Highly fermentable diets require the inclusion of adequate amounts of fiber to
reduce the risk of subacute rumen acidosis (SARA). To assess the adequacy of
dietary fiber in dairy cattle, the concept of physically effective neutral
detergent fiber (peNDF) has received increasing attention because it
amalgamates information on both chemical fiber content and particle size (PS)
of the feedstuffs. The nutritional effects of dietary PS and peNDF are complex
and involve feed intake behavior (absolute intake and sorting behavior),
ruminal mat formation, rumination and salivation, and ruminal motility. Other
effects include fermentation characteristics, digesta passage, and nutrient
intake and absorption. Moreover, peNDF requirements depend on the
fermentability of the starch source (i.e., starch type and endosperm
structure). To date, the incomplete understanding of these complex
interactions has prevented the establishment of peNDF as a routine method to
determine dietary fiber adequacy so far. Therefore, this review is intended to
analyze the quantitative effects of and interactions among forage PS, peNDF,
and diet fermentability with regard to rumen metabolism and prevention of
SARA, and aims to give an overview of the latest achievements in the
estimation of dietary fiber adequacy in high-producing dairy cattle. Recently
developed models that synthesize the effects of both peNDF and fermentable
starch on rumen metabolism appear to provide an appropriate basis for
estimation of dietary fiber adequacy in high-producing dairy cows. Data
suggest that a period lasting more than 5 to 6h/d during which ruminal pH is
<5.8 should be avoided to minimize health disturbances due to SARA. The
knowledge generated from these modeling approaches recommends that average
amounts of 31.2% peNDF inclusive particles >1.18mm (i.e., peNDF(>1.18)) or
18.5% peNDF inclusive particles >8mm (i.e., peNDF(>8)) in the diet (DM basis)
are required. However, inclusion of a concentration of peNDF(>8) in the diet
beyond 14.9% of diet DM may lower DM intake level. As such, more research is
warranted to develop efficient feeding strategies that encourage inclusion of
energy-dense diets without the need to increase their content in peNDF above
the threshold that leads to lower DM intake. The latter would require
strategies that modulate the fermentability characteristics of the diet and
promote absorption and metabolic capacity of ruminal epithelia of dairy cows
