Herbicide-treated Areas Research Articles

Effect of herbicides on pollinator foraging behavior and flower morphology in white clover (Trifolium repens L.)–infested turfgrass

AbstractThe recent decline in pollinator abundance is a cause of concern for sustaining global food production. Several common weeds of managed turfgrass systems attract honeybees and other wild pollinators. As turfgrass often requires treatment with insecticides that harm bees, best practices are needed to prevent bees from visiting weed-infested turf areas that will be treated for insect pests. Weed control tactics can protect pollinator exposure to insecticides by reducing the floral resources afforded to bees from turfgrass weeds. Three field studies were conducted in 2021 and 2022 to evaluate the effect of various herbicides and herbicide formulation constituents on pollinator foraging and white clover floral morphology in managed tall fescue turfgrass. Treatments included a nontreated control; MCPP; 2,4-D; dicamba; Trimec Classic™ (2,4-D, MCPP, dicamba); Speedzone™ (carfentrazone, 2,4-D, MCPP, dicamba); and an herbicide-formulation constituent (inert ingredients of Speedzone™). All response variables were evaluated for 8 d, starting from one day before treatment and ending 6 d after treatment (DAT). The herbicide formulation constituent did not alter white clover flower density, floral discoloration, floral quality, or insect visitation compared to nontreated plots. Herbicides reduced flower density and floral quality to the same extent, but MCPP discolored white clover floral tissue 16% per day and less than all other herbicides except dicamba. Floral quality completely declined in approximately 5 d following any herbicide treatment. Bee visitation to white clover–infested turf increased by 3 bees min–1 for every 100 white clover blooms m–2. Honeybees and other insects vacated herbicide-treated areas in less than 2 d, despite minimal effects on floral quality and density at that time. The data suggest that practitioners could apply insecticides 2 d after auxin herbicide treatment and avoid harm to pollinators, but additional work is needed to directly measure pollinator exposure following such treatments.

Phragmites australis invasion and herbicide-based control changes primary production and decomposition in a freshwater wetland

Wetlands are important global carbon sinks, an increasingly important ecosystem service. Invasive plants can disrupt wetland carbon budgets, although efforts to suppress invasive plants may also have unintended effects. Invasive Phragmites australis subsp. australis (European common reed) produces extensive monocultures that displace resident plant communities. In Long Point (Ontario, Canada), a glyphosate-based herbicide was used to control over 900 ha of P. australis. We determined how this ongoing management, and different environmental conditions, influence net primary productivity and decomposition rates. We compared above- and below-ground biomass, belowground:aboveground biomass ratios, standing dead stems, and litter in un-treated P. australis stands, in herbicide-treated areas, and in reference vegetation. We also conducted a reciprocal transplant experiment to measure the decomposition rates of plant litter. One-year post-treatment, control efforts dramatically reduced aboveground biomass (122 g/m2 ± 133 SD) compared to un-treated P. australis (1254 g/m2 ± 449 SD) and reference habitat (821 g/m2 ± 335 SD). An interaction between the plant community of the site and water depth predicted litter decomposition rates, with litter loss ranging from 5.75 to 74.65% across all species, and submerged litter decomposing faster. These results emphasize that secondary treatment, such as rolling, burning, or cutting, encourages the decomposition of dead P. australis stems, opening up space for the recovery of native plants. While herbicide-treated sites had low biomass immediately following treatment, aquatic vegetation began rapidly colonizing treated areas, suggesting that plant community recovery may restore the wetland carbon uptake in subsequent years.

