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In the first week of November, I received four different calls regarding how to control wild turnip in forages. The situations varied as they were in different forage crops and found in various forage growth stages, but each question dealt with the same weed, wild turnip. Since this weed quickly became an important topic of discussion, I decided that a Timely Information sheet describing the plant, growth habit, seedling and mature characteristics, and toxic properties may be helpful.
Wild turnip is synonymous with Birdsrape mustard, Argentine rape, canola, colza, field mustard, Polish rape, rape, rapeseed, turnip and Brassica napus (L.) Koch. It is in the Brassicaceae family which also contains wild garlic, wild mustard, and sheperd's purse as well as cabbage, kale, and cauliflower. The plant characteristics section is derived from Weeds of the South 2009.
Habitat and Origin
It is an erect, winter or summer annual herb; to 31.5 in tall; fields, pastures, and waste sites; introduced and cultivated as a commercial crop; native of Europe.
Cotyledons rounded to heard-shaped; leaves alternate, pubescent
Mature Plant Characteristics
Roots: fibrous from taproot; Stems: 12 - 31.5 in tall, usually branched at tip, sparsley pubescent; Leaves: alternate, 3 – 12 in long, 1 – 4 in wide; basal and lower leaves pinnately dissected, with 5-11 rounded segments; upper leaves entire to coarsely toothed, clasping, auriculate, progressively smaller apically; Inflorecences: elongate raceme, 4-20 brances; petals 4; 0.5 – 0.8 in wide, obovate, yellow; bud clusters compact, below uppermost florets. Fruits: slender pod, 1.8 – 3.0 in long, ascending, quadrangular, beak flattened. Seeds: round, 0.05 – 0.08 in diameter, smooth; color variable from yellow to yellowish brown, dark brown, or bluish black.
Special Identifying Features
Erect winter or summer annual; leaves clasping stem; flower yellow; 4-20 floret branches per plant.
Two types of sulfur-containing compounds limit the feeding value of these brassica crops which are glucosinolates and an amino acid, S-methylcysteine sulfoxide (SMCO). It is generally believed that the growth performance of ruminants grazing on brassicas is lower than predicted from their nutrient content due to SMCO.
When ruminants diet consists mainly of brassicas, they may develop a severe hemolytic anemia after 3-4 weeks, which shows Heinz bodies in the first clinical signs. This is the appearance of stainable granules within the red blood cells. In brassica anemia, the hemoglobin level falls from a normal level of about 11 g/100 ml to 8 g or lower. If the animals are removed from the brassica pasture, the hemoglobin levels return to normal in 3-4 weeks and the Heinz bodies disappear. If animals are left on the pasture, surviving animals make a spontaneous but incomplete recovery, followed by cycle of anemia and partial recovery. Other clinical signs include loss of appetite, diarrhea, and jaundice (Cheeke 1998).
The SMCO is a fairly rare aminio acid and is found mainly in brassicas, garlic, and onion which are all weeds with which we try and control during the late fall and early Spring (Benevenga et al. 1989). Cattle are much more sensitive to brassica-induced anemia than are sheep and goats (Greenhalgh et al. 1969), while non-ruminants are not affected because the SMCO is absorbed in the intestine, anterior to the site of microbial fermentation, the hindgut.
Not only is controlling brassicas early in the season while they are small easier to do, but it is physiologically better in order to decrease the chances of toxicity. The SMCO content of brassicas tends to increase with plant maturity. Through the winter, the toxicity increases. Nitrogen fertilization tends to increase the SMCO content. Forage brassicas have the advantage of plant breeding methods to develop cultivars low in SMCO. However, brassica weeds don't have this option. They will continue to grow from the original populations found in the field if not controlled.
Brassicas, particularly wild radish, but also wild mustard and wild turnip are found throughout Alabama. The best time to control these plants are when they are small and actively growing. If spraying them in the cotyledon up to 4 inches, 2,4-D ester or amine can easily control them. The school of thought is that 2,4-D is better on wild mustard and dicamba is better on wild radish. You can use combination products that contain both 2,4-D and dicamba for effective control of all of the brassicas and still be cost effective. However, this means spraying in the fall, oftentimes when grass is just getting established. Spraying 2,4-D is not a problem in an established pasture or hayfield, but it will cause injury to a newly established field. If a field has just been planted, it is better to wait until the brassica has grown some and the grass has gotten established (usually tillered), prior to making a herbicide application. By the time a grass is established, 2,4-D will not be enough to control the brassica. It will be early Spring before it is established and the Brassicas will be bolting so it will be necessary to use a stronger herbicide such as GrazonNxt, Pastora, Grazon P + D, Chapperell, or some other herbicide that can control annual, biennial, and perennial weeds.
Benevenga, N.J., G. L. case, and R. D. Steele. 1989. The relative feeding value of kale (Brassica oleracea) containing normal and low concentrations of S-methyl-L-cys-teine sulphoxide (SMCO). J. Agric. Sci. 102:635-643.
Bryson, C.T. and DeFelice, M.S., Ed., Weeds of the South. Athens and London: University of Georgia Press. 2009. Print.
Cheeke, P.R., J. S. Powley, H.S. Nakaue, and G. H. Arscott. 1983. Feed preference responses of several avian species fed alfalfa meal, high and low saponin alfalfa, and quinine sulfate. Can. J. Anim. Sci. 63:707-710.
Greenhalgh, J.F.D., G.A.M. Sharman, and J.N. Aitken. 1969. Kale anemia. I. The toxicity to various species of animal of three types of kale. Res. Vet. Sci. 10:64-72.
Timely Information number 15 - Brassicas.docx
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