1 Herbicide residues and yield effects from repeated floodirrigations of alfalfa with water containing monuron or simazine Y. W. Jame 1, A. J. Cessna 2,5, V. O. Biederbeck 1, R. Grover 3,4, A. E. Smith 3,4, and H. C. Korven 1,4 1 Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, Box 1030, Swift Current, Saskatchewan, Canada S9H 3X2; 2 Agriculture and Agri-Food Canada Research Centre, P.O. Box 3000, Lethbridge, Alberta, Canada T1J 4B1; 3 Agriculture and Agri-Food Canada Research Station, Regina, Saskatchewan, Canada. Received 30 September 1998, accepted 3 May Jame, Y.W., Cessna, A. J., Biederbeck, V. O., Grover, R., Smith, A. E. and Korven, H. C Herbicide residues and yield effects from repeated flood-irrigations of alfalfa with water containing monuron or simazine. Can. J. Plant Sci. 79: Surface runoff or irrigation water contaminated with herbicides may cause crop damage and result in inadvertent residues in the crop. Rotational crops may also be damaged due to the persistence of leached herbicides within the root zone. An 11-yr field experiment was carried out to address these issues through repeated flood-irrigations of alfalfa (Medicago sativa L. Roamer ) with water containing various concentrations (0, 10, 100 and 1000 µg L 1 ) of monuron or simazine. The experimental site, on an alluvial clay soil, was flood-irrigated for 8 yr with a total of 32 irrigations, then oat (Avena sativa L. Harmon ) was grown for 3 yr under dryland conditions as a sensitive bioassay crop. Based on alfalfa forage yield, repeated applications of water containing up to 100 µg L 1 of either monuron or simazine did not show any harmful effects on the crop. However, the 1000 µg L 1 treatments caused cumulative yield reductions with greatest deleterious effect being caused by simazine (up to 55% yield reduction). After 7 yr of irrigation, inadvertent residues of both herbicides were consistently detected in the crop, but only for the 1000 µg L 1 treatments. Average concentrations of monuron in the alfalfa foliage were 0.94 and 1.76 mg kg 1 for the first and second cuts, respectively, whereas corresponding values for simazine were 0.31 and 0.64 mg kg 1. Approximately 4 and 12% of the total amounts applied remained in the soil profile for monuron and simazine, respectively. Herbicide residues to 1.5 m soil depth decreased with increasing depth with half of the total being present in the top 0.15 m, and they were detectable only for the 100 and 1000 ug L 1 treatments. Only soil residues of simazine from the 1000 µg L 1 rate of application reduced oat yields extensively. These yield reductions occurred only during the first 2 yr under dryland production. Key words: Herbicide-contaminated irrigation water, crop damage, herbicide residues, alfalfa, simazine, monuron Jame, Y. W., Cessna, A. J., Biederbeck, V. O., Grover, R., Smith, A. E. et Korven, H. C Résidus d herbicide résultant de la pratique répétée de l irrigation de la luzerne par submersion avec de l eau contenant du monuron ou de la simazine. Effet sur le rendement de la luzerne. Can. J. Plant Sci. 79: L eau de ruissellement ou l eau d irrigation contaminée avec des herbicides peut endommager les cultures vivaces et y entraîner l accumulation de résidus si l on n y porte pas attention. Les cultures de rotation peuvent être endommagées, elles aussi, par suite de la persistance des herbicides lessivés dans la zone racinaire. L objet d une expérience au champ de 11 ans a été d examiner ces problèmes eventuels au moyen d irrigation par submersion répétée de la luzerne (Medicago sativa L. Roamer ) avec de l eau contenant diverses concentrations (0, 10, 100 et µg L 1 ) de monuron ou de simazine. L emplacement expérimental, sur argile alluviale était irrigué pendant 8 ans (32 irrigations en tout), après quoi de l avoine (Avena sativa L. Harmon ) était utilisée pendant 3 ans en régime pluvial comme indicateur biologique en raison de sa sensibilité. D après le rendement fourrager obtenu de la luzerne, des apports répétés d eau contenant jusqu à 