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Evaluation of wheat cultivars for seedling tolerance to atrazine
Field Crops Research, 14 (1986) 135--139
135
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
EVALUATION OF WHEAT CULTIVARS FOR SEEDLING TOLERANCE TO ATRAZINE
R.K. BACON, F.C. COLLINS and T.L. LAVY
University of Arkansas, Fayetteville, A R 72701 (U.S.A.) (Accepted 9 January 1986)
ABSTRACT
Bacon, R.K., Collins, F.C. and Lavy, T.L., 1986. Evaluation of wheat cultivars for seedling tolerance to atrazine.Field Crops Res., 14: 135--139. Winter wheat (Triticum aestivum L.) is sometimes planted after maize (Zea mays L.) or sorghum (Sorghum bicolor (L.) Moench) has been harvested. If the herbicide atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)~-triazine] was used on the maize or sorghum crop, the residue can reduce the stand of wheat. The objective of this research was to develop a screening technique to differentiate the seedling response of wheat cultivars to atrazine. Wheat was grown in the greenhouse in flats containing a Captina silt loam soil (fine-silty, mixed, mesic typic Fragiudult) treated with atrazine rates ranging from 0 to 1.0 mg kg-1. Survival ratings made after 4 weeks indicated that atrazine rates used to differentiate among genotypes should be less than 0.4 mg kg -1. To determine the proper concentration to use for screening cultivars, another experiment was conducted, using eight atrazine rates ranging from 0 to 0.4 mg kg-'. Based u p o n results from this study, 120 cultivars and lines were screened at 0.25 mg kg -1 atrazine for 4 weeks. 'TexRed', 'Maverick', AR 67-3-19, 'McNair 1813', and 'Potomac' had the highest survival ratings among genotypes. This screening procedure is an effective preliminary method to identify wheat genotypes with seedling resistance to atrazine.
INTRODUCTION
Winter wheat (Triticurn aestivum L.) is sometimes planted following the fall harvest of grain sorghum {Sorghum bicolor (L.) Moench) or maize (Zea mays L.). Atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine], which is commonly used as an herbicide to control grass and broadleaf weeds in both maize and sorghum, will severely injure the stand of wheat if sufficient residue persists in the soil. Sandy soils low in organic matter increase the chances of a residue problem for wheat, especially in years following a dry growing season for grain sorghum or maize (Wicks et al., 1969). Previous research has shown that cultivars of crop plants exhibit different degrees of tolerance to herbicides. Genotypic responses to atrazine have been reported in maize (Grogan et al., 1963; Anderson, 1964), cotton (Gossypium hirsutum L.) (Abernathy et al., 1979), flax (Linurn usitatissimum L.) (Ander-
0378-4290/86/$03.50
© 1986 Elsevier Science Publishers B.V.
136 son and Beherens, 1967; Comstock and Anderson, 1968), zoysia (Zoysia japonica Steud.) (Engel et al., 1968), sugarcane (Saccharum ssp.) (Peng and Yeh, 1970) and oats (Arena sativa L.) (Brinkman et al., 1980). Inheritance of tolerance to atrazine varies among crop species (Grogan et al., 1963; Comstock and Anderson, 1968; Machado et al., 1978). Since genetic variation for tolerance to atrazine exists in these species, it was thought likely that variation existed in wheat. The objective of this research was to devise a screening procedure that would detect genetic differences in tolerance to atrazine among wheat genotypes. Wheat cultivars and lines were then evaluated with the devised technique to identify genotypes tolerant to atrazine. MATERIALS AND METHODS Based on results from a preliminary test using atrazine rates of 0, 0.2, 0.3, 0.4, 0.6, 0.8, and 1.0 mg kg -1, an experiment was conducted to define the concentration which would differentiate wheat genotypes. Rates of 0, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, and 0.40 mg kg -1 were used to test the reaction of a seed composite of 75 hard and soft red winter wheat cultivars. The soft used was a Captina silt loam (a fine-silty, mixed, mesic typic Fragiudult) consisting of 366 g kg -1 sand, 528 g kg -~ silt, 106 g kg -1 clay. Organic matter