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Forage Nutritive Value and Palatability of 12 Common Annual Weeds¹

Our objective was to determine whether or not some broadleaf (forb) and grass weeds that commonly are found in newly established stands of perennial forage crops have sufficient quality to be considered satisfactory forages.

We determined the relative palatability of 12 weed species in comparison to oats (Avena sativa L.) in a grazing study with sheep. In each of 3 years, we estimated, by laboratory analyses, the nutritive value of these 12 weed species grown in field studies.

Six weed species, yellow foxtail [Setaria glauca (L.) Beauv.J, barnyardgrass [Echinochloa crusgalli (L.) Beauv.], green foxtail [Setaria viridis (L.) Beauv.], redroot pigweed (Amaranthus retroflexus L.), Pennsylvania smartweed (Potygonum pensylvanicum L.), and common lambsquarters (Chenopodium album L.) were as palatable to sheep as was oats. Four weed species, giant foxtail (Setaria faberi Herrm.), wild mustard [Brassica kaber (DC.) L.C. Wheeler var. pinnatifida (Stokes)], giant ragweed (Ambrosia trifida L.), and common cocklebur (Xanthium pensylvanicum Wallr.) were unpalatable. Two species, common ragweed (Ambrosia artemisiifolia L.) and velvetleaf (Abutilon theophrasti Medic.) were classed as interacters; i. e. some sheep found them palatable, whereas other sheep refused to graze them. Palatability was not associated with nutritive value, indicating that sheep lacked "nutritional wisdom."

Redroot pigweed, common lambsquarters, and common ragweed had nutrient composition and digestibility essentially equivalent to that of high quality alfalfa (Medicago sativa L.). Giant foxtail and Pennsylvania smartweed were less digestible than alfalfa. Giant foxtail, yellow foxtail, and barnyardgrass contained more acid detergent fiber and less crude protein than alfalfa. Ten of the 12 weed species were more digestible than oats forage. Nine of the 12 contained more crude protein than oats.

All of the 12 weed species contained adequate minerals to meet the requirements of ruminants, although the species differed considerably in mineral concentration. The grasses contained less than half of the calcium of the forbs, and the grasses usually contained less magnesium than the forbs.

Tests for anti-quality components (alkaloids, nitrate, and mineral unbalances) revealed only a few potential animal toxicities due to mineral imbalances of some of the weeds if they were fed as sole rations.

We conclude that some of the common annual weeds found in new stands of perennial forages growing in fertile soils do not decrease the nutritive value of hay or pasture if utilized at relatively early stages of maturation.

Key Words: In vitro digestible dry matter • Alfalfa • Oats forage • Forage establishment • Grasses • Forbs • Minerals • Fiber • Lignin • Silica

² Research agronomist and plant physiologist, ARS-USDA, and professors of agronomy, Agronomy and Plant Genetics Dep., Univ. of Minnesota, St. Paul, MN 55108.

Received for publication April 9, 1975.

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