No abstract

moisture use by spring wheat. TtLe accumulation of dry matter and N in whezLt (7. aestivum L. cv. Manitou) grown on stubble land in lysimeters at two moistLLre levels and seven rates of N was mt:asured and related to moisture use at five stages of developrnent. In the irrigated lyr;imeters, leaf areas, plant dry matter and N content increasedl with fertilizer N. In thr: dry lysimeters, low rainfall between the stLot blade and anthesis stages produced moisture stress; consequently, dry matter production and leaf area were depressed and plants lost significant amounts ofN at rates of N > 61.5 kg/ha. Rain in the latter part of the growing season permitted the plants to recover and by maturity plant dry matter and N content increased with N. Approximately twic,g as much plant dry matter was produced in the wet treatment as in the dry for all levels of N. Total evapotranspiration (ET) increased with applied N in the wet treatment, and was much higher than in the dry treatment (Fig. 2, top)

Pesticides can be transported to ground water more rapidly through preferential flowpaths than would be predicted from their physico‐chemical properties. The leaching rates of the herbicides 2,4‐D, bromoxynil, clopyralid, dicamba, diclofop, MCPA, and mecoprop were compared in this study on plots filled after harvest (conventional till, CT) and those that were not (fall tillage operation omitted, NT). The soil‐incorporated herbicides triallate and trifluralin were applied to the CT plot only. Herbicide was applied immediately prior to a leaching irrigation for salt removal, which represents a “worst‐case” scenario for pesticide leaching. Direct evidence of preferential flow was obtained when the herbicides, with the exception of triallate and trifluralin, were detected in the first water reaching the tile drains. Although the nonincorporated herbicides were transported preferentially at the same rate, the amounts transported depended on the solubility and adsorption coefficient (Koc) of the herbicide. Only 0.01% of the application of the least soluble herbicide, diclofop, was transported, compared with 0.46% of the most soluble herbicide, dicamba. Preferential flow was only slightly reduced by the tillage pass. The amounts of herbicide transported to the tile drain, however, were substantially reduced on the CT plot. The tillage effect was greatest for the more soluble and less strongly absorbed herbicides. There was no clear relationship between amounts transported in the year after application and reported persistence but herbicides with the longer half‐lives persisted in relatively greater amounts than the other herbicides.

Abstract-Farm ponds or dugout waters were monitored for residues of seven major herbicides used in the Canadian prairies from fall of 1987 to spring of 1989. The frequencies of confirmed detection of herbicides in water samples, depending on the time of sampling, in decreasing order were: 2,4-dichlorophenoxyacetic acid (2,4-D; 93-100%), diclofop (46-95%), bromoxynil (50-85%), 4-chloro-2-methyl-phenoxyacetic acid (MCPA; 33-70%), triallate (28-63%), dicamba (17-55%), and trifluralin (0-18%). The corresponding frequencies of quantifiable residues (Ն0.05 g/L) were lower, ranging from 75 to 86% for 2,4-D to 0 to 7% for dicamba. Median residues in all water samples were near or below the quantification limits of 0.05 g/L. Maximum residues varied widely and were (g/L): trifluralin (not detectable [ND]-0.11), bromoxynil (0.27-0.33), dicamba (ND-11.2), triallate (0.05-0.87), MCPA (0.12-1.97), 2,4-D (0.64-2.67), and diclofop (0.27-3.47). Maximum residues were seasonal and declined to near or below detection limits by the following sampling time. Median values were two to three orders of magnitude less than the corresponding maximum allowable concentration and interim maximum allowable concentration guidelines for drinking water in Canada and the United States. Maximum values were also less than these guidelines. Only the maximum values for residues of MCPA and 2,4-D approached the guidelines for these herbicides in water used for irrigation.

Despite its importance to the degradation of the land resource, few estimates of soil loss or soil erodibility for water erosion from cropland have been made for the semiarid Brown soil zone of western Canada. To estimate these, we calibrated the event-based Modified Universal Soil Loss Equation with measured sediment yield for three 5-ha fields near Swift Current, Saskatchewan. The calibration used predetermined C, LS, P, and warm-season K factors with all erositivities and the K factors for other seasons optimized. Using this calibrated equation with 31 yr of measured runoff data, mean annual sediment yield for a conventional-tillage spring wheat-fallow cropping system on an undulating landscape (0 to 5% slopes) was estimated to be 0.8 Mg ha−1 yr−1. Estimated erosion from fallow land from 16 March to 15 April constituted 86% of the mean annual sediment yield. Erosion occurring during rainfall-runoff were relatively unimportant, accounting for about 4% of the estimated total sediment yield. An exceptionally severe rainstorm was predicted to have caused erosion of many Mg ha−1but the frequency of such events on a given field is too low to greatly affect mean annual erosion. Apparent field-scale soil erodibility was least in the winter (November to 15 March) when the soil would typically be frozen to the surface and was greatest in April when the soil would typically be partially frozen. Apparent erodibility in the summer and in late March was intermediate between those values. Key words: Erosion, runoff, snow, Universal Soil Loss Equation