100 µg L 1 de monuron ou de simazine ne provoquaient pas d effets nocifs sur la culture. À la dose de µg L 1, toutefois, on observait des réductions du rendement cumulatif, les effets les plus nuisibles étant dus à la simazine (jusqu à 55 % de manque à produire). Au bout de 7 années d irrigation, des résidus des deux herbicides étaient régulièrement détectés dans la culture, mais seulement à la dose de traitement de µg L 1. Les concentrations moyennes de résidus dans le feuillage de la luzerne étaient, respectivement, de 0,94 et 1,75 mg kg 1 à la première et à la deuxième coupe pour le monuron et de 0,31 et 0,64 mg kg 1 pour la simazine. Approximativement 4 et 12 %, respectivement, des quantités totales utilisées des deux produits demeuraient dans le profil du sol. Les résidus d herbicide mesurés dans les 150 cm supérieurs du sol diminuaient à mesure qu on descendait dans le profil, la moitié du total se retrouvant dans les 15 premiers cm. Ils n étaient par ailleurs détectables qu aux doses de traitement de 100 et µg L 1. Seuls les résidus de la simazine résultant du traitement à µg L 1 provoquaient des baisses graves du rendement de l avoine, les réductions ne survenant cependant que dans les deux premières années de culture en régime pluvial. Mots clés: Eau d irrigation chargée d herbicide, endommagement des cultures, résidus d herbicide, luzerne, simazine, monuron 4 Retired. 5 Present address: National Water Research Institute, 11 Innovation Blvd., Saskatoon, Saskatchewan, Canada S7N 3H Abbreviations: Check, irrigation water containing no herbicide; M10, M100 and M1000, irrigation water containing 10, 100, and 1000 µg L 1, respectively; of monuron; S10, S100 and S1000, irrigation water containing 10, 100, and 1000 µg L 1, of simazine
2 640 CANADIAN JOURNAL OF PLANT SCIENCE Herbicide transport has been well documented in snowmelt runoff (Nicholaichuk and Grover 1983), rainfall runoff (Wauchope 1978), and irrigation runoff (Spencer and Cliath 1991; Cessna et al. 1994, 1996), as well as during the initial flush of treated irrigation canals (Smith et al. 1975; Anderson et al. 1978, 1986; Grover et al. 1980). Edge-offield herbicide concentrations in surface runoff generally tend to increase as the interval between herbicide application and surface runoff decreases (Wauchope 1978; Spencer and Cliath 1991). Depending on the formulation applied, the water solubility of the herbicide, and the intensity of the rainfall, event-averaged herbicide concentrations in rainfall runoff can exceed 1000 µg L 1, and total loss in runoff can exceed 5% of the amount applied (Wauchope 1978). In the case of irrigation runoff, event-averaged herbicide concentrations were generally <100 µg L 1 and total loss seldom exceeded 1% of the amount applied (Spencer et al. 1985; Cessna et al. 1994, 1996). The amounts of herbicides transported in rainfall or irrigation runoff are sub-lethal dosages to the crops to which they were applied. However, such dosages, which can also result from vapour (Breeze and van Rensburg 1992) and droplet drift (Fletcher et al. 1996; Wall 1996), improper sprayer tank cleanout (Derksen 1989), carryover of soil-persistent herbicides (Keeling et al. 1989; Ahrens and Fuerst 1990; Johnson et al. 1995) and contaminated irrigation water (Smith et al. 1975; Grover et al. 1980), may be phytotoxic to susceptible crops. Sub-lethal amounts of herbicides have been shown to cause injury or reduce yields in a variety of susceptible crops at application rates that were < 1% of recommended field rates (Derksen 1989: Fletcher et al. 1996) or 1 to 2% of recommended field rates (Derksen 1989; Eberlein and Guttieri 1994; Wall 1996). Thus, the movement of herbicide-contaminated surface runoff to adjacent crops or irrigation of crops with contaminated water has the potential to cause crop damage (Korven 1975) or result in inadvertent residues (Pringle et al. 1978; West and Parka 1992) in the crops. In addition, there is potential for damage to rotational crops due to leaching of persistent