Fiber Digestibility and Forage Fragility in Dairy Cattle

Mertens (1997) defined physically effective NDF (peNDF) as the fraction of NDF that stimulated chewing and contributed to a ruminal digesta mat. He also proposed a standard laboratory method for measuring peNDF that involved dry sieving and measuring the fraction of sample NDF retained on the ≥1.18-mm sieve. This large particle pool requires rumination and has a high resistance to escape from the rumen of sheep (Poppi et al., 1980). Mertens (1997) summarized a large data set that showed a positive correlation between peNDF measured by dry sieving, ruminal pH, and milk fat percentage in dairy cattle. Based on Mertens (1997) data set, the minimal requirement for peNDF in lactating dairy cows has been defined as approximately 21 to 23% of ration dry matter (DM) to maintain ruminal pH above 6.0 and milk fat above 3.4% for Holsteins. Since 1997, the peNDF system has become widely used within the CPM-Dairy and Cornell Net Carbohydrate Protein System (CNCPS) ration formulation models to predict the effect of forage particle size on cow chewing response and ruminal pH. A problem has been lack of an on-farm tool that accurately measures peNDF and results in physical effectiveness factors (pef) similar to those obtained by the standard dry sieving method. Recently, we developed the Z-Box to effectively measure peNDF for common forages and total mixed rations (TMR) in on-farm situations (Cotanch and Grant, 2006).