Agronomically-desirable nematode-resistant sugar beet cultivars are unavailable. In Wyoming and other areas sugar beets are grown in two- or three-year rotations. Longer rotations, which would help in the control of SBN, are not economically practical due to lack of adapted and profitable crops. SBN-resistant "trap crop" varieties are available and their use is currently a major control practice in Europe (Peterson 1992). Seed of nematode-resistant radish (Raphanus sativus L.) and mustard (Sinapsis alba L.) has recently become available in the United States. Technology for using these cultivars in United States sugar beet rotations has not been adequately developed. In western Nebraska, trap crop cultivars grew best when planted between mid-May and mid-Aug., when soil temperatures at 15 cm were above 20°C (Wilson et al. 1993). A major limitation of this method is the limited amount of time and growing degree days following harvest of the main crop. Full-season production is probably not justified economically as a routine practice. The objectives of this study were to determine the potential for biological control of SBN with SBN-resistant cultivars and the potential for utilizing these crops for fall grazing.
Another cooperator's field (Mosegard) was used to study the effect of grazing the SBN-resistant radish cultivar, `Adagio', on SBN control. Malt barley was harvested Aug. 2, 1994. The field was burned and on Aug. 4, seed (22 kg ha-1) was mixed with fertilizer (67 kg ha-1 N as ammonium nitrate) and applied with a spinner-broadcast type applicator. Replicated strips were left unseeded. The field was roller-harrowed to cover the seed and firm the seedbed. The field was furrow-irrigated twice. Sethoxydim at 0.31 kg ha-1 a.i. and oil concentrate at 2.3 l ha-1 was applied for the control of volunteer barley on Aug. 27, when barley was about 10 cm in height. The experimental design was a randomized complete block with five replications. Treatments were: (1) unseeded and ungrazed; (2) radish-seeded and fall-grazed; and (3) radish-seeded and fall growth plowed down, rather than grazed. Following evaluation of radish stand and growth, 150 lambs averaging 33.1 kg grazed on 1.8 ha from Oct. 13 until Nov. 12. Lambs were weighed to determine daily weight gain and gain ha-1. Following grazing the entire field was moldboard plowed. Sugar beets were planted on the entire field in 1995 with no nematicides applied to the study area and sugar beet yields and quality components were evaluated.
On the above two test sites, populations of SBN were determined before and after the radish crop, at sugar beet planting, at mid-season and at sugar beet harvesting. Two hundred cm3 of soil was processed through an elutriator and cysts, eggs, and juveniles counted. SBN counts were adjusted for efficiency of extraction and nematode population data were subjected to covariance analysis.
Trap crop mustard was evaluated for grazing potential at the Torrington Research and Extension Center, where there are no SBN. In 1993, following harvest of feed barley, loose straw was removed and `Metex' mustard was planted on Aug. 16 at 21 kg ha-1 with a disk drill into stubble averaging 20 cm in height. Soil analysis showed low phosphorus, therefore, 72 kg N and 93 kg P ha-1 were applied. The field was divided into three seeded blocks (0.15 ha each) and three unseeded blocks (0.05 ha each). Lambs grazed 0.11 ha of each seeded block. Eight lambs grazed each block from Oct. 29 until Nov. 12 and weight gain was determined. The entire field was spring-plowed and uniformly planted to sugar beets in 1994.
In 1994 the same grazing study was repeated on a different field of the Torrington R & E following winter wheat (Triticum aestivum L. emend. Thell) harvest. The methods were the same, except that phosporus fertility was higher and no phosphorus fertilizer was applied. Planting date was on July 26 and plots were grazed Nov. 4 to 14.
On the Hefenieder field SBN populations at sugar beet planting in April 1993 were below the estimated damage threshold (EDT = 2.8 eggs and/or juveniles cm-3) (Griffin 1981) (Table 2). SBN population remained lower than with the unseeded fallow treatment throughout the sugar beet season. At mid-season (July) the SBN population was 26% lower for radish-seeded, compared to conventional fallow plots. At sugar beet harvest in Sept. the SBN population was significantly (25%) lower for radish-seeded compared to unseeded plots. Aldicarb, band-applied along the seed row at sugar beet planting, did not significantly reduce SBN population, even at the full-label rate. While aldicarb application did not increase sugar beet yield, radish inclusion in the rotation the previous year increased sugar beet yield 8.78 Mg ha-1. Estimated cost of growing SBN-resistant mustard and radish is $192 and $206 ha-1. The yield increase with radishes on this field was well above the estimated 4.9 Mg ha-1 needed to offset the cost of growing radishes, based on the average price of sugar beets, $45.83 Mg-1, over the past five years.
Both trap-crop cultivars are high in forage quality and are readily consumed by lambs (Table 3). 'Metex' mustard is capable of high yields of topgrowth, as evidenced by the 1994 planting at the Torrington Research and Extension Center. Performance (average daily gain) was greater with mustard than with radish. Gains on mustard were nearly comparable to gains on hay and grain diets. With all three grazing studies the value of lamb produced ha-1 was greater than the cost of growing the crops. On the Mosegard field there was no difference in SBN populations with grazing vs plowdown of radishes. Radishes were not grazed until growth had stopped and soil temperatures were too low for SBN activity.
|Location/year||Preceeding crop||Trap crop planted||Planting date||Top growth (Mg/ha)||Growing degree days (base 4.4°C)||SBN reductionz (%)|
|Hefenieder/1992||Malt barley||Pegletta radish||July 28||1.56||1758||54|
|Mosegard/1994||Malt barley||Adagio radish||Aug. 4||2.52||1658||71|
|Torr. R&Ey/1993||Feed barley||Metex mustard||Aug. 12||1.90||1279||--|
|Torr. R&E/1994||Winter wheat||Metex mustard||July 26||8.56||2168||--|
|Sugar beet nematode population|
(No. eggs and/or juveniles cm-3 of soil)
|Treatment||Aug. 1992||Oct. 1992||Apr. 1993||July 1993||Sept. 1993||Sugar beet yield (Mg/ha)|
|Unseeded (fallow)||2.04 az||1.93 a||2.55 a||4.66 a||6.83 a||31.72 b|
|Radish seeded||2.90 a||1.33 a||2.48 a||3.43 a||5.10 b||40.50 a|
|Untreated (no aldicarb)||--||--||2.33 a||4.33 a||6.19 a||37.78 a|
|Half rate of aldicarb||--||--||2.78 a||4.30 a||5.39 a||35.16 a|
|Full rate of aldicarb||--||--||2.44 a||3.52 a||6.66 a||35.38 a|
|Forage yield (Mg/ha)|
|Crop||Totalz||Trap crop||Crude protein (g/kg)||Utilization (g/kg)||ADGy (kg)||Lamb gain/ha (kg)||Value of lambx ($/ha)|