was 9 g kg -1. At the 0.25 mg kg -~ concentration, the distribution coefficient, KD, of atrazine on soil was 0.79. Prior to the addition of atrazine, the soil was dried and screened through a 2-ram mesh sieve to ensure uniformity. Desired concentrations were produced b y applying a 0.0122 g 1-1 solution of 990 g kg -~ technical grade atrazine dissolved in methanol to 2 kg o f soil. After the methanol had evaporated, the soil was mixed thoroughly with the remaining 10.25 kg o f soil in the greenhouse flat to ensure uniform distribution of the herbicide in the soil. Temperature in the greenhouse was maintained at approximately 20°C. Soil moisture adequate for growth was maintained with periodic watering. Available sunlight (February--March) with no additional lighting was utilized. Each 48 cm × 35 cm × 9 cm flat was planted with 5 rows of the composite at the rate of 20 seeds per row. Seedling counts were made on each flat 2 weeks after planting when emergence was complete. After 4 weeks, when toxicity was more acute, survival readings were taken. A seedling was counted as a survivor if approximately 40% of its leaf area was green and the growing point appeared to be alive. Reaction to atrazine was expressed as percentage survival calculated by the number of seedlings at 4 weeks divided by the number of seedlings at 2 weeks. A concentration of 0.25 mg kg -~ atrazine was used in a cultivar screening study after this concentration was identified as the approximate threshold for 50% toxicity in wheat. One hundred and twenty cultivars and experimental lines of Triticum aestivum were tested in greenhouse flats b y planting 5 rows o f genotypes per flat at a rate o f 20 seeds per genotype. Seed for all
137
genotypes was less than 6 months old to ensure good germination of each entry. After 4 weeks, ratings for tolerance were made according to the method described earlier. The experiment was replicated four times using a randomized complete block design. Each replication consisted of 24 treated flats containing the 120 lines. A flat containing untreated soil and sown with five randomly chosen lines was placed with each replicate to serve as a control. The data from the cultivar screening were transformed using the arcsin square root transformation before an analysis of variance was conducted. All statistical information refers to analysis of the transformed data. Values reported as percentage survival were transformed back to the original scale and adjusted for bias (John and Quenouille, 1977). RESULTS AND DISCUSSION
The relationship between the concentration of atrazine and seedling survival was sigmoidal (Fig. 1). Very little damage was observed until the concentration reached 0.20 mg kg-1 atrazine; at that point 23% of the seedlings in the composite died. Seedling mortality increased dramatically until it reached 89% at the 0.30 mg kg -1 rate. Thereafter, seedling mortality increased at a much lower rate. Results of this study showed that 0.25 mg kg1 atrazine was the appropriate concentration for the soil used to approximate a 50% mortality rate for wheat cultivars. The 120 cultivars and lines exhibited significant variation (P < 0.0001) in tolerance to atrazine. The reaction of 10 of the cultivars is given in Table 1. A complete list of the genotypes' reaction to atrazine is available from the I00
8O
6O
40
2O
0
I
0
I
0.1
I
I
0.2
I
I
I
0.3
I
0.4
Concentration of Atrozine (rng kg-I)
Fig. 1. R e a c t i o n o f a w h e a t c o m p o s i t e g r o w n for f o u r w e e k s in a n a t r a z i n e - t r e a t e d C a p t i n a silt l o a m .
138
TABLE 1 Seedling survival of selected wheat cultivars to 0.25 mg kg-1 atrazine in greenhouse fiats
Cultivar
Classificationa
% Survivalb
Tex Red Maverick Potomac Rall Beau Delta Queen McNair 1003 Rosen Blueboy Dancer
HRWW HRWW SRWW HRWW SRWW SRWW SRWW SRWW SRWW SRWW
61.2 a 50.7 ab 45.2 a--c 37.9 a--d 30.0 a-e 21.0 b--f 17.6 c---f 13.2 d--f 9.0 ef 5.0 f
aHRWW: hard red winter wheat; SRWW: soft red winter wheat. bCultivars followed by the same letter are not significantly different at 0.05 level of probability using Duncan's Multiple Range Test.
