Q. I keep my horses on a small, 5-acre property. This summer my pastures have grown a lot of weeds. I’m not really sure what has changed—I bought the property two years ago, and last summer the weeds weren’t so bad. Where would the weeds have come from, and why are they getting worse?
A. Pasture weeds can be a frustrating problem. Not only are some toxic to horses but they also gradually choke out the desirable grasses and reduce the pasture’s nutritional value. Additionally, some weeds are high in sugars and not suitable for horses with metabolic conditions.
Where Weeds Come From
Weeds can arrive from several means. Some seeds travel long distances on the wind, while others might drop in the pasture with bird or animal feces or fur. Some seeds might also have come from the horse’s hay or forage, either passing through their digestive tract and out in feces in a still-viable form, or from hay that contains weeds, especially when that hay is fed loose on the ground. This is just one reason to check your forage for weeds.
It’s unlikely for seeds to come from grain mixes or concentrate feeds. Producers typically heat pellets to a point where any seeds are denatured, and straight grains such as oats and barley are screened for weed seeds. In fact, contamination level related to undesirable seeds is just one criterion by which grain is graded and priced once harvested.
Because you often can do little to prevent weed seeds from arriving in your pastures, it becomes a matter of managing your pastures so weeds are less able to become established. Weeds will often grow in areas that hold water. Check how level your pastures are and whether the weeds are growing in uneven areas. If this is the case, then you might want to rework that area of your pasture and reseed it with grass once you’ve leveled it or improved the drainage.
Certain soil conditions, especially poor soil, might also favor weeds. Think how well weeds do on the sides of freeways, where the soil is never fertilized or amended. You might want to test your soil and determine how to best fertilize and manage it to help establish strong grass that’s less favorable to weeds.
However, the main reason weeds become established is due to overgrazing. Overgrazing is a sign that you haven’t given the pastures enough time to recover from grazing stress. Always move horses off a pasture before the sward height (height of the grass in the pasture) gets below 4 inches. This leaves enough grass leaf to continue photosynthesizing and generating energy to use for growth. If grazed below this, the plant will have to dip into energy stores it has saved in its roots. Over time, this causes the roots to shrink, which in turn shrinks the overall plant size.
Without adequate leaf material, bare patches of ground appear between individual grass plants, and this enables weed seeds to germinate and grow. Often weeds have broad leaves that create shade, resulting in the grass having inadequate sunlight access. Sunlight is necessary for photosynthesis and energy production; when limited, the grass withers further.
Make sure you walk your pasture and look down on the ground, not just across it. Pastures will often appear quite thick when you look over them but looking down on the same pasture might reveal bare earth between grass plants. This means the pasture needs to rest.
Also check that your horses are grazing the pastures evenly rather than eating the same short grass and leaving large patches of long grass. This is another sign of overgrazing. Horses prefer shorter grass because it’s sweeter, and they will keep eating it in preference to the longer grass. To avoid this scenario, increase the number of horses grazing a pasture so they can’t be as picky, but remove them before the average pasture height gets below 4 inches. Alternatively, accept that you will need to mow the long areas to “freshen” up the pasture and then give it a good rest. Ideally, you want to rest your pastures until they have 8 to 12 inches of growth.
As far as the weeds already in your pasture, it’s important to identify what they are so you can determine how best to remove them. If you can’t identify the weeds, see if you can take a sample or send pictures to your local university extension office. Your extension agent should be able to advise you on the best way to eradicate common weeds in your area.
If the number of horses grazing an area is high, more even grazing will result, but you will need to remove more frequently to ensure that they don’t consume too much of the pasture grass.