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Weeds are coming for their pound of milk

Albert Adjesiwor for Progressive Forage Published on 28 September 2021
A weedy cornfield

If you have ever driven through southern Idaho in the summer, you will notice there is alfalfa and corn everywhere.

In fact, Idaho is the leading producer of alfalfa hay in the U.S. If you are like me (always looking at crop fields when driving), you would also notice most of the corn and alfalfa have a lot of weeds hiding in the canopy. This doesn’t surprise me, unsurprisingly.

One of the things weeds are really good at is adapting to whatever we throw at them. Yes, that’s right: Throughout human history, weeds have outlasted any control practice thrown at them. The dandelion tries to grow as close to the ground as possible to avoid the cutting blade of the mower, cheatgrass flowers early in the spring before you can even get out to spray it, lamb’s-quarters and pigweeds flower when they are just 2 inches tall so you can’t see them in the crop canopy, several weeds developed resistance to most of our weed killers and the list goes on.

Knowing weeds would almost always be present in crops, the question hay producers often ask is: Should we harvest the weeds? How much weed biomass is too much? We know weeds are bad for farm animals, but is there a tipping point? The short answer is: I don’t know. I have been asking these same questions for years. I did some internet searches on work done on this subject, but it turns out not a whole lot has been done. At first, I was shocked – but then I convinced myself that, just maybe, forage folks don’t like weed folks.

Weeds in a field

What I would attempt to do here is share one of the most interesting research projects conducted over the years regarding how weed biomass in alfalfa hay can reduce forage quality and value. Some work done in Minnesota years ago compared the forage value of weed-free alfalfa to alfalfa hay contaminated with seven different weeds. Five (redroot pigweed, lamb’s-quarters, yellow foxtail, giant foxtail and barnyardgrass) out of the seven weeds are very common in Idaho, so I thought this is most applicable to Idahoans. I did some simple math and realized that, on average, the weed biomass mixed with alfalfa reduced protein by 43% and increased fiber by 30% compared to the weed-free alfalfa. When I plugged these numbers into a spreadsheet, I estimated this could potentially reduce milk production by close to 12%. These are just estimates, but we would all agree these are huge numbers. The question is: Why is this the case?

Let’s look at a few things.

  • Protein: Alfalfa has a fairly high amount of protein. This is also true for some weeds (like common lamb’s-quarters and kochia). However, it rarely happens that there’s only one weed present in your hay. Most weeds, especially grassy weeds, have very low protein. If your hay has a lot of grassy weeds like foxtail, cheatgrass or barnyardgrass, expect a huge reduction in protein.

  • Fiber: When weeds are very small, they are soft and have very low fiber. By the way, this is also true for alfalfa. As the weeds grow bigger, the stem component increases relative to leaves. This increases the amount of fiber in your hay. One thing worth mentioning is: Weeds can grow really fast, so this increase in fiber can happen rather quickly. If your hay is mixed with a lot of those high-fiber weed stems, expect your fiber to increase. This would eventually ding your forage digestibility. Corn chopped for silage is harvested late in the summer, and this gives weeds a lot of time to grow big and tall. Thus, it is more likely weeds present in corn silage will often have high fiber concentration.

It is also important to mention that weeds contribute to forage biomass or yield. Thus, if you are feeding your hay to your cows, it ultimately comes down to quantity versus quality. However, not all weeds are created equal, so it is important to know what kind of weed you are dealing with.

Some weeds can also poison your livestock, so there’s that too. Regardless of what type of weed you have, it is important to “start clean.” Forage crops like corn and alfalfa are very competitive with weeds, but it is important to give them a head start by making sure winter weeds like cheatgrass, buttercup, shepherd’s-purse, china lettuce and early emerging spring weeds (like kochia and common lamb’s-quarters) are controlled. Conventional hay growers have a lot of herbicide options to get the job done. For organic growers, spring cultivation can help to a certain extent. If you don’t start clean and stay clean, the weeds will always get their pound of milk and meat.  end mark

PHOTO 1: A weedy cornfield near Kimberly, Idaho, shows what can hide beneath the canopy.

PHOTO 2: Weeds not only affect palatability; they will also increase fiber concentration and lower forage digestibility. Photos by Albert Adjesiwor.

Albert Adjesiwor
  • Albert Adjesiwor

  • Extension Weed Management
  • University of Idaho Kimberly Research & Extension Center
  • Email Albert Adjesiwor

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