Forages and Weeds

Jerry Doll Extension Weed Scientist

Forage harvest in 2002 is here (I saw the first field of alfalfa cut in Dane Co. this week). Some fields are abundant with yellow (dandelion or yellow rocket) or white (shepherd's purse or pennycress) flowers. While it is too late to do anything to suppress these weeds now, some attention to their impact on forage quality and drying time is in order.

As a general rule, weeds in the mustard family (yellow rocket, pennycress, shepherd’s purse) are relatively low in feed value compared to alfalfa. When alfalfa is harvested in the bud stage, weeds are less detrimental to feed value than when alfalfa is harvested in the early bloom stage. In general, protein content and digestibility of forage weeds decline more rapidly as plants mature than they do in alfalfa. Thus it makes sense to harvest weedy field first because weeds will have less impact on forage quality if harvested sooner rather than later.

Dandelions seem particularly abundant in some areas this year. Many are in the seed stage in southern Wisconsin now. Any plant that has flowered had dropped considerably in protein content and digestibility. For dandelions, it is the drop in protein content that is most dramatic. For four years I sampled the dandelions in the windrows of alfalfa in the first, second and third cuttings at Arlington. Dandelions averaged 4% less protein than alfalfa in the first cutting but their protein content was equal to or greater than alfalfa in the later harvests. The protein content of the lower dandelion leaves not cut by the cutter bar and left in the field was well below that of the dandelions in the windrow which is not surprising as these would be the oldest leaves on the plant.

What impact will dandelions have on overall protein content in the forage? Dandelions (nor other forage weeds) do not normally decrease total biomass so the return on investment has to come from improved quality. Let’s assume dandelions comprise 25% of the harvested biomass (such a field would look very yellow) and that the yield is 3 ton/acre of dry matter in the first cutting. One-fourth of the biomass (dandelion) has 18% protein and the alfalfa has 22%. If we had pure alfalfa at 22% protein, we’d harvest 660 lb/a protein. But the 25% dandelion component has 18% protein (135 lb/a) and the 75% alfalfa component has 22% protein (495 lb/a) for a total of 630 lb protein/a. How much does it cost to replace 30 lb of protein with a protein supplement? Much less that the $20 to 25/acre it would cost to control the dandelions. Thus we are often left with two choices in forages when dandelions are the dominant weed: live with them or rotate the field to corn for a year or two.

Lastly, we have known for several years that dandelions are wetter than alfalfa at the time of harvest. This is also the case for white cockle, curly dock, and broadleaf plantain. This could cause forage in heavily infested fields to dry more slowly if the crop will be handled as dry hay. Dandelions and other weeds should not increase the drying time needed to make silage. Harvesting weedy fields as silage rather than hay eliminates the risk that weeds will delay the drying time and it seems that some weeds are more palatable when handled as silage than as dry hay.

May 2002