Carrying Capacity Estimation of Herbicide-Treated and Untreated Palm Oil Plantation for Bali Cows

This study aims to estimate the carrying capacity of oil palm plantations with the use of herbicides based on the forage availability for Bali cows. The research was performed in palm oil plantation owned by PTPN V Riau, from March 2016 until March 2017. The parameters observed were 1) Forage production that grows between palm trees at 6 weeks cutting age. The forage sample was taken from 5 point of 1x1m2, then subjected to proximate analysis. Data were analyzed with one-way random design, 2) Forage consumption; obtained by observation of 5 pregnant Bali cow and 7 non-pregnant for seven consecutive days 3). Estimated carrying capacity for Bali cows, obtained from the calculation of forage production (/ha/year) divided by forage consumption. The result of the research showed that 1). The production of dry matter (DM) forage in herbicide-treated areas was 689.55 kg/ha/year, not significantly different compared to untreated areas (622.33 kg/ha/year). Crude protein (CP) forage content of forage obtained from herbicide-treated area was significantly higher (p<0.05) than untreated area, whereas the total content of DM and total digestible nutrients (TDN) was not significantly different. DM consumption of pregnant Bali cow was on average 3.68±0.29 kg/head/day or 1,343.20±105.85 kg/head/year and non-pregnant 4.02±0.36 kg/head/day or 1,467.30±131.4 kg/head/year. The estimated carrying capacity on herbicide-treated (0.51 head of pregnant cow/ha), did not show any significant difference compared to untreated area (0.46 head/ha). For non-pregnant, the carrying capacity of herbicide-treated area (0.47 head/ha) was not different with non-treated area (0.42 head/ha). It was concluded that the use of herbicide on palm oil plantation had no effect on the carrying capacity of the Bali cow.

Fox squirrel response to forest restoration treatments in longleaf pine

Restoration of the longleaf pine (Pinus palustris; LLP) ecosystem and its associated fauna is a principal goal of many land-management agencies in the southeastern United States. Prescribed fire and herbicide application are 2 common methods of LLP restoration. We employed a multi-scale approach to investigate how occurrence of fox squirrels (Sciurus niger) was influenced by fire frequency and herbicide application in LLP communities of northern Florida. We sampled 9-point, 4-ha grids of camera traps with 106 grids in fire treatments, 23 herbicide treatment grids, and 27 control grids. We evaluated a priori models for occurrence of fox squirrels at point, 4-ha patch, and home-range scales, and the influence of fire and herbicides on vegetation structure. Fox squirrel occurrence was positively associated with densities of turkey oak (Quercus laevis) at the patch scale, which were significantly less abundant in herbicide-treated areas. Fox squirrel occurrence was negatively correlated with fire interval and positively correlated with oak densities at a localized point scale. Additionally, fox squirrel point occurrences declined over time since the last fire. Fire produced habitat more favorable for fox squirrels than did herbicide treatments.

Overcoming barriers to seedling regeneration during forest restoration on tropical pasture land and the potential value of woody weeds.

Combating the legacy of deforestation on tropical biodiversity requires the conversion to forest of large areas of established pasture, where barriers to native plant regeneration include competition with pasture grasses and poor propagule supply (seed availability). In addition, initial woody plants that colonise pasture are often invasive, non-native species whose ecological roles and management in the context of forest regeneration are contested. In a restoration experiment at two 0.64 ha sites we quantified the response of native woody vegetation recruitment to (1) release from competition with introduced pasture grasses, and (2) local facilitation of frugivore-assisted seed dispersal provided by scattered woody plants and artificial bird perches. Herbicide pasture grass suppression during 20 months caused a significant but modest increase in density of native woody seedlings, together with abundant co-recruitment of the prominent non-native pioneer wild tobacco (Solanum mauritianum). Recruitment of native species was further enhanced by local structure in herbicide-treated areas, being consistently greater under live trees and dead non-native shrubs (herbicide-treated) than in open areas, and intermediate under bird perches. Native seedling recruitment comprised 28 species across 0.25 ha sampled but was dominated by two rainforest pioneers (Homalanthus novoguineensis, Polyscias murrayi). These early results are consistent with the expected increase in woody vegetation recruitment in response to release from competitive and dispersive barriers to rainforest regeneration. The findings highlight the need for a pragmatic consideration of the ecological roles of woody weeds and the potential roles of “new forests” more broadly in accelerating succession of humid tropical forest across large areas of retired agricultural land.

