Production and utilization of crop residues as mulch and effective weed management are two central elements in the successful implementation of Conservation Agriculture (CA) systems in southern Africa. Yet, the challenges of crop residue availability for mulch or the difficulties in managing weed proliferation in CA systems are bigger than a micro-level focus on weeds and crop residues themselves. The bottlenecks are symptoms of broader systemic complications that cannot be resolved without appreciating the interactions between the current scientific understanding of CA and its application in smallholder systems, private incentives, social norms, institutions, and government policy. In this paper, we elucidate a series of areas that represent some unquestioned answers about chemical weed control and unanswered questions about how to maintain groundcover demanding more research along the natural and social sciences continuum. In some communities, traditional rules that allow free-range grazing of livestock after harvesting present a barrier in surface crop residue management. On the other hand, many of the communities either burn, remove, or incorporate the residues into the soil thus hindering the near-permanent soil cover required in CA systems. The lack of soil cover also means that weed management through soil mulch is unachievable. Herbicides are often a successful stopgap solution to weed control, but they are costly, and most farmers do not use them as recommended, which reduces efficacy. Besides, the use of herbicides can cause environmental hazards and may affect human health. Here, we suggest further assessment of the manipulation of crop competition, the use of vigorously growing cover crops, exploration of allelopathy, and use of microorganisms in managing weeds and reducing seed production to deplete the soil weed seed bank. We also suggest in situ production of plant biomass, use of unpalatable species for mulch generation and change of grazing by-laws towards a holistic management of pastures to reduce the competition for crop residues. However, these depend on the socio-economic status dynamics at farmer and community level.

Glyphosate-resistant weeds are difficult to manage and can serve as hosts for pests that threaten cultivated crops. Chrysodeixis includens (Walker) (Lepidoptera: Noctuidae) is one of the main polyphagous pests of soybean in Brazil that can benefit from weeds' presence during season and off-season. Despite its pest status, little is known about C. includens survival and development on alternative hosts, including those resistant to glyphosate. Therefore, we assessed the biology, reproduction, preference, and survival at different feeding periods of C. includens on seven glyphosate-resistant weeds (Sumatran fleabane, Italian ryegrass, sourgrass, goosegrass, smooth pigweed, wild poinsettia, hairy beggarticks) commonly found in Brazilian agroecosystems, under laboratory conditions. Our results showed that C. includens survival and reproduction were similar on soybean and wild poinsettia. Survival and reproduction were lower on smooth pigweed and hairy beggarticks. Also, these plants prolonged the larval stage. Larvae did not pupate when fed on sourgrass, goosegrass, Italian ryegrass, and Sumatran fleabane. However, on Sumatran fleabane their biomass was higher. The mean generation time was lower on wild poinsettia. This weed was preferred to soybean. An antifeeding factor was observed on Sumatran fleabane. Larvae fed for 11 days on soybean, wild poinsettia and smooth pigweed developed into pupae. In agricultural systems, farmers must pay attention to the management of these weeds, especially wild poinsettia, smooth pigweed, and hairy beggarticks, to interrupt the cycle of this pest, since these plants can serve as main sources of infestation for the soybean crop.

Climate change and soil fertility decline are major threats to smallholder farmers' food and nutrition security in southern Africa, and cropping systems that improve soil health are needed to address these challenges. Cropping systems that invest in soil organic matter, such as no-tillage (NT) with crop residue retention, have been proposed as potential solutions. However, a key challenge for assessing the sustainability of NT systems is that soil carbon (C) stocks develop over long timescales, and there is an urgent need to identify trajectory indicators of sustainability and crop productivity. Here we examined the effects of NT as compared with conventional tillage without residue retention on relationships between soil characteristics and maize (Zea mays L.) productivity in long-term on-farm and on-station trials in Zimbabwe. Our results show that relationships between soil characteristics and maize productivity, and the effects of management on these relationships, varied with soil type. Total soil nitrogen (N) and C were strong predictors of maize grain yield and above-ground biomass (i.e., stover) in the clayey soils, but not in the sandy soils, under both managements. This highlights context-specific benefits of management that fosters the accumulation of soil C and N stocks. Despite a strong effect of NT management on soil C and N in sandy soils, this accrual was not sufficient to support increased crop productivity in these soils. We suggest that sandy soils should be the priority target of NT with organic resource inputs interventions in southern Africa, as mineral fertilizer inputs alone will not halt the soil fertility decline. This will require a holistic management approach and input of C in various forms (e.g., biomass from cover crops and tree components, crop residues, in combination with mineral fertilizers). Clayey soils on the other hand have greater buffering capacity against detrimental effects of soil tillage and low C input.