authors. ' T e x R e d ' , 'Maverick', AR 67-3-19, 'McNair 1813' a n d ' P o t o m a c ' had the highest survival ratings. T h e percentage survival was rather low for m os t genotypes, as only seven had a survival rating o f 40% or greater, and 73 had a survival rating o f less than 20%. Although n o t every g e n o t y p e was included in a flat with unt reat ed soil, the five used as controls in each replication appeared adequate to show t h a t factors o t h e r than atrazine were n o t responsible for seedling death. Only 4 seedlings died o u t o f the 355 seedlings t hat emerged in the c o n t r o l flats. T he plants grown in the c o n t r o l flats were taller and had m ore tillers than plants in the treated soft. Differences in germination rate were removed by expressing atrazine tolerance as percentage survival. Several o f th e m or e tolerant genotypes such as T e x R e d , Maverick, and NR 31-74, are hard red wheats. O f the genotypes used in this study, 66 were classified as soft red and 18 as hard red wheats. T o determine if the hard wheats were mo re t ol e r ant than the soft wheats, one degree o f f r e e d o m from the analysis o f variance was partitioned o u t of the genotypic variation and a functional F-test was made. Hard wheats were significantly more tolerant than soft wheats, with mean survival rates o f 28.5% and 19.0%, respectively. The precision o f the screening pr oc e dur e was relatively low (CV = 47.9%). Most o f the r andom variability could pr ob abl y be accounted for by differences in light intensity, t em pe r at ur e and soil moisture. Greenhouse facilities which allow b ett er cont r ol of these factors should improve the precision o f the procedure. Although differences were f o u n d in the g e n o t y p e screening, an acceptable level o f tolerance f or field conditions remains to be identified. With over 35 000 genotypes o f wheat in the World Collection, a m ore t horough search,
139
similar to the one undertaken in flax (Anderson and Behrens, 1967), could identify more tolerant lines. Using the described technique might be more efficient for an initial screening of a large number of wheat genotypes than the field trials used for other species. Using methanol as a carrier to mix atrazine directly in the soil provides a more precise method for treating small amounts of soil than spraying on the plant or soil surface or spraying and incorporating. Greater precision in application of atrazine is needed for small greenhouse containers than for field plots. A uniform treatment under a controlled environment allows a smaller number of seedlings per genotype to be used. Once potentially tolerant lines are identified, field trials would be necessary to test for tolerance under field conditions. ACKNOWLEDGEMENTS
The authors thank CIBA-GEIGY Corporation for providing the technical grade atrazine. This paper is published with permission of the director of the Arkansas Agricultural Experiment Station.
REFERENCES Abernathy, J.R., Keeling, J.W. and Ray, L.L., 1979. Cotton cultivar response to propazine and atrazine. Agron. J., 71: 929--931. Anderson, R.N., 1964. Differential response of corn inbreds to simazine and atrazine. Weeds, 12: 60---61. Anderson, R.N. and Behrens, R., 1967. A search for atrazine resistance in flax. Weeds, 15: 85--87. Brinkman, M.A., Langer, D.K., Harvey, R.G. and Hardie, A.R., 1980. Response of oats to atrazine. Crop Sci., 20: 185--189. Comstock, V.E. and Anderson, R.N., 1968. An inheritance study of tolerance to atrazine in a cross of flax (Linum usitatissimum L.). Crop Sci., 8: 508--510. Engel, R.E., Funk, C.R. and Kinney. D.A., 1968. Effect of varied rates of atrazine and simazine on the establishment of several zoysia strains. Agron. J., 60: 261--262. Grogan, C.O., Eastin, E.F. and Palmer, R.D., 1963. Inheritance of susceptibility of a line o f maize to simazine and atrazine. Crop Sci., 3: 451. John, J.A. and Quenouille, M.H., 1977. Experiments: Design and Analysis. Charles Griffin and Company, London, 2nd edn., pp. 263--264. Machado, S.V., Bandeen, J.D., Stephenson, G.R. and Lavigne, P., 1978. Uniparental inheritance of chloroplast atrazine tolerance in Brassica campestris. Can J. Plant Sci., 58: 977--981. Peng, S.Y. and Yeh, H.J., 1970. Determination of the varietal tolerance of sugarcane to pre-emergence diuron and atrazine. Weed Res., 10: 218--229. Wicks, B.A., Fenster, C.R. and Burnside, O.C., 1969. Herbicide residue in soil when applied to sorghum in a winter wheat--sorghum--fallow rotation. Agron. J., 61: 721-724.