Evaluación de los efectos del glifosato y otros plaguicidas en la salud humana en zonas objeto del programa de erradicación de cultivos ilícitos

The herbicide glyphosate is administered aerially by the Program to Eradicate Illicit Crops Program and is undertaken in rigorous compliance with the Environmental Management Plan. The effects of the glyphosate herbicide and other aerially applied insecticides were measured to determine possible impact on human health. In 2006-2006, a survey was taken of 112 individuals living in herbicide-treated areas of the Colombian provinces of Huila, Tolima, Putumayo, Guaviare, Santander, Antioquia, Magdalena and La Guajira. Samples of blood were examined for presence of acetylcholinesterase and organochlorine insecticides; urine was analyzed for glyphosate and its metabolites. Fifty percent (50%) of the individuals sampled acknowledged the use of control chemicals as part of their work. The mean exposure time to the chemicals was 84.4 months, with a mean daily exposure of 5.6 hours. The most commonly used pesticides were of category I--extremely hazardous. In individuals sampled for glyphosate (39.6% of the total), 64.3% indicated the use of this herbicide at ground level in agricultural work. A statistically significative relationship was found between the use of glyphosate at ground level, and the concentration levels of glyphosate in the urine samples (odds ratio=2.54, 95% CI: 1.08 to 6.8). These data did not show a relationship between the aerial sprayings of glyphosate for illicit crops eradication and an impact on human health, nor with occupational exposure to this and other chemicals (insecticides) with a high levels of toxicity.

Soil nitrogen mineralisation changes rapidly when pine is planted into herbicide-treated pasture—the first two years of growth

The conversion of hill country pastures to exotic forest plantations has occurred rapidly in New Zealand over the last 10 years. This land-use change affects properties of the mineral soil, especially nitrogen (N) cycling. Here we studied the changes in soil in situ N mineralisation, in leaching, and microbial biomass, when Pinus radiata D. Don is planted into pasture; sheep were used to reduce pasture herbage before planting, and then herbicide was used to control the rank growth of grass around the trees. Total net N mineralisation in soil (0–100 mm) under pasture was 325 kg/ha.year. Net N mineralisation in the herbicide-treated areas was about double that under pasture or rank grass in the first 2 months. The concentration of nitrate-N in soil solution in the herbicide-treated areas was then 81 mg/L, compared with <2 mg/L under pasture. During this time, the pH under the pine trees decreased very rapidly as nitrification increased. Soil nitrification and nitrate leaching were enhanced for 15 months after application of herbicide. After this 15-month period, soil total carbon (C) was slightly lower (P < 0.10) under trees than rank grass, whereas microbial C and N were markedly lowest (P < 0.05) under the trees. Over this period, about 18 kg N/ha was leached to 250 mm in the tree-planted area. The data also give additional insights into processes that occur when soil cores in buried bags are used to estimate N mineralisation.

Broom Snakeweed Control and Seed Damage after Herbicide Applications

Broom snakeweed (Gutierrezia sarothrae [Pursh] Britt. & Rusby) is a major weed problem in the southwestern U.S. because it is toxic to livestock and suppresses forage productivity. In this study, broom snakeweed control, seed production and viability were determined after broadcast spraying in 1997 and 1998 with metsulfuron ({2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl) amino]carbonyl]amino]sulfonyl]benzoic acid}; 0.03 kg a.i. ha-1) and picloram ((4-amino-3, 4, 6-trichloro-2-pyridine-carboxylic acid); 0.28 kg a.i. ha-1). In 1997, plants were sprayed every 2 weeks beginning 1 October when snakeweed was in mid-flower and seed fill and continued until 15 December when seeds were being dispersed. Broom snakeweed control was not different by spray date and averaged 98% with picloram and 77% with metsulfuron. Only plants sprayed on 1 October 1997 with either herbicide had significantly less seed viability than nonsprayed plants, but seed production was not different. In 1998, herbicide applications were repeated at 2 week intervals for 6 weeks beginning on 1 September when snakeweed was in early-flower and seed development. Broom snakeweed control with picloram (average 88%) was consistently high across all spray dates, whereas, control with metsulfuron (average 25%) was always poor. Both herbicides reduced seed production by an average of 99, 95, and 38% when applied on 1 and 15 September 1998 and 1 October 1998, respectively, but seed production was not different among sprayed and nonsprayed plants after these dates. In the spring of 1999, broom snakeweed seedlings were common in all areas previously sprayed in 1997, but few seedlings established in plots sprayed in 1998. In the spring of 2001, the number of newly emerged broom snakeweed seedlings observed in nonsprayed and herbicide-treated areas was the same, irrespective of spray year, herbicide type or date applied. Data indicate that herbicide applications made at flower when seed is in early fill can provide satisfactory plant control and lower seed production. Spraying after seed has reached physiological maturity does not affect seed production or viability. In this study, results were inconclusive for determining if timed herbicide applications in autumn can be used to minimize later broom snakeweed establishment. DOI:10.2458/azu_jrm_v55i6_mcdaniel