Food and nutrition insecurity in southern Africa call for improvements in traditional agriculture systems. Conservation Agriculture (CA) based on minimum soil disturbance, permanent soil cover and crop diversification has been implemented as a strategy to maintain yields while safeguarding the environment. However, less focus has been placed on potential synergistic benefits on nutrition security. Maize-based systems may increase household income through selling but may not lead to proportionate reduction in malnutrition. Crop diversification in CA systems can have a direct impact on the nutritional status of farm households due to improved dietary diversity. Here we assess how the integration of grain legumes, cowpeas and soybeans, in maize-based CA systems either as intercrops or rotational crops affects maize grain yield and stability, total energy yield, protein yield and surplus calories after satisfying the daily requirement per household. The experiments were carried out from 2012 to 2020 (nine consecutive cropping seasons) in six eastern Zambian on-farm communities using 966 observations. Results show that intercropping compromises maize yields with marginal yield penalties of −5% compared to no-till monocropping. However, intercropped yields were more stable across environments. Total system caloric energy and protein yield were highest in intercropping systems due to higher productivity per unit land area owing to the additive contribution of both maize and legumes. Total system caloric energy and protein yield reached yearly averages of 60 GJ ha−1 and 517 kg ha−1, respectively, for the intercropping system as compared to 48 GJ ha−1 and 263 kg ha−1 in monocropped maize systems. Tillage-based monocrop resulted in the least stable yields. Our results suggest that intercropping maize with grain legumes in CA systems is a promising option for smallholder farming households to improve dietary diversity, dietary quality and stability of yields thus contributing to sustainable agriculture intensification while maintaining food and nutrition security.

Stochastic plateau production functions provide improved fertilizer recommendations based on multi-year agronomic experiments where weather and other stochastic variables change over time. This research assesses the profitability of no-tillage corn production in northeastern Colorado and determines economically optimal nitrogen fertilizer rates. It also proposes an alternative parameterization of the linear response stochastic plateau model which provides a robustness check against traditional parameterizations. Results show the current use of nitrogen fertilizer in the area exceeds estimated economically optimal levels. This suggests that a reduction in nitrogen use could increase expected profits and simultaneously reduce environmental costs.

Tillage regimes can influence weed population dynamics and, consequently, the choice of appropriate weed management practices. Studies were conducted in 2016 and 2017 in a long-term (36-yr) grain sorghum [Sorghum bicolor (L.) Moench ssp. bicolor] experiment at Texas A&M University, College Station, to determine the impact of long-term no-till (NT) and conventional till (CT) systems on weed species dynamics. Higher densities of johnsongrass [Sorghum halepense (L.) Pers.], prostrate spurge [Chamaesyce humistrata (Engelm. ex A. Gray) Small], waterhemp [Amaranthus tuberculatus (Moq.) Sauer], and henbit (Lamium amplexicaule L.) were recorded in the NT system compared with the CT system. Further, the NT system showed greater weed diversity (Shannon-Wiener index, H = 0.8) and species richness (S = 6.2), compared with the CT system (H = 0.6, S = 4.2). Seedling emergence of some dominant weed species was also delayed in the NT system. In the CT system, 50% emergence of S. halepense (8.5 C base temperature) and waterhemp (10 C base temperature) occurred at 59 and 63 growing degree days (GDD), respectively, whereas 68 and 75 GDD, respectively, were required in the NT system. Further, a greater proportion (61%) of the viable seedbank was present at the top 5 cm of the soil in the NT system compared with the CT system (46%). Overall, findings from this 36-yr-long tillage experiment have revealed that the NT system had greater weed densities (especially of the perennial weed S. halepense) and a high proportion of weed seeds (particularly small-seeded annuals) on the topsoil layer, corroborating some earlier reports that were based on short-term investigations. Findings indicate that growers transitioning to NT systems should be mindful of potential shifts in weed species dominance and develop appropriate management tactics.

