By Jerad Jaborek
Cattle and other ruminants are uniquely designed with the ability to digest the fibrous plant cell wall of forages. However, the inclusion of forages in finishing diets is often kept at a low concentration (<12% of diet dry matter) due to the lesser digestibility and energy density that typically result in lesser average daily gain (ADG) compared with grains. Other factors such as cost per unit of energy, storage, processing, and sorting of diets, including forages, present additional economic and operational challenges that pose barriers for forage inclusion in finishing diets.
Feedlot cattle can grow well on grain-based diets with little forage inclusion. However, lengthy feeding periods of grain-based diets with little forage may compromise digestive health (including the liver) and eventually ADG of feedlot cattle. Researchers have been searching for the minimal requirements of fiber in cattle finishing diets while still maintaining health and growth.
The chemical components contributing to the incompletely digestible components of feeds include neutral detergent fiber (NDF) and acid detergent fiber (ADF), while a part will remain undigested (undigestible fiber). However, feedstuffs and diets with similar fiber concentrations can still differ in ADG, feed intake, rumination, ruminal pH and the occurrence of digestive disorders for beef cattle. David Mertens, Founder and CEO of Mertens Innovation and Research LLC, proposed effective NDF (eNDF) as the fiber's ability to maintain milkfat production and animal health. Further refinement of eNDF led to the development of the term physically effective NDF (peNDF), which is defined by the physical characteristics of the fiber in feedstuffs, such as particle size, and how well it stimulates rumination and the establishment of a floating mat of digesta in the rumen.
The physical size of feed particles is important for ruminant digestion as feed particles must be reduced to a small enough size to pass from the reticulorumen to the rest of the digestive tract. The size reduction of consumed feed particles occurs when cattle ruminate, thus chewing their cud. Chewing provides additional saliva that can help buffer the ruminal pH and help mitigate durations of low ruminal pH that contribute to ruminal acidosis and other digestive disorders.
A vertical sieving technique was developed to sort diets and individual feedstuffs by particle size, leading to the development and refinement of the Penn State Particle Separator (PSPS). The PSPS consists of four stackable boxes containing sieves of 19, 8, and 4 mm. Finer particles, such as grains, are collected on the solid surface of the bottom box. The 4 mm sieve is intended to represent the threshold size of particles leaving the reticulorumen. Estimated peNDF consists of the percentage of feed particles remaining in the upper three boxes (> 4 mm) multiplied by the NDF% of the diet. The NDF% of the diet can be determined from a feed sample collected and sent for nutrient analysis testing at a commercial laboratory. While there are some factors that may influence the accuracy of the peNDF estimate using this technique, it offers producers a relatively easy method for assessing physically effective fiber that can be done on farm.
Source : msu.edu