herbicides into the root zone. Our study addresses each of these concerns with respect to surface runoff or irrigation water contaminated with herbicides. Two herbicides were selected for study. One of the herbicides is the s-triazine compound, simazine, which is a broad-spectrum herbicide used in Canada for the control of weeds in deep-rooted crops, such as those in orchards, vineyards, tree and ornamental nurseries, woodlands and shelter belts, and for vegetation control in industrial sites and other non-crop areas such as right-of-way (Pest Management Regulatory Agency (PMRA) 1999). In the Canadian prairies, the use of simazine is recommended for weed control in established legumes, such as alfalfa and bird s-foot trefoil (PMRA 1999; Saskatchewan Agriculture and Food (SAF) 1999), and in established shelterbelts (Prairie Farm Rehabilitation Administration 1994; SAF 1999). It is also used for algae and vegetation control in farm dugouts or ponds and irrigation and drainage systems (Saskatchewan Environment and Resource Management 1993; PMRA 1999). Simazine is classified as a relatively persistent (Jury et al. 1987) and moderately mobile (Jury et al. 1987; Gustafson 1989) herbicide. The other herbicide was the urea compound, monuron. Although there are no currently registered uses of monuron in Canada, this herbicide was included for study because it is more mobile (Jury et al. 1987) and less persistent (Smith et al. 1975; Grover et al. 1980; Gomezde-Barreda et al. 1991) in soil than simazine, and becuase its use had been registered for the control of vegetation in irrigation ditches (Korven 1975) when this study was initiated. The study was designed to determine the effect of repeated flood-irrigations with simazine- or monuron-contaminated water on the forage yield of alfalfa (perennial crop) and the presence of inadvertent herbicide residues in the alfalfa forage. In addition, the accumulation of herbicide residues in the root zone was determined and phytotoxic effects of those residues assessed using oat as a sensitive rotational crop grown under dryland conditions. MATERIALS AND METHODS Crop Seeding and Herbicide Application The experimental area was located on an alluvial clay, an Orthic Brown Chernozem (Aridic Haploboroll) at the Semiarid Prairie Agricultural Research Centre, Swift Current, Saskatchewan. The soil was uniform in texture down to 1.5 m depth with approximately 21% sand, 37% silt, and 42% clay. The surface layer (0 0.3 m) had a ph of 7.0 and contained approximately 1.7% organic carbon. In the summer of 1972, the area was seeded to Roamer alfalfa and irrigation commenced in Seven treatments, consisting of irrigation of alfalfa with water containing no herbicide or 10, 100 and 1000 µg L 1 of simazine or monuron, were replicated four times and designated as check, S10, S100, S1000, M10, M100 and M1000. The 28 plots, each 2 m 2 m and separated with 2- m pathways, were arranged in a randomized complete block design. Each plot was enclosed by a shallow dike to facilitate surface flood irrigation and to retain the irrigation water until infiltrated into the soil. The long-term average rainfall received in Swift Current, Saskatchewan during a growing season (May August) is about 200 mm, whereas the average seasonal consumptive use of water by alfalfa is 600 mm. Based on this premise, 400 mm of flood-irrigation water was applied annually to each plot to make up the deficit. The total amount of applied water was distributed evenly over four irrigations, one each in May, June, July, and August. For each irrigation, herbicide was mixed with water in a 1000-L tank at the desired concentration and the required amount of irrigation water was then distributed onto the plots. The total amount of simazine and monuron applied annually with the irrigation water was 0.04, 0.4 and 4.0 kg a.i. ha 1 at the 10, 100 and 1000 µg L 1 concentrations levels, respectively. The plots were flood-irrigated with herbicide-treated water for a total of 8 consecutive years from 1973 to In the fall of 1980, the alfalfa was worked with a cultivator and incorporated by rototilling. In spring of 1981, 1982 and 1983, the experimental site was seeded to dryland oat as a sensitive bioassay crop.
