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Livestock Sciences Section, Research Centre Agriculture and Agri-Food Canada, Lethbridge, AB, T1J 4B1, Canada
Corresponding author:
K. A. Beauchemin; e-mail:
beauchemin{at}agr.gc.ca.
Effects of forage particle size measured as physically effective NDF and ratio of alfalfa silage to alfalfa hay of diets on feed intake, chewing activity, particle size reduction, salivary secretion, ruminal fermentation, and milk production of dairy cows were evaluated using a 4 x 4 Latin square design with a 2 x 2 factorial arrangement of treatments. The diets consisted of 60% barley-based concentrate and 40% forage, comprised either of 50:50 or 25:75 of alfalfa silage:alfalfa hay, and alfalfa hay was either chopped or ground. Various methods were used to determine physically effective NDF content of the diets. Cows surgically fitted with ruminal and duodenal cannulas were offered ad libitum access to these total mixed diets. The physically effective NDF content of the diets was significantly lower when measured using the Penn State Particle Separator than when measured based on particles retained on 1.18-mm screen. Intake of DM was increased by increasing the ratio of silage to hay but was not affected by physically effective NDF content of diets. Eating time (hours per day) was not affected by the physically effective NDF content of diets, although cows spent more time eating per unit of DM or NDF when consuming high versus low alfalfa hay diets. Ruminating time (hours per day) was increased with increased physically effective NDF content of the diets. Rumen pH was affected more by changing dietary particle size than altering the ratio of silage to hay. Feeding chopped hay instead of ground hay improved ruminal pH status: time during which ruminal pH was above 6.2 increased and time during which ruminal pH was below 5.8 decreased. Milk production was increased by feeding higher concentrations of alfalfa silage due to increased DM intake, but was not affected by dietary particle size. Feed particle size, expressed as mean particle length or physically effective NDF was moderately correlated with ruminating time but not with eating time. Although physically effective NDF and chewing time were not correlated to mean rumen pH, they were negatively correlated to the area between the curve and pH 5.8, indicating a positive effect on reducing the risk of acidosis. Milk fat content was correlated to rumen pH but not to physically effective NDF or chewing activity. These results indicate that increasing physically effective NDF content of the diets increased chewing activity and improved rumen pH status but had limited effect on milk production and milk fat content.
Key Words: physically effective NDF • chewing behavior • rumen pH • dairy cow
Abbreviation key: AH= alfalfa hay, AS= alfalfa silage, AS:AH= ratio of alfalfa silage to alfalfa hay, FPS= forage particle size, MPL= mean particle length, pe= physically effective, pef= physical effectiveness factor, pefM= physical effectiveness factor from tabular values of chewing time, pef P>1.18= physical effectiveness factor determined as a percent of DM remaining on a 1.18-mm screen, pefPS= physical effectiveness factor calculated as the sum of the DM proportions retained on the two sieves of the Penn State Particle Separator, peNDF= physically effective NDF, peNDFM= physically effective NDF measured as the NDF content of the TMR multiplied by pefM, peNDFP>1.18= physically effective NDF measured as the NDF content of the TMR multiplied by pefP>1.18, peNDFPS= physically effective NDF measured as the NDF content of the TMR multiplied by pefPS, PSPS= Penn State Particle Separator
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