135
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
EVALUATION OF WHEAT CULTIVARS FOR SEEDLING TOLERANCE TO ATRAZINE
R.K. BACON, F.C. COLLINS and T.L. LAVY
University of Arkansas, Fayetteville, A R 72701 (U.S.A.) (Accepted 9 January 1986)
ABSTRACT
Bacon, R.K., Collins, F.C. and Lavy, T.L., 1986. Evaluation of wheat cultivars for seedling tolerance to atrazine.Field Crops Res., 14: 135--139. Winter wheat (Triticum aestivum L.) is sometimes planted after maize (Zea mays L.) or sorghum (Sorghum bicolor (L.) Moench) has been harvested. If the herbicide atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)~-triazine] was used on the maize or sorghum crop, the residue can reduce the stand of wheat. The objective of this research was to develop a screening technique to differentiate the seedling response of wheat cultivars to atrazine. Wheat was grown in the greenhouse in flats containing a Captina silt loam soil (fine-silty, mixed, mesic typic Fragiudult) treated with atrazine rates ranging from 0 to 1.0 mg kg-1. Survival ratings made after 4 weeks indicated that atrazine rates used to differentiate among genotypes should be less than 0.4 mg kg -1. To determine the proper concentration to use for screening cultivars, another experiment was conducted, using eight atrazine rates ranging from 0 to 0.4 mg kg-'. Based u p o n results from this study, 120 cultivars and lines were screened at 0.25 mg kg -1 atrazine for 4 weeks. 'TexRed', 'Maverick', AR 67-3-19, 'McNair 1813', and 'Potomac' had the highest survival ratings among genotypes. This screening procedure is an effective preliminary method to identify wheat genotypes with seedling resistance to atrazine.
INTRODUCTION
Winter wheat (Triticurn aestivum L.) is sometimes planted following the fall harvest of grain sorghum {Sorghum bicolor (L.) Moench) or maize (Zea mays L.). Atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine], which is commonly used as an herbicide to control grass and broadleaf weeds in both maize and sorghum, will severely injure the stand of wheat if sufficient residue persists in the soil. Sandy soils low in organic matter increase the chances of a residue problem for wheat, especially in years following a dry growing season for grain sorghum or maize (Wicks et al., 1969). Previous research has shown that cultivars of crop plants exhibit different degrees of tolerance to herbicides. Genotypic responses to atrazine have been reported in maize (Grogan et al., 1963; Anderson, 1964), cotton (Gossypium hirsutum L.) (Abernathy et al., 1979), flax (Linurn usitatissimum L.) (Ander-
0378-4290/86/$03.50
© 1986 Elsevier Science Publishers B.V.
136 son and Beherens, 1967; Comstock and Anderson, 1968), zoysia (Zoysia japonica Steud.) (Engel et al., 1968), sugarcane (Saccharum ssp.) (Peng and Yeh, 1970) and oats (Arena sativa L.) (Brinkman et al., 1980). Inheritance of tolerance to atrazine varies among crop species (Grogan et al., 1963; Comstock and Anderson, 1968; Machado et al., 1978). Since genetic variation for tolerance to atrazine exists in these species, it was thought likely that variation existed in wheat. The objective of this research was to devise a screening procedure that would detect genetic differences in tolerance to atrazine among wheat genotypes. Wheat cultivars and lines were then evaluated with the devised technique to identify genotypes tolerant to atrazine. MATERIALS AND METHODS Based on results from a preliminary test using atrazine rates of 0, 0.2, 0.3, 0.4, 0.6, 0.8, and 1.0 mg kg -1, an experiment was conducted to define the concentration which would differentiate wheat genotypes. Rates of 0, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, and 0.40 mg kg -1 were used to test the reaction of a seed composite of 75 hard and soft red winter wheat cultivars. The soft used was a Captina silt loam (a fine-silty, mixed, mesic typic Fragiudult) consisting of 366 g kg -1 sand, 528 g kg -~ silt, 106 g kg -1 clay. Organic matter was 9 g kg -1. At the 0.25 mg kg -~ concentration, the distribution coefficient, KD, of