3 JAME ET AL. EFFECTS OF HERBICIDE-CONTAMINATED WATER ON ALFALFA 641 Soil Bulk Density Measurements Soil bulk densities were determined by the core method (Blake and Hartge 1986). Core samples were obtained at 0.3-m increments using a 5-cm i.d. probe, which was forced into the soil and removed hydraulically. Each 0.3-m long core sample was cut into 0.15-m segments and the segments were placed into air-tight containers to minimize drying. Four cores were taken from the study site to a depth of 1.5- m to determine the average bulk densities at various depths in the soil profile. Soil and Plant Sampling and Statistical Analyses No forage was harvested in 1973, the year of alfalfa establishment. From 1974 through 1980 alfalfa was cut twice each year; once in late June and again in mid-august. Oat dry matter and grain were harvested at maturity from the entire plots in 1981, 1982 and Each year the yields were subjected to analysis of variance and differences among treatment means were tested with Duncan s multiple range test at the 5% level of significance (Little and Hills 1978). During the 1979 growing season, samples of alfalfa tissue from the first and second cuttings were taken for residue analysis. The dried plant tissue was milled (1-mm screen) prior to analysis. In late September of 1979, soil samples for residue analysis were taken from the , , , , and 1.2- to 1.5-m depths. All plant and soil samples were maintained at 1 2 C until analyzed. Laboratory Analyses of Herbicide Residues Simazine Residues in Soil Soil (20 g) was extracted with 10% aqueous acetonitrile (60 ml) and partitioned into methylene chloride using the procedures described by Smith et al. (1975). A Hewlett- Packard Model 5733A gas chromatograph, equipped with a Model 18789A nitrogen-phosphorus alkali-ionization detector, was operated with a 1.2-m 4-mm i.d. column packed with 5% Dexsil on mesh Chromsorb W, HP using conditions described previously (Cessna et al. 1985). With a column temperature of 215 C, the retention time for simazine was 3.8 min. Monuron Residues in Soil Soil (25 g) was extracted with methanol (50 ml) using the procedures described by Smith et al. (1975). The extract residue, redissolved in 2 ml of 1:1 methylene chloride/ hexane, was transferred to a Florisil (9 ml; deactivated with 5% water) column and the column eluted with 15% acetone in hexane (70 ml). The last 55 ml of eluate was concentrated to dryness using a rotary evaporator and then taken up in 10.0 ml of 60% aqueous methanol prior to HPLC analysis. Recoveries from fortified soil were 96.7 ± 4.8% and 87.2 ± 5.1%, respectively, at 0.5 (n = 4) and 0.05 mg kg 1 (n = 4). A Perkin Elmer Model 601 liquid chromatograph, equipped with the Model LC-55 variable wavelength UV- VIS detector (245 nm) and a Valco 9070 sample injector (50 µl loop), was operated with a 10 µ C 18 column (25 cm 4mm i.d.; Waters Bondapak). With a column temperature of 60 C and mobile phase (50:50 methanol/water; vol/vol) flow rate of 0.75 ml min 1, the retention time for monuron was 5.9 min. Monuron Residues in Alfalfa Milled alfalfa tissue (5 g) was extracted with methanol (50 ml) according to procedures described previously (Cessna 1988), except that rather than blending, the extraction mixture was shaken for 1 h and the extract taken to a volume of 100 ml. The extract (20 ml, equivalent to 1 g plant tissue) was partitioned into methylene chloride and the methylene chloride extract concentrated to dryness (Cessna 1988). The extract residue, redissolved in 3 ml of 25% methylene chloride in hexane, was transferred to an acidic alumina (10 ml; deactivated with 15% water) colum and the column washed with 25% methylene chloride in hexane (45 ml) and then eluted with 35% methylene chloride in hexane (65 ml). The last 40 ml of eluate was concentrated to dryness and