Macro-organisms contribute significantly to soil fertility improvement. The influence of conservation agriculture (CA) in southern Africa on their populations has not yet been fully understood. Thus, the objective of the current study was to evaluate the effects of CA and conventional tillage on below ground biological activity in a CA long-term trial in Monze, Zambia from 2011 to 2013. The study had ten treatments which differed by tillage systems (conventional ploughing, planting basins and direct seeding) and crop diversification intensity (sole cropping, 2- or 3-year crop rotations) involving maize, cotton and sunn hemp. These factors were combined to create rotation-tillage (RotTill) treatments. Sampling of macrofauna was done once per year using a metal frame measuring 0.25 m2, hand-sorted to 30-cm depth. RotTill treatments had a significant effect on earthworms (Lumbricus terrestris), termites (Coptotermes formosanus), dung beetles (Scarabaeus viettei) and centipedes (Lithobius forficatus). Earthworms and termites were more abundant in CA treatments than in conventionally ploughed (CP) treatments. Biota diversity was generally higher in CA treatments than in CP controls. Conventional mouldboard ploughing generally reduced macrofauna, thus affecting biological soil fertility and the beneficial effect of the interactions of these organisms with the soil. CA treatments had the highest maize grain yields throughout the study period. Based on the results, reduced tillage systems and crop rotations increase biological activity shown by increased densities of termites and earthworms.

The semiarid Espinal in central Argentina, being recently transformed from natural semiarid grasslands into agriculture, represents an interesting scenario to understand the early stages of weed community assembly and its relationship with crop identity and management. Our aim was to characterize the weed communities in corn (Zea mays L.) and soybean [Glycine max (L.) Merr.], the main crops of the Espinal region, under the dominant rainfed conditions. Weed surveys were carried out in 53 fields, and farmers were interviewed to collect information about crop management. Floristic composition was compared within and between crops by calculating the additive partition of the abundance-based Bray-Curtis dissimilarity. We compared the frequency and mean cover of functional groups between crops through generalized linear models. Finally, canonical correspondence analysis was carried out to analyze the associations between floristic composition and agronomic variables. Mean alpha and gamma diversity was greater in corn (10.0 and 80 species, respectively) than in soybean (7.6 and 46 species, respectively). Furthermore, species composition of weed communities was more similar among soybean fields than among either cornfields or fields of both crops. Hence, floristic differences between crops are potentially the result of different microenvironmental heterogeneity above- and belowground, with corn likely to be more permissive to weed establishment compared with soybean. The higher frequency of annual, dicotyledonous, and native species, and the high proportion of rare species, mostly native, suggest a strong legacy of the original vegetation that thrived in these recently cultivated systems. The functional composition was also affected by agronomic management, with sulfur, nitrogen, and grass herbicide application being the most important factors related to the floristic composition of weed communities. This early description can be used as a starting point for studies concerning trajectories, mechanisms, and processes of weed communities related to environment and management.

Climate variability and declining soil fertility pose a major threat to sustainable agronomic and economic growth in Zambia. The objective of this study was to assess crop yield, land and labor productivity of conservation agriculture (CA) technologies in Eastern Zambia. On-farm trials were run from 2012–2015 and farmers were replicates of a randomized complete block design. The trials compared three CA systems against a conventional practice. Yield and net return ha−1 were determined for maize and legume yield (kg ha−1) produced by ridge and furrow tillage, CA dibble stick planting, CA animal traction ripping and direct seeding. The dibble stick, ripline and direct seeding CA systems had 6–18, 12–28 and 8–9% greater maize yield relative to the conventional tillage system, respectively. Rotation of maize with cowpea and soybean significantly increased maize yields in all CA systems. Intercropping maize with cowpea increased land productivity (e.g., the land equivalent ratio for four seasons was 2.01) compared with full rotations under CA. Maize/cowpea intercropping in dibble stick CA produced the greatest net returns (US$312-767 ha−1) compared with dibble stick maize-cowpea rotation (US$204-657), dibble stick maize monoculture (US$108-584) and the conventional practice (US$64-516). The net-return for the animal traction CA systems showed that maize-soybean rotations using the ripper were more profitable than the direct seeder or conventional ridge and furrow systems. Agronomic and economic benefits of CA-based cropping systems highlight the good potential for improved food security and agricultural productivity for smallholder farmers.