Broom snakeweed control and seed damage after herbicide applications

Broom snakeweed (Gutierrezia sarothrae [Pursh] Britt. Rusby) is a major weed problem in the southwestern U.S. because it is toxic to livestock and suppresses forage productivity. In this study, broom snakeweed control, seed production and viability were determined after broadcast spraying in 1997 and 1998 with metsulfuron ({2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl) amino]carbonyl]amino]sulfonyl]benzoic acid}; 0.03 kg a.iha-1) and picloram ((4-amino-3, 4, 6-trichloro-2-pyridine-carboxylic acid); 0.28 kg a.iha-1). In 1997, plants were sprayed every 2 weeks beginning 1 October when snakeweed was in mid-flower and seed fill and continued until 15 December when seeds were being dispersed. Broom snakeweed control was not different by spray date and averaged 98% with picloram and 77% with metsulfuron. Only plants sprayed on 1 October 1997 with either herbicide had significantly less seed viability than nonsprayed plants, but seed production was not different. In 1998, herbicide applications were repeated at 2 week intervals for 6 weeks beginning on 1 September when snakeweed was in early-flower and seed development. Broom snakeweed control with picloram (average 88%) was consistently high across all spray dates, whereas, control with metsulfuron (average 25%) was always poor. Both herbicides reduced seed production by an average of 99, 95, and 38% when applied on 1 and 15 September 1998 and 1 October 1998, respectively, but seed production was not different among sprayed and nonsprayed plants after these dates. In the spring of 1999, broom snakeweed seedlings were common in all areas previously sprayed in 1997, but few seedlings established in plots sprayed in 1998. In the spring of 2001, the number of newly emerged broom snakeweed seedlings observed in nonsprayed and herbicide-treated areas was the same, irrespective of spray year, herbicide type or date applied. Data indicate that herbicide applications made at flower when seed is in early fill can provide satisfactory plant control and lower seed production. Spraying after seed has reached physiological maturity does not affect seed production or viability. In this study, results were inconclusive for determining if timed herbicide applications in autumn can be used to minimize later broom snakeweed establishment.

THE EFFECT OF VEGETATION MANAGEMENT ON BREEDING BIRD COMMUNITIES IN BRITISH COLUMBIA

Removal of deciduous vegetation in managed conifer forests is a major component of most silviculture programs in North America. We examined the effect of removing 90-96% of the volume of deciduous trees on breeding bird communities in young conifer plantations during four years. Trees were removed by two treatments: manual thin- ning (manual treatment), and manual thinning plus application of a herbicide, glyphosate (herbicide treatment). The control and two treatments were each replicated three times. During the three post-treatment years, the herbicide-treated sites remained depauperate of deciduous vegetation while the manually thinned sites experienced regrowth of deciduous trees. Number of species declined, total number of individuals increased, and common species dominated after herbicide treatment. Number of species, total number of individuals, and evenness increased after manual treatment. Turnover of bird species was highest in the herbicide-treated areas and lowest in control areas. Residents, short-distance migrants, ground gleaners, and conifer nesters increased significantly after herbicide treatment. De- ciduous nesters and foliage gleaners increased in abundance (nonsignificantly) in control and manually thinned areas. Warbling Vireos (Vireo gilvus), which are deciduous specialists, declined in areas treated with herbicide and may be particularly susceptible to glyphosate application. Although treated areas exhibited similar increases in the total number of birds, nesting success of open-cup nesting species was significantly lower in the herbicide-treated than in manually thinned areas. We suggest that habitat variability may be a mechanism for producing nested subset structure of bird community composition. Overall, the com- position of bird communities became more homogeneous after herbicide treatment, and it showed little change after manual thinning.

