost impossible.   

From the negative relationship between forage quality and fiber content, ruminant nutritionists have learned that chemical analysis based on the detergent system reveals many significant relationships between fiber levels and potential forage intake and digestibility.  Estimated feed values (TDN or RFV: relative feed values) are frequently used for evaluation of forage quality for marketing.

In Louisiana livestock production, bermudagrass is one of the most important forage resources, and more than 35% of forage samples submitted at this laboratory are bermudagrass hay. Each grass species maintains a different range of CP and TDN. Warm-season grasses (bermudagrass, bahiagrass, etc.) CP and TDN values are usually lower than cool-season grass (ryegrass, oats, wheat, etc.).

In this topic (Fig. 3), we will discuss only bermudagrass samples because it is the most common forage resource in Louisiana and Mississippi. Forage quality analysis results appearing on the database indicate that many producer samples of bermudagrass forage are harvested with low TDN. However, those low TDN forage samples contain adequate CP levels to reach the minimum level of CP required for growing heifers. (Each purple dot represents producer sample TDN analyses. Blue dots represent CPs.)  Bermudagass needs to be harvested before the grass matures (28 to 42 days regrowth) for optimum quality.