the residue taken up in 1.0 ml of 60% aqueous methanol for HPLC analysis using conditions described above. Recoveries from fortified alfalfa tissue were 79.3 ± 10.4% at 0.50 mg kg 1 (n = 6). Simazine Residues in Alfalfa Alfalfa tissue (5 g), in a mixture of chloroform (50 ml) and water (50 ml) contained in a 300-mL Erlenmeyer flask, was extracted by shaking for 1 h using a wrist-action shaker. After filtration under reduced pressure, the extract was transferred to a 125-mL separatory funnel and the chloroform layer passed through anhydrous sodium sulfate (50 ml) into a 100-mL volumetric flask, along with a 35-mL chloroform wash of the sodium sulfate, and taken to volume with chloroform. The extract (20 ml; equivalent to 1 g plant tissue) was then concentrated to dryness using a rotary evaporator. The extract residue, redissolved in 3 ml of 15% methylene chloride in hexane, was transferred to an acidic alumina (5 ml; deactivated with 15% water) column and the column was washed with 15% methylene chloride in hexane (25 ml) and then eluted with 25% methylene chloride in hexane (60 ml). The last 40 ml of eluate was concentrated to 1 ml prior to gas chromatographic analysis using the conditions described above. Recoveries from fortified alfalfa tissue were 83.0 ± 3.8% and 79.6 ± 7.9% at 1.0 (n = 4) and 0.1 (n = 4) mg kg 1, respectively. RESULTS AND DISCUSSION A total of 28, 2.8 and 0.28 kg of each herbicide had been applied in 2800 mm of flood-irrigation water to alfalfa plots receiving the 1000, 100 and 10 µg L 1 treatments, respectively, during the first 7 yr (1973 to 1979). Both herbicides leached into the soil profile with the infiltrating irrigation water. Since simazine and monuron are taken up by the roots as the main route of entry into plants (Eue 1971; Kearney and Kaufman 1976), the effect of the herbicides on alfalfa forage yield, inadvertent herbicide residues in alfalfa, and yield of oat as a rotational crop would depend, to a large extent, on the magnitude of herbicide residues in the root zone.
4 642 CANADIAN JOURNAL OF PLANT SCIENCE Table 1. Residual concentrations of simazine and monuron in the soil profile of irrigated plots following 28 irrigations over 7 yr (1973 to 1979) with water containing herbicide concentrations of 10, 100, and 1000 µg L 1 Treatments z Soil Bulk depth density S10 S100 S1000 M10 M100 M1000 (m) (Mg m 3 ) µg kg 1 kg ha 1 µg kg 1 kg ha 1 µg kg 1 kg ha 1 µg kg 1 kg ha 1 µg kg 1 kg ha 1 µg kg 1 kg ha ± 5 y ± ± <5 x 6 ± ± <10 x 40 ± ± <5 12 ± ± <10 <10 83 ± <5 10 ± ± <10 <10 43 ± <5 5 ± ± <10 <10 30 ± <5 <5 16 ± <10 <10 38 ± <5 <5 17 ± Total z S10, S100 and S1000 are plots irrigated with water containing 10, 100 and 1000 µg L 1 of simazine, respectively; M10, M100 and M1000 are plots irrigated with water containing 10, 100 and 1000 µg L 1 of monuron, respectively. y Average of four replicates, mean ± standard deviation. x Limits of quantification. Table 2. Effects on alfalfa dry matter yield of repeated irrigation with water containing concentrations of simazine or monuron at 10, 100 and 1000 µg L 1 Year Treatment z (Mg ha 1 ) y Check x 11.07bc 10.56ab 14.52a 12.90a 12.57ab 9.64ab 11.95b S ab 11.03ab 14.85a 13.20a 14.09a 10.43a 15.14a S abc 12.66a 13.05a 13.70a 11.37bc 9.64ab 11.76b S c 7.98c 9.52b 7.90b 5.77d 4.31d 5.81d M a 12.15ab 14.10a 13.22a 13.05ab 10.08ab 11.29b M ab 11.49ab 12.96a 12.97a 12.01abc 9.46ab 11.76b M bc 10.18b 12.08ab 11.74a 10.30c 7.53b 9.44c z S10, S100 and S1000 are plots irrigated with water containing 10, 100 and 1000 µg L 1 of simazine, respectively; M10, M100 and M1000 are plots irrigated with water containing 10, 100 and 1000 µg L 1 of monuron, respectively, and Check are plots irrigated with water containing no herbicide. y Total from two cuttings yr 1. x Establishment year, no forage was harvested. a d Means (within a given year) followed by the same letter are not significantly different at the 0.05 probability level as determined by Duncan s multiplerange test. Herbicide Residues in Soil At the highest rate of