Potential of Trap‐Crop Radish for Fall Lamb Grazing

After small grains are harvested as hay, grain, or silage in many irrigated areas it is possible to produce a second crop. Brassica species and radish ( Raphanus sativus L.) establish quickly and produce high‐quality fall and winter forage. Recently, nematode‐resistant radish cultivars (trap crops) have become available and studies show that they are an effective alternative to nematicides for control of the sugarbeet nematode (SBN, Heterodera schachtii Schmidt). A 2‐yr study was conducted to determine the potential of allowing lambs ( Ovis aries ) to graze nematode‐resistant radish. At Powell, WY, ‘Adagio’ trap‐crop radish and ‘Purple top’ forage turnip ( Brassica rapa L.) were grown in a replicated and randomized complete block design. Studies lasted 10 wk in 1994 and 12 wk in 1995. Lambs either grazed radish or turnip (a host of the SBN) continuously or they grazed radish and turnip, then were transferred to a feedlot diet. Additional groups of lambs received the feedlot diet for the entire study period. At Manderson, WY, 50 lambs were assigned to each replicate of radish for a 20‐d period in 1994. SBN‐resistant radish was comparable to SBN‐susceptible turnip in forage production, quality, and acceptability and weight gain by fall‐grazing lambs. Lambs grazing radish and turnip, without concentrate, produced acceptable market size and carcass grade; but they required longer than feedlot lambs to reach similar weight. Grazing of trap‐crop radish shows promise for recovering expenses of growing the crop, providing further incentive for including this crop in sugarbeet ( Beta vulgaris L.) rotations. Research Question Sugarbeet nematode‐resistant radish trap crops are substituted for nematicides in several western states. Grazing these crops would be highly desirable because fall grazing does not influence pest control. However, the effect of anti‐quality components associated with nematode resistance, on animal performance is unknown. This study was conducted to determine the effect of fall grazing on lamb performance. Literature Summary Nematode‐resistant trap crops, recently developed in Europe, provide an environmentally safe alternative to pesticides for controlling the sugarbeet cyst nematode in the U.S. Anti‐quality components (glucosinolates and S‐methyl cysteine sulfoxide) associated with nematode resistance are greater in nematode‐resistant than in susceptible cultivars. Favorable lamb performance was obtained in grazing studies in Ireland and in the U.S. (Washington) with nematode‐susceptible fodder radish. However, results of grazing studies with newly developed nematode‐resistant trap crop cultivars have not been reported. Study Description This field study was conducted on the Univ. of Wyoming Research and Extension Center in Powell, and at the Mosegard Farm near Manderson, WY, during 1994 and 1995. Both sites are in the sugarbeet‐growing area of northwestern Wyoming. Nematode‐resistant ‘Adagio’ radish and nematode‐susceptible ‘Purple top’ turnip were planted in a pea‐oat stubble at Powell, and Adagio radish was planted in a barley stubble at Manderson. Radish and turnip were planted at Powell on 20 July 1994 and on 2 August 1995, and at Manderson on 4 August 1994 and 9 August 1995. Both sites were furrow irrigated. Soil type was a Garland clay loam at Powell and a Lostwells clay loam at Manderson. Forage samples were collected before and after grazing so we could determine utilization. Radish and turnip shoots and turnip fleshy roots were sampled and dry matter yield and forage composition determined. At Manderson, 50 lambs were assigned on a weight basis, to each of three replicate radish plots. Lambs grazed during the fall of 1994 only. At Powell during 1994 and 1995, 15 lambs (three replicates) either grazed radish or turnip the entire grazing period or were transferred to a feedlot diet after 5 wk in 1994 or 6 wk in 1995. Additional groups of lambs received a feedlot diet for the entire study period. Water and mineral supplement were offered free choice. Applied Question What is the performance of lambs grazing sugarbeet nematode‐resistant radish? Radish shoot yield prior to grazing ranged from 1.3 to 2.5 tons dry matter/acre, compared to 0.9 to 1.5 tons/acre for turnip shoots. Crude protein prior to grazing ranged from 11.0 to 20.9% in radish, compared with 11.5 to 12.1% for turnip. Lambs utilized radish and turnip forage similarly. Average daily gains at Powell in 1994 were 0.28 lb for radish, 0.26 lb for turnip, 0.51 lb for feedlot, 0.39 lb for radish‐feedlot and 0.43 lb for turnip‐feedlot diets. Average daily gains for the same diets in 1995 were 0.32 lb for radish, 0.37 for turnip, 0.33 for feedlot, 0.28 for radish‐feedlot, and 0.30 lb for turnip‐feedlot. Lamb performance at Manderson for radish grazing was 0.37 lb/d. Lamb carcass weight was greater for feedlot lambs than for lambs grazing radish or turnip in 1994. There Full scientific article from which this summary was written begins on page 559 of this issue. were no differences among diets in carcass weights in 1995. Dressing percentage was greater for lambs on feedlot diet and least with lambs grazing turnip in 1994. There were no differences in 1995. Fat depth of lambs in the feedlot was greater than for lambs grazing turnip in 1994; however, there were no differences in 1995. Lambs grazing radish and turnip produced carcasses with acceptable yield and quality grade without need of grain. Recommendations Trap crop radish can be planted as a second crop in late July to early August and used to finish lambs in late fall and early winter. Desirable lamb gains can be expected, reducing animal feed costs while controlling the sugarbeet cyst nematode. Nematode‐susceptible forage turnip (host) should be replaced with nematode‐resistant radish to control the sugarbeet cyst nematode in sugarbeet rotations.