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Particle size of the ration: Does it matter?

Marcia Endres

Dairy cows have a requirement for physically effective fiber; i.e., long particles that stimulate cud chewing and help keep rumen pH at normal levels. However, could we be promoting ration sorting behavior at the feed bunk if particles are too long and dry?

During last month’s 4-State Dairy Nutrition and Management Conference held in Dubuque, IA, I presented a talk about total mixed ration (TMR) particle size distribution and its effect on cow sorting behavior and performance. The following is a summary of the information presented.

Can cows sort the ration? Leonardi and Armentano from the University of Wisconsin-Madison evaluated the sorting behavior of 24 cows housed in tiestalls. They fed TMR diets containing 60% concentrate and either 40% chopped alfalfa hay or 20% alfalfa hay (chopped or long) and 20% alfalfa silage. Hay was either lower or higher quality, resulting in 6 treatment combinations. The authors determined the effect of quantity, quality and length of hay on sorting behavior and also the variation in sorting among cows.

From the study, they concluded that quality of hay did not affect sorting activity. The intake of the upper screen material (longer particles) as a percentage of predicted intake was the most variable. For 4 cows the intake of longer particles was between 60 and 70% of predicted intake, for 11 cows it was 71 to 80%, for 5 cows it was 81 to 90%, for 2 cows it was 91 to 100%, and for 2 cows it was 101 to 110%. Sorting of the middle screen was less likely. This variation in individual cow sorting behavior may have special significance in a freestall situation where cows can move along the length of the feed bunk and sort for finer particles throughout the day. The researchers also reported that diets with more hay resulted in more sorting. They cautioned that using dry hay or straw may increase the average particle size of the ration on paper but may underestimate the average particle size consumed by the cows.

In a Minnesota study last summer and early fall, we collected TMR particle size data from 50 freestall dairy herds using the ‘new’ Penn State Particle Separator (3 sieves and a bottom pan). We evaluated the initial TMR (as delivered to the cows), the TMR refusal and 3 other samples during the day (at intervals of 3-4 hours). The results showed there was a large variation among farms on the percentage of particles retained on the various screens. There was an average for the initial TMR of 10.8% on the upper screen and 12.6% on the bottom pan. The average for the TMR refusal was 22.7% for the upper screen and 8.1% for the bottom pan. The difference between initial and refusal indicates there was some degree of sorting, on average, on these farms.

Let’s take a look at the individual farm results. There are some farms where little or no sorting took place (see Figure 1) and some farms where quite a bit of sorting was done by the cows (see Figure 2). The charts in Figures 1 and 2 were done using the Penn State Particle Separator Datasheet available on the Internet ( Particle distribution is plotted using lognormal graphing paper, with sieve size on the X-axis and cumulative percentage of material that falls under each sieve on the Y-axis. The goal is to have each point in, or very close, to the recommended target rectangles, and the lines to be very close together. In both figures, the darker line and point refer to the initial TMR particle size and the lighter point and line refer to the refusal particle size. Forage to concentrate ratio in these two dairies was not very different. However, in Example Farm 1, where no sorting occurred, no dry hay was being fed to the cows. Also notice that particle size of the TMR meets the target goals (points are inside the target rectangles). On Example Farm 2, where cows did a lot of sorting, 16% of the ration dry matter was dry hay (the highest amount of all farms we visited) and the initial particle size for the upper screen is outside the target goal (more than recommended). We found a correlation in our study between amount of dry hay in the diet and ration sorting (the more hay in the diet, the more sorting by the cows).

The results of our study and the study in Wisconsin indicate that it is necessary to process dry hay (and straw) to reduce the amount of sorting. Penn State ’s current recommendations are for 2 to 8% of the TMR particles on the upper screen of the separator and 30 to 50% of the particles on the middle screen. It appears that these are good targets if we want to avoid sorting by the cows, and it is especially important to check particle size when feeding dry hay or straw.

Published in Dairy Star July 9, 2005

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