application, both herbicides were detectable throughout the soil profile down to the 1.2- to 1.5-m depth (Table 1), whereas in the plots treated at 100 µg L 1, residues were detectable only to shallower depths. With the 10 ug L 1 rate of application, monuron was not detected at any depth and simazine was detected only in the uppermost layer (0- to 0.15-m depth) of soil. With the 1000 µg L 1 application rate, both simazine and monuron had leached, over the 7-yr study period, at least to the 1.2- to 1.5-m depth in the soil profile (Table 1). Leaching to this and possibly greater depths is supported by Di and Aylmore (1997) who, using a model formulated to calculate the fraction of pesticide remaining and travel time as the pesticide leaches deeper into the soil profile, predicted that the mean travel time for simazine to reach the 1.5-m depth was 1.1 yr. When the extent of leaching of the two herbicides for the 100 µg L 1 application rate is compared, it is evident that monuron leached to greater depth (0.6 to 0.9 m) than simazine (0.15 to 0.3 m) under the conditions of our study. Thus, the leaching of monuron to greater depth reflects the greater mobility of monuron in soil (Jury et al. 1987). The soil concentration profiles for the 1000 µg L 1 application rate indicate that approximately half of the amount of each herbicide in the soil profile was retained in the upper (0 to 0.15 m) layer of soil. This pattern of leaching has been observed for simazine, monuron and other herbicides in citrus grove (Gomez-de-Barreda et al. 1991) and uncropped soils (Gomez-de-Barreda et al. 1996) following several years of herbicide treatment. Conversion of the soil profile residue data for the 1000 µg L 1 rate of application to units of kg ha 1 shows that, of the total amount of each herbicide applied over the 7 yr, about 4% of the monuron and 12% of the simazine remained in the soil profile, equivalent to 1.13 and 3.27 kg ha 1, respectively (Table 1). These results indicate that, under the conditions of our study, monuron was degraded in the soil more rapidly than simazine. More rapid degradation of monuron also occurred in citrus grove soils with a long history of monuron and simazine application (Gomez-de-Barreda et al. 1991) and in irrigation ditches that had been treated with monuron and simizine at soil sterilant rates (Smith et al. 1975; Grover et al. 1980). Alfalfa Yield Alfalfa yield reduction effected by irrigation with water containing either herbicide at 1000 µg L 1 was apparently cumulative (Table 2). With the S1000 treatment, significant
5 JAME ET AL. EFFECTS OF HERBICIDE-CONTAMINATED WATER ON ALFALFA 643 Table 3. Herbicide residues in alfalfa after 7 yr (1973 to 1979) of repeated irrigations with water containing simazine or monuron concentrations of 10, 100 and 1000 µg L 1 Treatments z S10 S100 S1000 M10 M100 M1000 Replicate 1st Cut 2nd Cut 1st Cut 2nd Cut 1st Cut 2nd Cut 1st Cut 2nd Cut 1st Cut 2nd Cut 1st Cut 2nd Cut Simazine concentration (mg kg 1 ) (dry weight basis) Monuron concentration (mg kg 1 ) (dry weight basis) 1 <0.05 y <0.05 <0.05 < <0.10 y <0.10 <0.10 < <0.05 <0.05 < <0.10 <0.10 <0.10 < <0.05 < <0.10 < < <0.05 <0.05 < <0.10 <0.10 < Mean ± Trace Trace 0.31± 0.64± Trace Trace 0.94± 1.67± SD z S10, S100 and S1000 are plots irrigated with water containing 10, 100 and 1000 µg L 1 of simazine, respectively; M10, M100 and M1000 are plots irrigated with water containing 10, 100 and 1000 µg L 1 of monuron, respectively. y Limits of quantification. yield reduction first occurred in 1975 (25%) and continued to decrease until 1978 (55%). After the 1978 growing season, there were no addition reductions in yield. A similar pattern of alfalfa yield reduction occurred with the M1000 treatment, except that no significant yield reduction was evident in 1974 or However, although a yield reduction of 17% was first observed in 1976, a significant yield reduction of this order of magnitude was not observed until After 1976, alfalfa yields from the M1000 treatment remained constant