atrazine on soil was 0.79. Prior to the addition of atrazine, the soil was dried and screened through a 2-ram mesh sieve to ensure uniformity. Desired concentrations were produced b y applying a 0.0122 g 1-1 solution of 990 g kg -~ technical grade atrazine dissolved in methanol to 2 kg o f soil. After the methanol had evaporated, the soil was mixed thoroughly with the remaining 10.25 kg o f soil in the greenhouse flat to ensure uniform distribution of the herbicide in the soil. Temperature in the greenhouse was maintained at approximately 20°C. Soil moisture adequate for growth was maintained with periodic watering. Available sunlight (February--March) with no additional lighting was utilized. Each 48 cm × 35 cm × 9 cm flat was planted with 5 rows of the composite at the rate of 20 seeds per row. Seedling counts were made on each flat 2 weeks after planting when emergence was complete. After 4 weeks, when toxicity was more acute, survival readings were taken. A seedling was counted as a survivor if approximately 40% of its leaf area was green and the growing point appeared to be alive. Reaction to atrazine was expressed as percentage survival calculated by the number of seedlings at 4 weeks divided by the number of seedlings at 2 weeks. A concentration of 0.25 mg kg -~ atrazine was used in a cultivar screening study after this concentration was identified as the approximate threshold for 50% toxicity in wheat. One hundred and twenty cultivars and experimental lines of Triticum aestivum were tested in greenhouse flats b y planting 5 rows o f genotypes per flat at a rate o f 20 seeds per genotype. Seed for all
137
genotypes was less than 6 months old to ensure good germination of each entry. After 4 weeks, ratings for tolerance were made according to the method described earlier. The experiment was replicated four times using a randomized complete block design. Each replication consisted of 24 treated flats containing the 120 lines. A flat containing untreated soil and sown with five randomly chosen lines was placed with each replicate to serve as a control. The data from the cultivar screening were transformed using the arcsin square root transformation before an analysis of variance was conducted. All statistical information refers to analysis of the transformed data. Values reported as percentage survival were transformed back to the original scale and adjusted for bias (John and Quenouille, 1977). RESULTS AND DISCUSSION
The relationship between the concentration of atrazine and seedling survival was sigmoidal (Fig. 1). Very little damage was observed until the concentration reached 0.20 mg kg-1 atrazine; at that point 23% of the seedlings in the composite died. Seedling mortality increased dramatically until it reached 89% at the 0.30 mg kg -1 rate. Thereafter, seedling mortality increased at a much lower rate. Results of this study showed that 0.25 mg kg1 atrazine was the appropriate concentration for the soil used to approximate a 50% mortality rate for wheat cultivars. The 120 cultivars and lines exhibited significant variation (P < 0.0001) in tolerance to atrazine. The reaction of 10 of the cultivars is given in Table 1. A complete list of the genotypes' reaction to atrazine is available from the I00
8O
6O
40
2O
0
I
0
I
0.1
I
I
0.2
I
I
I
0.3
I
0.4
Concentration of Atrozine (rng kg-I)
Fig. 1. R e a c t i o n o f a w h e a t c o m p o s i t e g r o w n for f o u r w e e k s in a n a t r a z i n e - t r e a t e d C a p t i n a silt l o a m .
138
TABLE 1 Seedling survival of selected wheat cultivars to 0.25 mg kg-1 atrazine in greenhouse fiats
Cultivar
Classificationa
% Survivalb
Tex Red Maverick Potomac Rall Beau Delta Queen McNair 1003 Rosen Blueboy Dancer
HRWW HRWW SRWW HRWW SRWW SRWW SRWW SRWW SRWW SRWW
61.2 a 50.7 ab 45.2 a--c 37.9 a--d 30.0 a-e 21.0 b--f 17.6 c---f 13.2 d--f 9.0 ef 5.0 f
aHRWW: hard red winter wheat; SRWW: soft red winter wheat. bCultivars followed by the same letter are not significantly different at 0.05 level of probability using Duncan's Multiple Range Test.