A 3-yr (1982, 1983, and 1984) study was conducted to determine the relationship between tillage and six weed control treatments for soybean [Glycine max (L.) Merr. ‘Williams 79′] production on silt loam (Udollic and Mollic Ochraqualfs). Conventional tillage consisted of spring moldboard plowing and secondary tillage with a combination tool just before planting. No-tillage consisted of a foliar application of glyphosate [N-(phosphonomethyl)glycine] at the time of planting. Weed control treatments included combinations of no weed control with and without soybean plants, preemergence herbicide application only, postemergence herbicide application only, and combined preemergence and postemergence applications with and without additional handweeding. In 1982, a year of above-normal seasonal rainfall, conventional tillage had significantly greater soybean grain yields than no-tillage for all weed control treatments except the preemergence-only treatment. Yields within tillage systems and between weed control treatments in 1982 were not significantly different because adequate rainfall reduced the effect of weed competition for soil moisture. Soybean seed yields in 1983 and 1984 in no-tillage were equal to or significantly greater than those of conventional tillage. No-tillage treatments had greater soil moisture conservation and soil moisture availability resulting in less plant water stress during podfilling in periods of drought in 1983 and 1984. In all 3 yr, conventional tillage had significantly greater early weed growth than no-tillage in the treatments with and without soybean plants where no preemergence or postemergence herbicides were used. Comparing treatments with and without soybean plants indicated an average increase of 36 and 38% weed control for no-tillage and conventional tillage, respectively.

Field violet germination occurred during May to early July and again during September to November in Nova Scotia (N.S.), Canada. Germination occurred from October until early April in the North Carolina (NC) piedmont. Germination was stimulated by tillage. Seed collected in N.S. and in NC germinated optimally in the dark under a temperature regime of 15/5 C, on an 8/16 h thermoperiod. Germination at constant temperatures decreased with increasing temperatures, from 5 to 25 C. Seed collected in NC was capable of germinating at a higher constant temperature of 25 C than seed collected in N.S. Germination of seed collected in NC was partially inhibited by constant irradiance with a photosynthetic photon flux density of 100 μM m−2 s−1 (400 to 700 nm). Seed did not germinate when exposed to constant red or far-red irradiance.

Two strains of crownvetch (Coronilla varia L. # CZRVA) rhizobia were cultured in vitro with various rates of atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine] and bifenox [methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate]. Growth, measured turbidimetrically over 48 h, was similar for both strains. Atrazine and bifenox significantly reduced bacterial growth after 14 and 36 h, respectively, only at the highest concentrations tested (463 μM atrazine and 292 μM bifenox). Since growth of crownvetch rhizobia was apparently not affected by rates of atrazine or bifenox above reasonable soil solution concentrations, it is likely that herbicidal effects on nodulation were due to toxicity to the host plant rather than toxicity to these bacteria. In a growth chamber experiment, total nodule activity (TNA) and carbon dioxide exchange rate (CER) were measured simultaneously in an effort to distinguish direct atrazine effects on nodule function from indirect effects due to inhibition of photosynthesis and a resulting decrease in photosynthate supply to nodules. When 5 and 50 mg atrazine per kg soil were applied to intact plants, CER was severely reduced within 24 h, but similar reductions in TNA were not observed until 48 h after treatment. Total nodule activity was reduced similarly by atrazine and defoliation; the application of atrazine to defoliated plants did not inhibit TNA more than did defoliation alone. The data indicate that reductions in crownvetch nodule activity by atrazine are due to inhibition of photosynthesis or other processes rather than direct toxicity to N fixation.

Soils from long-term tillage plots at three locations in Ohio were sampled to determine composition and size of weed seed banks following 25 yr of continuous no-tillage, minimum-tillage, or conventional-tillage corn production. The same herbicide was applied across tillage treatments within each year and an untreated permanent grass sod was sampled for comparison. Seed numbers to a 15-cm depth were highest in the no-tillage treatment in the Crosby silt loam (77 800 m–2) and Wooster silt loam (8400 m–2) soils and in the grass sod (7400 m–2) in a Hoytville silty clay loam soil. Lowest seed numbers were found in conventional-tillage plots in the Wooster soil (400 m–2) and in minimum-tillage plots in the Crosby (2200 m–2) and Hoytville (400 m–2) soils. Concentration of seeds decreased with depth but the effect of tillage on seed depth was not consistent among soil types. Number of weed species was highest in permanent grass sod (10 to 18) and decreased as soil disturbance increased; weed populations were lowest in conventional tillage in the Hoytville soil. Common lambsquarters, pigweeds, and fall panicum were the most commonly found seeds in all soils. Diversity indices indicated that increased soil disturbance resulted in a decrease in species diversity. Weed populations the summer following soil sampling included common lambsquarters, pigweeds, fall panicum, and several species not detected in the seed bank.

