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.