Post-fire herbicide sprays enhance native plant diversity

Following catastrophic fire, broad-spectrum herbicides such as hexazinone are often used to control shrubs and forbs that compete with planted conifers. This practice encourages rapid growth and reduces mortality of conifers. Although the initial effect is to reduce native plant species richness, recovery is rapid and plant diversity exceeds that in untreated areas within 8 years of application. Success of native forb and grass species in herbicide-treated areas appears to be due to early suppression of otherwise dominant shrubs.

Effect of nitrogen fertiliser on temporal and spatial variation of mineral nitrogen and microbial biomass in a silvopastoral system

This paper describes a field study to assess the effect of increasing the frequency of split applications of N fertiliser on the pattern of plant uptake, soil N availability, and microbial biomass C and N. Measurements were taken during the growing season in different positions relative to young trees (Prunus avium L.) in an upland silvopastoral system in its first year after establishment. At fertiliser rates of 72 and 144 kg ha-1 N applied as NH4NO3, increasing the number of split applications increased N uptake by the pasture. Mineral forms of soil N measured 2 weeks after application indicated that residual NH inf4 sup+ -N and total mineral N were also greater in this treatment on certain dates. Soil NO inf3 sup- -N was positively correlated with the soil moisture content, and nitrification reached a maximum in early May and declined rapidly thereafter except within the herbicide-treated areas around the trees where soil moisture had been conserved. Results of the study suggest that high NO inf3 sup- -N in herbicide-treated areas was probably caused by mineralisation of grass residues and low uptake by the tree rather than by preferential urine excretion by sheep sheltering beside the trees. Mean microbial biomass C and N values of 894 and 213 kg ha-1, respectively, were obtained. Microbial C was slightly increased by the higher frequency of split applications at 144 kg ha-1 N and was probably related to the greater herbage production with this treatment. Microbial N was not significantly affected by the N treatments. Both microbial biomass C and N increased during the growing season, resulting in the net immobilisation of at least 45 kg ha-1 N which was later released during the autumn.

Response of wildlife to forest herbicide applications in northern coniferous ecosystems

Reviewed studies of the effects of forest herbicide applications on wildlife often lacked replication, pretreatment information, and (or) were conducted for only one or two growing seasons after treatment. Because of these problems, as well as the use of dissimilar sampling techniques, study conclusions have sometimes been contradictory. A review of eight studies of the effects of herbicide treatments on northern songbird populations in regenerating clearcuts indicates that total songbird populations are seldom reduced during the growing season after treatment. Densities of species that use early successional brushy, deciduous cover are sometimes reduced, while densities of species which commonly use more open areas, sometimes increase. A review of 14 studies of the effects of herbicide treatments on small mammals indicates that like songbirds, small mammal responses are species specific. Some species are unaffected, while some select and others avoid herbicide-treated areas. Only studies that use kill or removal trapping to study small mammal responses show density reductions associated with herbicide treatment. It seems that some small mammal species may be reluctant to venture into disturbed areas, although residents in those areas are apparently not affected by the disturbance. Fourteen relevant studies examined the effects of conifer release treatments on moose and deer foods and habitat use. Conifer release treatments reduce the availability of moose browse for as long as four growing seasons after treatment. The degree of reduction during the growing season after treatment varies with the herbicide and rate used. Deer use of treated areas remains unchanged or increases during the first growing season after treatment. Eight years after treating a naturally regenerated spruce–fir stand browse was three to seven times more abundant on treated than on control plots (depending on the chemical and rate used). Forage quality (nitrogen, ash, and moisture) of crop trees increased one growing season after the soil-active herbicide simazine was applied to control competition around outplanted 3-year-old balsam fir seedlings.