authors. ' T e x R e d ' , 'Maverick', AR 67-3-19, 'McNair 1813' a n d ' P o t o m a c ' had the highest survival ratings. T h e percentage survival was rather low for m os t genotypes, as only seven had a survival rating o f 40% or greater, and 73 had a survival rating o f less than 20%. Although n o t every g e n o t y p e was included in a flat with unt reat ed soil, the five used as controls in each replication appeared adequate to show t h a t factors o t h e r than atrazine were n o t responsible for seedling death. Only 4 seedlings died o u t o f the 355 seedlings t hat emerged in the c o n t r o l flats. T he plants grown in the c o n t r o l flats were taller and had m ore tillers than plants in the treated soft. Differences in germination rate were removed by expressing atrazine tolerance as percentage survival. Several o f th e m or e tolerant genotypes such as T e x R e d , Maverick, and NR 31-74, are hard red wheats. O f the genotypes used in this study, 66 were classified as soft red and 18 as hard red wheats. T o determine if the hard wheats were mo re t ol e r ant than the soft wheats, one degree o f f r e e d o m from the analysis o f variance was partitioned o u t of the genotypic variation and a functional F-test was made. Hard wheats were significantly more tolerant than soft wheats, with mean survival rates o f 28.5% and 19.0%, respectively. The precision o f the screening pr oc e dur e was relatively low (CV = 47.9%). Most o f the r andom variability could pr ob abl y be accounted for by differences in light intensity, t em pe r at ur e and soil moisture. Greenhouse facilities which allow b ett er cont r ol of these factors should improve the precision o f the procedure. Although differences were f o u n d in the g e n o t y p e screening, an acceptable level o f tolerance f or field conditions remains to be identified. With over 35 000 genotypes o f wheat in the World Collection, a m ore t horough search,
139
similar to the one undertaken in flax (Anderson and Behrens, 1967), could identify more tolerant lines. Using the described technique might be more efficient for an initial screening of a large number of wheat genotypes than the field trials used for other species. Using methanol as a carrier to mix atrazine directly in the soil provides a more precise method for treating small amounts of soil than spraying on the plant or soil surface or spraying and incorporating. Greater precision in application of atrazine is needed for small greenhouse containers than for field plots. A uniform treatment under a controlled environment allows a smaller number of seedlings per genotype to be used. Once potentially tolerant lines are identified, field trials would be necessary to test for tolerance under field conditions. ACKNOWLEDGEMENTS
The authors thank CIBA-GEIGY Corporation for providing the technical grade atrazine. This paper is published with permission of the director of the Arkansas Agricultural Experiment Station.
REFERENCES Abernathy, J.R., Keeling, J.W. and Ray, L.L., 1979. Cotton cultivar response to propazine and atrazine. Agron. J., 71: 929--931. Anderson, R.N., 1964. Differential response of corn inbreds to simazine and atrazine. Weeds, 12: 60---61. Anderson, R.N. and Behrens, R., 1967. A search for atrazine resistance in flax. Weeds, 15: 85--87. Brinkman, M.A., Langer, D.K., Harvey, R.G. and Hardie, A.R., 1980. Response of oats to atrazine. Crop Sci., 20: 185--189. Comstock, V.E. and Anderson, R.N., 1968. An inheritance study of tolerance to atrazine in a cross of flax (Linum usitatissimum L.). Crop Sci., 8: 508--510. Engel, R.E., Funk, C.R. and Kinney. D.A., 1968. Effect of varied rates of atrazine and simazine on the establishment of several zoysia strains. Agron. J., 60: 261--262. Grogan, C.O., Eastin, E.F. and Palmer, R.D., 1963. Inheritance of susceptibility of a line o f maize to simazine and atrazine. Crop Sci., 3: 451. John, J.A. and Quenouille, M.H., 1977. Experiments: Design and Analysis. Charles Griffin and Company, London, 2nd edn., pp. 263--264. Machado, S.V., Bandeen, J.D., Stephenson, G.R. and Lavigne, P., 1978. Uniparental inheritance of chloroplast atrazine tolerance in Brassica campestris. Can J. Plant Sci., 58: 977--981. Peng, S.Y. and Yeh, H.J., 1970. Determination of the varietal tolerance of sugarcane to pre-emergence diuron and atrazine. Weed Res., 10: 218--229. Wicks, B.A., Fenster, C.R. and Burnside, O.C., 1969. Herbicide residue in soil when applied to sorghum in a winter wheat--sorghum--fallow rotation. Agron. J., 61: 721-724.