Effects of Annual Weed Control on Alfalfa Forage Quality

Establishment of alfalfa (Medicago sativa L.) alone rather than with an oat (Avena sativa L.) companion crop usually results in a reduction in dry matter production. Use of herbicides far weed control during alfalfa seedling establishment also reduces yield since weeds are harvested along with the alfalfa and contribute to forage yield. Thus, if there are benefits from seeding alfalfa alone using herbicides for weed control they would only be expressed in terms of animal consumption, nutritional value, digestability, and finally animal growth or milk production. Consequently, a study was conducted to determine the benefits of annual weed control during alfalfa establishment. Alfalfa was sown into field test plots (Plano silt loam, 3.5 to 4% organic matter, Typic Argindoll, fine, silty, mixed mesic) in April 1976 with an oat companion crop and alone with and without herbicide treatments. Herbicide treatments included were S-ethyl dipropylthiocarbamate (EPTG) + 4-(2,4-dichlorophenoxy) butyric acid (2,4-DB), and Nbutyl-N-ethyl-α,α,α-trifluoro-2,6-dinitro-p-toluidine (benefin). Weeds were also allowed to grow in the absence of an alfalfa or oat seeding. The oat companion crop was harvested at the early dough stage and the alfalfa treatments at first flower, all as chopped hay. The stand of weeds was harvested at the same time as the alfalfa. The oat hay, weeds, and alfalfa established with and without herbicides, were analyzed in laboratory studies for in vitro digestible dry matter (IVDDM), neutraldetergent fiber (NDF), aciddetergent fiber (ADF), crude protein, and nitrate. A sheep feedingdigestion trial was used to determine the palatability, digestible dry matter (DDM), digestible crude protein (DCP), digestible neutraldetergent fiber (DNDF) and digestible aciddetergent fiber (DADF) of the forages, and N balances of the sheep fed the forages. The four forages were ranked as follows in order of increasing quality as measured by animal intake and digestible dry matter: oat, hay, weed forage, untreated alfalfa, herbicide-treated alfalfa. The lower quality of the untreated alfalfa in comparison with the herbicide treated alfalfa was attributed to the fact that weeds constituted 50% of the dry weight of the untreated alfalfa stituted 50% of the dry weight of the untreated alfalfa forage. Chemical analysis of the weeds indicated that the Pennsylvania smartweed (Polygonum pensylvanicum L.), she herds-purse (Capsella bursa-pastoris (L.) Medic), and yelpow foxtail (Setaria lutescens (Weigel) Hubb.) had the greatest influence in lowering the forage quality. These three weeds, which composed 25% of the untreated alfalfa forage, had the lowest % crude protein and IVDDM and the highest % CWC and ADF. The digestion trial also showed the annual weeds, as a whole, to be the lowest in DCP, DNDF, and DADF. These results confirm that seeding alfalfa alone using herbicides for weed control is beneficial, and that feeding forage thus produced will improve livestock performance.