Nutritional Quality of Browse after Brush Management on Cross Timbers Rangeland

We evaluated seasonal changes in browse quality 5-6 years after experimental manipulations to control unwanted woody vegetation using combinations of herbicide and fire on cross timbers rangeland in central Oklahoma. The study area consisted of two 32-ha replications of untreated controls and 4 brush treatments (tebuthiuron and triclopyr used singly or in combination with periodic prescribed burning); herbicides were applied in 1983 and fires initiated in 1985. Nutritional quality of blackberry (Rubus spp.), coralberry, (Symphoricarpos orbiculatus Moench), rough-leaf dogwood (Cornus drummondii Meyer), elm (Ulmus spp.), greenbrier (Smilax spp.), hackberry (Celtis spp.), and smooth sumac (Rhus glabra L.) were assessed by measuring crude protein, in vitro dry matter digestibility, neutral detergent fiber, acid detergent fiber, and moisture content. Crude protein concentrations of browse were 14% higher on herbicide-treated areas compared to untreated controls and 11% higher on triclopyr treatments compared to tebuthiuron treatments. In vitro dry matter digestibility was 9% higher on herbicide-treated areas compared to untreated controls. Fiber constituents and moisture content were not influenced by brush treatments. Prescribed burning combined with herbicide applications did not improve the quality of browse. Our results indicate that browse quality can be improved for white-tailed deer by applications of tebuthiuron or triclopyr and improvements persist for up to 6 years post treatment.

Growth and persistence of perennial ryegrass and white clover direct-drilled into a paspalum-dominant dairy pasture treated with glyphosate

Abstract Paspalum-dominant pastures suffer from poor winter/spring herbage production, which can be corrected by direct-drilling cool-season-active species such as ryegrass. The growth and persistence of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) direct-drilled into a paspalum (Paspalum dilatatum Poir.)-dominant, rotationally grazed dairy pasture, and their effects on production, were studied over 5 years. Treatments were original pasture (control) and herbicide treatment with 0, 0.72, 1.44, and 2.16 kg a.i./ha of glyphosate (Roundup® herbicide, 36% glyphosate) before direct-drilling. Observations on tagged ryegrass and white clover plants found establishment plant numbers for ryegrass were 74% higher (788 plants/m2) in herbicide-treated than in untreated areas (452 plants/m2). Half of all tagged ryegrass plants disappeared in the first year after which losses were much reduced. The establishment and survival of tagged white clover plants were poor. Paspalum gradually re-infested the herbicide-treated areas and by the end of the fourth year had reached pretreatment levels of 30–40% of dry matter (DM). Application of glyphosate before drilling produced a 38% increase in winter/spring herbage accumulation in the first year, but this advantage declined and was non-significant by the end of the fourth year.

Crop Responses to Hexazinone, Imazapyr, Tebuthiuron, and Triclopyr

Field investigations were conducted for 2 yr to characterize the responses of wheat, kidney bean, field corn, squash, okra, potato, and banana to soil-applied hexazinone, imazapyr, tebuthiuron, and triclopyr at rates used for control of woody perennial plants. Test species were planted at seven selected intervals through the first and second years after herbicide applications to assess residual herbicide activity. Tebuthiuron treatments of 2.2 kg ai ha−1were not tolerated by any bioassay species planted 436 d after application (DAA). Only potato was tolerant of residual hexazinone 436 DAA. All species except banana generally tolerated residual imazapyr and triclopyr by the second growing season. Indigenous plants repopulated all herbicide-treated areas, except those treated with tebuthiuron, by the third growing season. Plots treated with tebuthiuron were 90% free of indigenous plants 40 mo after herbicide application.

