Source: Agriculture and Agri-Food Canada
Wen Yang and Karen Beauchemin • Agriculture and Agri-Food Canada Research Center, Lethbridge, Alberta
Take home message
The physical effectiveness factor (pef) is determined as the proportion of the TMR (as-fed) retained on the 0.75” and 0.31” screens of the Penn State Particle Separator. The peNDF is determined by multiplying the pef by the NDF content (DM basis) of the diet.
Increasing the peNDF content of the diet by feeding longer chopped forages improves rumen pH, and helps avoid acidosis. The reduction in acidosis by increasing the peNDF content of the diet was greater for barley than for corn diets, because the incidence of acidosis is higher in barley diets.
Manipulating forage particle length to reduce acidosis is beneficial in low forage diets, but there is no need to consider particle length of forages for diets containing more than 60% forage (DM basis).
Subacute ruminal acidosis is a common and economically important metabolic disorder in dairy cattle. It is caused by low pH (< 5.8) in the rumen due to an accumulation of fermentation acids. Salivary buffers help neutralize these acids. Salivation increases during chewing, thus, increasing the time that cows spend chewing is thought to decrease ruminal acidosis.
To determine whether the requirement of lactating dairy cows for physically effective fibre (peNDF) varies with grain source or forage to concentrate ratio.
Materials and methods
Barley and corn grains were each used in separate feeding studies with lactating dairy cows. Alfalfa silage, chopped short (5/16”) or long (3/4”), was the forage in both studies. In each study, four diets were formulated using the two cuts (short and long) of alfalfa silage combined with two ratios of forage to concentrate (35:65 or 60:40, DM basis). The particle size of the diets was quantifi ed using the Penn State Particle Separator  as follows:
• pef = % particles > 0.75” + % particles > 0.31”
• peNDF = NDF content (DM basis) × pef
Results and discussions
Content of peNDF in the TMR (Table 1)
The pef and peNDF contents of the TMRs were increased with increasing forage chop length, as well as with increasing forage to concentrate ratios of the diets.
Intake, chewing and ruminal pH (Table 2)
Feed intake was reduced by 10% when proportion of forage in the diet was increased from 35 to 60%.
Increasing peNDF content of the diets did not affect eating time, but it generally increased ruminating time and thus total chewing time. Increasing peNDF increased mean ruminal pH from 5.86 to 6.17 for barley diets, and from 5.99 to 6.26 for corn diets, for cows fed low forage diets. However, increasing peNDF had minimal effect on ruminal pH for cows fed high forage diets.
Cows fed barley diets had longer periods of low ruminal pH (i.e., pH<5.5) than cows fed corn diets. Increasing forage particle length of diets reduced the amount of time pH<5.5 by 25%.
Increasing forage particle size efficiently reduced ruminal acidosis for cows fed barley diets. Cows fed high forage or corn grain diets had minimal risk of acidosis when alfalfa silage was fed as the sole forage.
Improvement of ruminal pH by increasing physically effective fibre of the diet was not caused by increased chewing time. Rather, improved ruminal pH status was due to a change in feed fermentability and fermentation patterns in the rumen caused by an increase in the physically effective fibre content of the diet.
 Mertens, D.R. 2000. Feedstuffs. Pages 11-14, April 10, 2000.
 Lammers, B.P., D.R. Buckmaster, and A.J. Heinrichs. 1996. J. Dairy Sci. 79: 922.
This research was supported by Dairy Farmers of Canada (Ottawa, ON), and Agriculture and Agri-Food Canada’s Matching Investment Initiative.