Department of Plant Sciences, University of Wyoming, Laramie, WY 82071-3354, USA

Background and objectives
The sugar beet nematode (SBN), Heterodera schachtii, is one of the most damaging pests of sugar beet, Beta vulgaris, in the USA. SBN is currently controlled with a combination of a 2- to 3-year crop rotation and the application of a nematicide or soil fumigant. However, due to public concern about environmental contamination from pesticides, there is an urgent need for alternative nematode control methods. Longer rotations out of sugar beet may not be economical in Wyoming due to the lack of other adapted and profitable cash crops. Recent breeding programmes in Germany have resulted in the development of nematode-resistant trap-crop cultivars of radish, Raphanus sativus, and mustard, Sinapis alba, which are currently utilized in several European countries [1]. The primary objective of our studies was to determine the overall feasibility of using German-bred trap-crop radish in sugar beet rotations in Wyoming.

Materials and methods
Experiments were located on eight sugar beet fields. Following the harvest of primary rotation crops, plots were either seeded with Adagio radish or left unseeded (main plots) and sugar beet planted the following spring. Subplots the following spring consisted of three rates of aldicarb. Aldicarb rates were 0, 15.7 kg/ha (half labelled rate) and 31.4 kg/ha (full labelled rate). The granular formulation was banded beside the row at sugar beet planting. In one experiment, radish plots were either grazed with lambs or ungrazed.

Results and conclusions
Sugar beet yield was significantly increased from trap cropping at three locations. Yield increase occurred following corn, Zea mays harvested as silage (5.8 Mg/ha increase; Pi=1.5, 622 growing degree days (GDDs), base 4.4 C, and following malt barley, Hordeum vulgare, (8.7 Mg/ha increase at one site; 977 GDDs, Pi=2.9) and (10.9 Mg/ha increase at another site; 918 GDDs, Pi=14.6). Significant, but lower beet yield increases with aidicarb occurred at two of the three locations (3.0 and 4.0 Mg/ha). Volunteer barley following a hail storm near harvest at one location, and lack of available irrigation water after seeding at another location, resulted in failure to obtain stands of radish. Also, at one site sugar beet yields were very low due to severe hail. In general, trap-crop radish was more productive and reduced SBN to a greater extent when grown following malting barley than following corn or dry beans (Phaseolus vulgaris). At sites where irrigation water was available, volunteer barley was controlled, and GDDs exceeded 621, significant and economic increases in sugar beet yields were obtained. Also, lambs grazing radish prior to plough down gained 0.18 kg/day with no detrimental effect on nematode control. Value of lamb gain (251 kg/ha) offset costs of growing and grazing trap-crop radish.

1. Cooke D, 1991. British Sugar 59, 44-47.