Three steps to sizing a TMR mixer

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If you want to bake a perfect batch of chocolate chip cookies, you've got to blend the right amount of ingredients in a bowl. However, if the bowl is too small, carefully-measured ingredients tumble over the side. That can result in a less-than-perfect cookie.

The same thing can happen when you combine a total-mixed ration (TMR) in a mixer that's too small. However, when ingredients tumble over the side, you don't end up with an imperfect batch of cookies. Instead, you end up delivering an inconsistent and improperly-mixed TMR to your cows.

To avoid over- or under-filling a TMR mixer, use these three steps to help you choose a mixer size which meets your dairy's needs.

Step 1. Estimate dry matter intake

Before you can determine the size of the mixer needed on your dairy, estimate the dry matter intake of your highest-producing group of cows - the group most likely to have the highest dry matter intake on your dairy - and the group consuming the least amount of dry matter, such as a dry cow group.

Your nutritionist can give you the pounds of dry matter intake for both group sizes. However, you don't need exact numbers to size a mixer, "you just have to be close," says Dennis Buckmaster, agricultural engineer at Penn State University. To estimate dry matter intake for lactating cows, use the chart on page 54. The chart does not provide dry matter intake for non-lactating animals, so assume daily dry matter intake to be about 2.5 percent of bodyweight for non-lactating animals, such as close-up dry cows and heifers.

Step 2. Determine TMR batch sizes

After you've determined dry matter intakes, use the information to help you calculate the largest and smallest batch sizes you will mix each day.

Again, assume your highest-producing group of cows will eat the most of all your animal groups, so they will need the largest batch size. Then, assume a group of close-up dry cows and transition cows will be fed the smallest batch size.

To keep things simple, assume your ration's dry matter content averages 60 percent, and you plan to feed two batches per animal group. Each cow in your "high group" eats about 50 pounds of dry matter per day, while each of the dry and transition cows consumes about 30 pounds of dry matter per day.

Perform the following calculations to determine the pounds of dry matter per batch for the largest batch and the smallest batch you will need to mix:
Largest batch mixed for your "high group":
50 lbs. dry matter/cow/day
x 75 cows in the group
÷ 2 batches per day
= 1,875 pounds of dry matter per batch

Smallest batch mixed for your "dry group":
30 lbs. dry matter/cow/day
x 50 cows in the group
÷ 2 batches per day
= 750 pounds dry matter per batch

Thus, your smallest and largest batch sizes will be 750 pounds and 1,875 pounds of dry matter, respectively.

Step 3. Select mixer size

Now that you know the largest and smallest batch sizes you need to mix each day, you can determine mixer size, measured in cubic feet. To do so, determine the pounds of feed on an "as-fed" basis and then divide that number by the ration density, listed as pounds per cubic foot. Use the following calculations as your guide:

Largest batch: 1,875 pounds dry matter at 60 percent dry matter.
1,875 pounds dry matter
÷ 0.6
= 3,125 pounds as-fed

A typical TMR density is 17 pounds per cubic foot. So, 3,125 pounds as-fed
÷ 17 pounds per cubic foot
= 184 cubic feet

Smallest batch: 750 pounds dry matter at 60 percent dry matter
750 pounds dry matter
÷ 0.6
= 1,250 pounds as-fed

So, 1,250 pounds as-fed
÷ 17 pounds per cubic foot
= 74 cubic feet

Thus, you need a mixer which will blend batches ranging in size from 74 cubic feet to 184 cubic feet. Ask the manufacturer to help you determine which model will hold both of these batch sizes and blend them uniformly, says David Kammel, extension agricultural engineer at the University of Wisconsin-Madison.

However, if you plan to expand herd size, don't be afraid to choose a mixer with a larger capacity, Kammel adds. That way, you won't get caught with a mixer that's too small if you expand in the future.

And, a larger capacity mixer can help you avoid the temptation to over-fill the mixer, a situation which results in spilled feed and an inconsistent ration. For example, certain types of mixers, such as reel-type mixers, may not blend a ration effectively if they are overfilled, explains Sandy Stokes, extension dairy specialist at Texas A&M University. "Make consistency to the animals the priority," she adds.

Follow these three steps to help you determine the appropriate mixer size for your dairy.


How to test a TMR mixer

You've got your eye on two TMR mixers. Which one should you buy? To find out which mixer will give you the best mix, conduct an on-farm test using each of the mixers, suggests David Kammel, extension agricultural engineer at the University of Wisconsin-Madison.

Here's how to conduct a test on your dairy:
1. Mix the largest batch size which you currently feed.
2. Deliver the ration using one of the mixers.
3. Pull 1- to 2-quart samples along the length of the bunk - the beginning, middle and end - after the feed is delivered. Don't sift the feed before you pull a sample.
4. Pull two or more samples at each location and average the results.
5. Measure the uniformity of the samples. For example, you can use a particle separator box to check uniformity of particle length or use a nutritional analysis test to determine crude protein uniformity.
6. The next day, repeat steps 1 to 5 using the second mixer.
7. On the third and fourth days, repeat steps 1 to 6, mixing the smallest batch instead of the largest batch listed in step 1.
8. Ask your nutritionist to help you determine the significance of the test results.



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