at approximately 80% of those in the check plots and no further reduction of forage yield was observed. Annual applications of relatively persistent herbicides, which carry over in the soil until the next growing season, will result in a gradual build-up in the soil until a stabilized herbicide concentration is reached. The magnitude of the buildup is determined by both the application rate and the percent of yearly carryover (Smith 1982) or persistence of the herbicide in soil. The alfalfa yield reductions we observed (Table 2) would have been closely related to herbicide accumulation in soil; that is, as herbicide soil concentrations increased, alfalfa yields would have decreased, but once herbicide soil concentrations had stabilized, alfalfa yields would have stabilized as well. Based on the cumulative alfalfa yield reductions for the 1000 µg L 1 treatments, it would appear that a stabilized concentration of monuron in the soil may have been reached during the fourth year of treatment, whereas for the more persistent simazine (Table 1), a stabilized concentration occurred during the sixth year. The greater yield reductions by simazine (~55% versus ~20%, Table 2) reflect the greater concentrations of this herbicide in the soil profile at all depth intervals (Table 1). No significant effects on alfalfa yield were noted with either the S100 or M100 treatments (Table 2). Thus, the stabilized concentrations of the two herbicides within the soil profile were not phytotoxic to the crop at the 100 µg L 1 rate. There were also no significant adverse effects on yields by the 10 µg L 1 treatments of either herbicide. On the contrary, the M10 treatment in 1974 and the S10 treatment in 1980 did significantly out-yield the check plots. Although the yield difference between check plots and the 10 µg L 1 treatments was significant only in these two instances, the mean forage yields for the S10 treatment were higher than those for the corresponding checks every year, and those for the M10 treatments were higher in all but 2 yr (Table 2). Stimulation of plant growth by application of subtoxic amounts of herbicides has been observed previously for simazine (Wiedman and Appleby 1972) and for the urea herbicides diuron (Wiedman and Appleby 1972) and isoproturon [data of Kemp and Caseley (1987) analyzed by Brain and Cousens (1989] and has been described by Brain and Cousens (1989) using a symmetrical sigmoid equation. Herbicide Residues in Alfalfa Foliage Significant concentrations of both herbicides had accumulated in the root zone of the soil after 7 yr and 28 irrigations with the 1000 µg L 1 treatments (Table 1). The reduction of alfalfa forage yields (Table 2) by the 1000 µg L 1 treatments indicated that phytotoxic amounts of the herbicides were taken up by the crop. Alfalfa plant tissues taken from the S1000 and M1000 treatments in 1979 showed readily detectable herbicide residues (Table 3). Herbicide concentrations in plants from monuron-treated plots were approximately threefold of those from simazine-treated plots. Since simazine concentrations in the soil profile were approximately three times those of monuron, this difference in concentration in the plant tissue most likely reflects the greater water solubility of monuron [230 mg L 1 at 25 C (Pesticide Manual 1987) versus 6.2 mg L 1 at 20 C (Pesticide Manual 1997) for simazine]. It suggests that a greater proportion of the monuron was present in soil in the solution phase, whereas the bulk of simazine in this clay soil was in the adsorbed phase. Concentrations of both herbicides in the crop also increased about twofold from the first cut to the second cut (Table 3). Since the primary mechanism of uptake of these two herbicides is thought to be by passive mass-flow where the herbicides, dissolved in the soil solution, enter with the water taken up by the roots, this increase of herbicide concentration in the alfalfa plants was probably due to the progressive accumulation of herbicide residues in soil that occurred during the 1979 growing season with repeated irrigations. Phytotoxic amounts of monuron and simazine were not taken up by the alfalfa plants from the 10 and 100 µg L 1