Barley grass control with herbicides in subterranean clover pasture. 1. Effect on pasture in the year of spraying

Barley grass (Hordeum spp.) in a subterranean clover pasture was controlled with herbicides in the time between the 2-3 leaf stage and flowering. The effects on subsequent pasture production and composition were measured and the interaction between grazing and herbicides was also assessed. Early removal of barley grass reduced dry matter production in winter by an average of 41%. It also allowed an incursion of Indian hedge mustard (Sisymbrium orientale L.). which was suppressed by grazing but comprised an average of 23% ground cover in ungrazed pasture. Loss of total pasture production decreased as the date of herbicide application was delayed from June to September. Broadleaf species compensated for the early removal of barley grass, so that total dry matter production in spring in herbicide-treated areas was higher than in the unsprayed pasture. Fluazifop-p at 32, 53, and 106 g a.i/ha gave, respectively, 94, 91, and 94% reduction in barley grass seedhead density when applied at the 2-3 leaf stage, and 64, 88, and 98% reduction when applied at mid jointing. Application of this herbicide at 53 or 106 g a.i/ha for barley grass control can be delayed until late winter, thereby maintaining winter feed production but avoiding declining palatability and awned barley grass seeds.

Concentrations of nitrate in soil water following herbicide treatment of tree planting positions in an upland agroforestry system

Concentrations of nitrate have been measured in soil water samples collected during the first year of an agroforestry trial at an upland site in Wales. Nitrate concentrations were higher in water samples collected from herbicide-treated areas around trees than from undisturbed pasture between the trees. These differences were statistically significant fo samples collected during the summer but not for those from the main winter leaching period. The enhanced nitrate concentrations that were measured beneath the tree planting positions may have implications for the nutrition of the trees but, at the planting densities included in the trial (100 and 400 trees/ha), are estimated to represent only a small increase in the nitrate content of water draining from the agroforestry area as a whole, relative to that from unplanted pasture. There was some evidence that preferential deposition of urine, but not of dung, contributed to the increased nitrate contents in the soil around the planting positions.

Runoff and Erosion Following Mechanical and Chemical Control of Creosotebush (Larrea tridentata)

Runoff and sediment yield were monitored from 1983 through 1986 on a range site dominated by creosotebush. The site was rootplowed and seeded, treated with tebuthiuron at 36 kg ai ha-1, or left untreated. Runoff from rootplowed and herbicide-treated plots was no different from untreated plots for 1 yr, but sediment yield from treated plots was lower than that from the untreated plots. Rootplowing and seeding increased sediment yield in the second year (1984), whereas treating with herbicide decreased sediment yield. Runoff and sediment yield during 1985 and 1986 were greatest from the untreated areas and least from the rootplowed and herbicide-treated areas. Total cover was nearly the same for each treatment. The untreated plots had 13% shrub cover; the others had none. Between shrubs in untreated plots, bare ground occupied large interconnected areas that contributed to the higher runoff and erosion rates.

Wild-Proso Millet (Panicum miliaceum) Control in Central Great Plains Irrigated Corn (Zea mays)

Wild-proso millet control in furrow-irrigated corn was evaluated in Colorado and Nebraska in 1986 and 1987. No single herbicide alone controlled wild-proso millet all season. In Colorado, EPTC applied preplant incorporated followed by cyanazine plus pendimethalin applied early postemergence controlled 94% of wild-proso millet. Acetochlor applied preemergence followed by cyanazine plus pendimethalin applied early postemergence controlled wild-proso millet better all season (93%) than alachlor (81%) or metolachlor (71%) followed by the same early postemergence strategy. Successful Colorado wild-proso millet management treatments (>85% season-long control) increased corn yields an average of 3260 kg ha-1compared to the untreated control. To obtain wild-proso millet control of 90% or more in Nebraska in 1986, alachlor, cycloate, EPTC, and metolachlor applied had to be combined with cyanazine plus pendimethalin applied early postemergence. Average corn yields in herbicide-treated areas in Nebraska were 2980 kg ha-1higher than those recorded in the untreated control.