Small Chop.
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“The cow will sort it out.”

Every farmer I have ever argued with about chop length has said some version of the same thing. “The cow will sort it out.”

She will. That is exactly the problem.

Here is what sorting actually does to you. She eats what she likes and leaves the rest, and the long straw is what she leaves. So the first cow to the bunk gets one ration, and the cow behind her gets what is left of it. Nobody eats the diet you built.

You did not feed your herd one diet. You fed them four.

This is not an opinion. It has been measured, at a university, and the numbers are worth your time.

What Guelph actually did

At the Elora Dairy Research Facility, Professor Trevor DeVries and his group at the University of Guelph wanted to know what happens to a cow when you change one thing in her ration and one thing only: how short you chop the straw.

Not the ingredients. Not the energy density. Not the mixer. Just the chop.

So they took wheat straw and ran it through a tub grinder. And here is the part I did not expect when I first read the paper: the machine is named in the methods. It was a HayBuster tub grinder, out of Jamestown, North Dakota. The same tub grinder we have been building for sixty years is the one the university reached for when they needed to control particle size for a research trial.

Half the cows got straw chopped through a 2.5 cm (1 in) screen. The other half through a 10 cm screen. Everything else identical. Thirty-six percent corn silage, thirty-five percent dry cow pellet, twenty-nine percent wheat straw. Forty cows, enrolled at dry-off about forty-five days before they were due to calve, then followed straight through calving and into lactation.

One machine. Two screens. Forty cows. That is the whole experiment.

What the cows did

The cows on short straw ate more. 15.6 kg of dry matter per day against 15.0 kg for the long straw group. In a dry cow that is a real difference, and dry cow intake is not a number anyone should shrug at.

But intake is not the interesting one. The interesting one is consistency.

In the week before calving, every dairy farmer knows what happens. Intake falls off a cliff. It is normal. It is also the moment your cow is most exposed, and the size of that drop is one of the better predictors we have of whether she is about to get sick.

The short-chop cows held their intake steadier into calving. The long-chop cows dropped faster. The short-chop cows also ate 0.4 kg more per meal, ate faster, and took 0.6 fewer meals a day to get it done. Fewer trips to the bunk, more in her each trip.

And they sorted less. Both groups sorted against the long particles, because that is simply what cows do. But the cows on the long chop sorted to a greater extent. In the companion study, the sorting index for the longest particles was 94.2 percent on the long chop and 99.7 percent on the short. A hundred percent means she ate the ration you built. 99.7 is about as close to a straight line as this kind of data gets.

Then it showed up in the blood. Three weeks after calving, the long-chop cows had higher beta-hydroxybutyrate. Higher BHB means deeper negative energy balance, and that means more ketosis, more metabolic disease, more vet bills, more cows that never quite get going in their lactation.

Their rumens told the same story. Mean rumen pH was not different between the groups. What was different was the rate of decline: in the first week after calving, the long-chop cows’ rumen pH fell away faster. Their rumens had a harder landing.

The chop length in the dry pen was still being paid for, or paid out on, three weeks into lactation.

And the milk

The DeVries lab ran a second study on the other side of calving. Forty-one multiparous Holstein cows, followed through the first twenty-eight days after calving, on a lactating TMR with nine percent wheat straw. One group’s straw through a 2.5 cm (1 in) screen. The other through a 5 cm (2 in) screen.

The cows on the shorter straw produced milk with less fluctuation day to day. And over those first twenty-eight days, they tended to produce 75 kg more milk, cumulatively.

Straw did not become more nutritious. Straw is straw. What changed is that she ate what you actually mixed for her, in the same amount, every day, and her rumen never had to whipsaw between a fibre day and a grain day.

Where small goes wrong

The short-chop diet produced more fines and dust than the long-chop diet. And the cows sorted against them. Also the Guelph finding

That surprised the research team. Cows normally pick out the small particles, because the small ones are usually the tasty ones. Not here. Here the finest fraction was pulverised straw and dust, and the cows wanted nothing to do with it.

Their recommendation was to use a chopping technique that minimises over-processing, so you are not filling the bunk with fines.

So the honest instruction from the research is not “make it smaller.” It is:

And that is a harder engineering problem than it sounds, which is exactly why it is worth talking about.

Anything can beat a bale to pieces. A flail can. A mixer with knives can, eventually. The difficult thing, the thing that actually separates machines, is delivering a particle that is short and uniform and not pulverised, load after load, in wet straw and dry straw and the brittle miserable straw you get in a bad year.

That is what a hammer mill running against a screen is for. The screen is the referee. Material stays in the mill until it is small enough to pass, and the moment it passes, it is out and gone before it can be ground a second time. Material that is already the right size does not sit in there getting hammered into powder. That is the entire design philosophy, and it is why we tell operators to run a screen about half their target particle size rather than reaching for the smallest screen in the rack and hoping.

It is also why the machine has more than one screen. A 2 to 4 cm feeding chop and a bedding chop under 2 cm are different jobs, and on an H-1135 an operator swaps between them in about ten minutes.

Guelph found the lever. They also found the trap at the end of it. Size matters, and so does the machine you trust to decide it.

Sixty years of the same argument

We have been building tub grinders since 1966. Sixty years, out of Jamestown, North Dakota. In that time the arguments against pre-chopping have not changed much, and neither have the results.

What has changed is that the research caught up. What a HayBuster customer could have told you from the seat of a loader in 1985, the University of Guelph has now measured with rumen boluses, blood samples and a scale under every cow.

She eats more. She eats the ration you actually built, instead of the one she assembled herself. She comes through the worst two weeks of her year in better metabolic shape. And the milk leans your way.

All of it from one decision. How short, and how consistently, you cut her feed.

Download this article

The whole piece as a print-ready PDF. Every number, every photo, both studies cited. You are welcome to share it within your company, with your nutritionist, or with your dealer. No email needed.

The research behind this article

1. Havekes, C.D., Duffield, T.F., Carpenter, A.J., DeVries, T.J. (2020). Effects of wheat straw chop length in high-straw dry cow diets on intake, health, and performance of dairy cows across the transition period. Journal of Dairy Science, 103(1): 254 to 271. University of Guelph, Elora Dairy Research Facility. The Materials and Methods section names the machine used to chop the straw: a HayBuster tub grinder, Jamestown, ND. doi:10.3168/jds.2019-17033
2. Coon, R.E., Duffield, T.F., DeVries, T.J. (2018). Effect of straw particle size on the behavior, health, and production of early-lactation dairy cows. Journal of Dairy Science, 101(7): 6375 to 6387. doi:10.3168/jds.2017-13920
3. Havekes, C.D. and DeVries, T.J. (2020). Particle size matters for high-straw dry cow diets. Progressive Dairy, via Cornell Cooperative Extension. Read it
4. Penn State Extension. Penn State Particle Separator and Total Mixed Rations for Dairy Cows. Read it
5. NDSU Extension. Alternative Feeds for Ruminants and Develop Strategies to Reduce Feed Waste. Read it

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