6 644 CANADIAN JOURNAL OF PLANT SCIENCE Table 4. Dryland oat yields over 3 yr from plots previously irrigated for 8 yr (1973 to 1980) with water containing monuron or simazine at 10, 100 or 1000 µg L Total dry Grain Total dry Grain Total dry Grain Treatments z weight weight weight weight weight weight (Mg ha 1 ) Check 7.85a 3.27a 8.50a 3.88ab 5.34b 2.91a S a 3.49a 8.95a 4.38ab 5.54b 3.12a S a 3.57a 8.43a 3.86ab 5.83ab 2.96a S1000 0b 0b 2.58b 0.61c 6.81a 3.46a M a 3.41a 9.79a 4.79a 5.34b 3.08a M a 3.69a 9.36a 4.52ab 5.80ab 3.17a M a 3.21a 8.06a 3.70b 6.33ab 3.54a z S10, S100 and S1000 are plots previously irrigated with water containing 10, 100 and 1000 µg L 1 of simazine, respectively; M10, M100 and M1000 are plots irrigated with water containing 10, 100 and 1000 µg L 1 of monuron, respectively, and Check are plots irrigated with water containing no herbicide. a c Means (within a given year) followed by the same letter are not significantly different at the 0.05 probability level as determined by Duncan s multiplerange test. treatments (Table 2) because of lower herbicide concentration in the soil (Table 1). As a consequence, alfalfa tissue samples taken from the S10 or M10 plots contained no detectable herbicide residues, whereas plants from S100 and M100 plots contained trace amounts of the corresponding herbicides. Pringle et al. (1978) found similar residue accumulation in alfalfa flood-irrigated with water also containing 10 and 100 µg L 1 of simazine. Dryland Oat Yield Neither oat dry matter production nor grain yield were affected by previous monuron irrigation treatments in the 3 yr following the final herbicide application (Table 4). Similarly, yields were not affected by the 10 or 100 µg L 1 simazine treatments. Thus, any uptake of herbicide residues carried over in the soil from these treatments (Table 1) was not sufficient to produce a significant phytotoxic response in the rotational crop. However, this was not the case with the 1000 µg L 1 simazine treatment (Table 4). Total simazine residues in the root zone of these plots after 7 yr of herbicide application were equivalent to 3.27 kg ha 1 (Table 1) and, although the crop germinated well in the first year following the cessation of irrigation, all plants died at about the threeleaf growth stage. Thus, with this treatment, repeated application in irrigation water resulted in carryover of simazine in the root zone in sufficient concentration that uptake by oat produced acute phytotoxicity. In this clay soil, carryover of phytotoxic concentrations of simazine extended more than one growing season as was evident from the 84% yield reduction of oat grain in However, by the third year following the cessation of irrigation, simazine concentrations in the soil from the S1000 treatment had declined sufficiently to no long cause yield reduction (Table 4). CONCLUSIONS Based on alfalfa yields from the experimental plots, we concluded that repeated flood-irrigations with water containing either the herbicides simazine or monuron eventually resulted in steady-state concentrations of these herbicides being accumulated in the soil. Reductions in alfalfa forage yield and in subsequent dry matter and grain yield of the rotational dryland oat crop reflected the magnitude of the steadystate concentrations of these herbicides in the root zone. At the 1000 µg L 1 rate, the steady-state concentrations of simazine and monuron in the root zone (0- to 1.5-m depth) were equivalent to 3.27 and 1.13 kg ha 1, respectively, and resulted in corresponding alfalfa yield reductions of 55 and 20%. These treatments also resulted in the uptake of inadvertent residues of both herbicides into the alfalfa forage. Maximum residues of monuron and simazine were detected in forage from the second cut and were 1.67 and 0.64 mg kg 1, respectively. 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