Estimating physically effective fiber

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The Penn State Particle Separator (PSPS) provides a tool to quantitatively determine the particle size of forages and total mixed rations (TMR). The updated 2013 version of the PSPS adds the ability to estimate physically effective fiber (peNDF) to this tool. The concept of measuring feed particle size using a standard method is not new.

The American Society of Agricultural and Biological Engineers’ (ASABE) standard for particle size analysis and distribution has been available for many years. Unfortunately, the ASABE method is a cumbersome laboratory procedure that is impractical for farm use. The objective of developing the PSPS was to mimic the complex lab method with a simpler, on-farm method. The newest refinement allows for an estimation of the quantity (%) of a forage or TMR that provides physically effective fiber to the dairy cow.

The goal of analyzing TMR particle size is measuring the distribution of feed and forage particles the cow actually consumes. The focus is not only on particles greater than a particular size, but also on the overall distribution of feed particles consumed by the cow. Measuring TMR samples fresh from the feed bunk before the cows eat or sort the feed is recommended. Mixing and distribution equipment can reduce particle size of feeds and forages and need to be accounted for by evaluating the actual diet being fed.

Physically Effective Fiber or peNDF

Measurement of peNDF has become widely used in dairy cattle nutrition and research. The original system of peNDF was developed by Dr. Dave Mertens of the US Dairy Forage Research Center. It measured fiber particle size using dry samples of feed in a 3-dimensional, vibrating sieve system (Ro-Tap separator; measuring smallest particle dimension) and had neutral detergent fiber (NDF) on the whole sample. Most forage is now measured in a 2-dimensional sieve device (such as the PSPS; measuring particle length). Using peNDF in ration balancing and troubleshooting, along with a single NDF value, may prove to be a useful tool in troubleshooting rations and cow performance.

The original peNDF procedure used 1.18 mm as the critical size at which feed particles are considered physically effective for dairy cows. This number originated from earlier research that determined 1.18 mm was a threshold particle size determined with both cattle and sheep (at maintenance intakes) for greatly increased resistance to particles leaving the rumen and that less than 5% of fecal particles were generally retained on a 1.18-mm sieve.

Researchers from Penn State and other universities have studied particle size of diets and their impact on rumen metabolism, and have clearly shown in recent studies that the critical threshold for feed particles escaping the rumen of high producing cows is significantly greater than 1.18 mm and more in the range of 4 mm. While there is no one perfect sieve size to measure particles for all diets and all forages, the data from four independent labs show that the 4-mm sieve is more accurate for the high producing dairy cow for estimating peNDF.

The original 1996 PSPS contained sieves of 0.75 and 0.31 inches (19 and 8 mm). The 0.75-inch (19-mm) sieve was designed to capture forage or feed particles that would be buoyant in the rumen (form the forage mat) and provide material that would require substantial additional cud chewing by the cow. In theory this would supply additional buffering to the rumen and help modify rumen pH. The 0.31-inch (8-mm) sieve collects primarily forage particles that will be part of the forage mat in the rumen, but will be broken down faster with less cud chewing and will hydrate in the rumen faster to allow more rapid rumen microbial breakdown. Both the amount of cud chewing required and the hydration rate will depend on the digestibility of the forage contained in this fraction.

The newest sieve now in the 2013 PSPS, is a 0.16-inch (4-mm) sieve. Feed particles found on this sieve will primarily be small forage pieces that are often, but not necessarily, high fiber in nature. Initially these particles will likely be trapped in the forage mat of the rumen, but they can be broken down easily with minimal rumination or by rapid microbial action. Typically they will hydrate quite rapidly and will not remain trapped in the fiber mat for a long period of time. In either event these feed particles will have a small, yet significant, impact on buffering the rumen. This sieve is designed to allow estimation of peNDF.

The peNDF can be estimated by adding the amount of feed on the top three sieves (all ≥ 4 mm) and multiplying by the NDF content of the feedstuff. This is an estimated value, as the NDF content and digestibility of each fraction are unknown. In addition, some portion of the contents on the smallest (0.16-inch; 4-mm) sieve will likely contain grain or rapidly digested carbohydrates and may need to be discounted from the value. Furthermore, peNDF by itself will not guarantee that the diet is well balanced and that rumen pH will be correct, as diet starch or total carbohydrate levels have been shown to significantly impact rumen pH in high producing cows with high dry matter intakes.

Feeding management plays an important role in the particle length needs of the cow. Guidelines for TMR for high producing dairy cows are 2 to 8 percent of the particles in the upper sieve, 30 to 50 percent in the middle sieve, 10 to 20 percent on the 4-mm sieve, and no more than 30 to 40 percent in the bottom pan. Sixty to 70 percent of the TMR should be classified as physically effective, to be multiplied by the NDF content of the diet or forage. Only forage and high fiber byproducts should be classified as effective fiber sources. Further evaluations of this new method to estimate peNDF are continuing.

Guidelines for Particle Size

Achieving adequate ration particle size requires using recommended guidelines for forages and TMR.

Table 1. Corn silage, haylage, and TMR particle size recommendations for lactating cows.

Screen Pore Size (inches) Particle Size (inches) Corn Silage Haylage TMR
Upper Sieve 0.75 > 0.75 3 to 8 10 to 20 2 to 8
Middle Sieve 0.31 0.31 to 0.75 45 to 65 45 to 75 30 to 50
Lower Sieve 0.16 0.16 to 0.31 20 to 30 30 to 40 10 to 20
Bottom Pan
< 0.16 < 10 < 10 30 to 40

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