As dandelions bloom in spring, many farmers ask if they should control weeds in established alfalfa stands. The answer is usually “no” if you’re going to feed the hay or haylage yourself and probably “yes” if you’re going to market the hay. Hay marketers are paid premiums for pure alfalfa; those growing and feeding alfalfa can use the energy and protein whether it comes from alfalfa or weeds.

Weeds generally don’t cause stand thinning; they come in to fill holes in alfalfa stands. Thus, when you see dandelions flowering in a field and spray to remove them, those holes remain.

Controlling weeds in grain crops gives us a yield benefit. But with forages, since we harvest the total biomass, we get little or no yield benefit, as the accompanying table shows. Yet a herbicide application did increase the percentage of alfalfa within the alfalfa-quackgrass mix. That improved crop quality by lowering neutral detergent fiber (NDF) and raising the forage’s protein content.

Some winter-annual weeds, such as chickweed, field pennycress, pepperweed, wild garlic and wild onion, however, can thin alfalfa stands. Winter annuals germinate in fall, develop rosettes (like dandelion leaves) and overwinter. In spring they rapidly grow upright stems (bolting) and produce seed. Once plants bolt, herbicides aren’t very effective and stands can thin.

A fall dormant application of herbicide should be made because winter-annual weeds start growing earlier in spring than alfalfa. By the time weed and alfalfa growth are seen in spring, it’s too late to chemically control weeds. They’re mainly a problem if an alfalfa stand is not thick and growing well. If you have a problem, scout the field in fall and then apply a dormant application of herbicide.

The alfalfa-quackgrass table shows that controlling weeds in established stands can improve forage quality. Broadleaf weeds are generally high in quality at immature stages but mature faster than alfalfa and will be low in forage quality when alfalfa is harvested.

Grasses generally have higher fiber than alfalfa and can reduce intake in a dairy ration if forage NDF is increased too much. On the other hand, some grass – up to 30% of the hay or haylage dry matter – may be beneficial for dairy. Greater amounts will likely not be a problem for dairy heifers, steers, horses or sheep, for example, which are on a lower nutrition plane.

Cut weedy fields early to minimize a forage quality reduction. If hay or haylage is harvested before a grassy weed has headed or lambsquarters or other weeds have gone to seed, the impact of weed presence can be reduced. Dandelions, often the most visible in an alfalfa field, are high in forage quality and seldom justify spraying.

Cutting weedy forage early can also reduce feed refusal. Immature lambsquarters will be consumed by cattle; mature, hardened stems will not. Weed refusals are less of a problem in chopped haylage than in hay.

Some weeds such as lambsquarters or chickweed dry slower than alfalfa and, while not a problem for haylage, they can delay hay drying. If weeds are spotty in fields, some portions of windrows will be weedy and wetter than other portions, which can lead to moldy spots in bales.

In general, if you see weeds in alfalfa, the stand has reduced yield potential and you should think about turning the stand over, taking the legume credits for the next crop and replanting a new seeding of alfalfa. If the stand has less than 55 stems per square foot, the low alfalfa stand density is limiting yield and, again, a herbicide won’t increase yield.

Herbicides can be beneficial when applied to weedy alfalfa, but first make sure that they will pay.