Recent changes in technology, governmental regulation and scrutiny, and public opinion have motivated the agricultural community to examine current management practices from the perspective of how they fit into a sustainable agricultural framework. One aspect which can be incorporated into many existing farming systems is plant residue management (e.g., reduced tillage, cover crops). Many residue management systems are designed to enhance accumulation of plant residue at the soil surface. The plant residue covering the soil surface provides many benefits, including protection from soil erosion, soil moisture conservation by acting as a barrier against evaporation, improved soil tilth, and inhibition of weed emergence. This review summarizes recent literature (ca. last 25 yr) concerning the effects of plant residue management on the soil environment and how those changes impact herbicide interactions.

The effect of different levels of tillage and weed management on population, distribution, and germination of weed seed was evaluated in three tillage systems at Arlington and Hancock, WI, in 1989 and 1990. Over 60% of all weed seed in the top 19 cm of soil were found in the top 1 cm in no-tillage at both sites. As depth increased, concentration of weed seed declined logarithmically in no-tillage. In chisel plowing, over 30% of seed were in the top 1 cm and seed concentration decreased linearly with depth. Moldboard plowing had uniform distribution of weed seed in the top 19 cm of soil. Preemergence metolachlor plus atrazine decreased weed seed population by 50% compared with no treatment over all tillage systems. One year of the herbicide treatment plus handweeding to assure weed-free conditions did not reduce seed numbers in chisel plowing or moldboard plowing compared to herbicide alone. Seed numbers with no-tillage and weed-free conditions decreased by 40% relative to herbicide alone. Common lambsquarters germination was 40% greater in moldboard plowing and chisel plowing compared with no-tillage. Germination was highest in seed taken from 9 to 19 cm deep in moldboard plowing and from 0 to 9 cm deep in chisel plowing.

The herbicides 2,4-D [(2,4-dichlorophenoxy) acetic acid], dalapon (2,2-dichloropropionic acid), glyphosate [(N-phosphonomethyl)glycine], paraquat (1,1’-dimethyl-4,4’-bypiridinium ion), and pronamide [3,5-dichloro-(N-1,1-dimethyl-2-propynyl)benzamide] were applied at selected rates and dates to quackgrass [Agropyron repens (L.) Beauv. ♯ AGRRE]-infested alfalfa (Medicago sativa L.) sod in 1979 and 1980 to suppress vegetation for subsequent establishment of alfalfa without tillage. The influence of herbicides on weed control, alfalfa seedling density and visual stand, and forage yield and quality was assessed for 3 yr. During establishment, all herbicides except 2,4-D effectively controlled quackgrass. Glyphosate applied broadcast increasingly suppressed the existing sod as the rate was increased from 0.6 to 1.6 kg ai/ha. The effectiveness of herbicides as measured by total (old and new) plant density was closely related to the suppression of the sward. While the substitution of new alfalfa plants for old ones was not necessary under dense (40 plants/m2) alfalfa stands, completely controlling the vegetation with glyphosate resulted in sustained productivity throughout the experimental period. After 3 yr, alfalfa percentage and yield were highest when glyphosate was broadcast at 1.6 kg/ha. Improvement in forage quality, as estimated by the concentrations of crude protein and neutral-detergent fiber, was closely related to grass control.

Cover crop residues are not widely used for weed control because, as a stand-alone tactic, they do not effectively suppress all weeds and their duration of weed control is too short. Field experiments were conducted in 1995 and 1996, under both irrigated and rainfed conditions, to quantify Amaranthus spp., Setaria spp., and soybean emergence and growth in residues of fall-planted, spring-killed barley, rye, triticale, wheat, and hairy vetch. For both weed species, seedling emergence was reduced 3 wk after soybean planting by rye and wheat residues (≥ 2, 170 kg ha−1) in 1996. In 1996, Amaranthus spp. canopy volume was reduced 38 to 71% by residues 3 wk after planting. Likewise, Setaria spp. canopy biomass was reduced 37 to 97% in residues 5 wk after planting over both years. The response comparison index was used to identify frequency by which weed growth was placed at a disadvantage relative to soybean growth. Amaranthus spp. and Setaria spp. growth suppressions 3 to 5 wk after planting indicate potential times for intervention with other integrated weed management tactics such as reduced postemergence herbicide rates and interrow cultivation.

Horseweed emergence and survival were evaluated in no-tillage soybean and corn at Rosemount, MN, and Ames, IA, from the fall of 1992 through the summer of 1995. Most of the horseweed at both locations emerged in the fall. Winter survival of fall-emerged seedlings ranged from 59 to 91%. Timing and extent of horseweed emergence in spring varied by year and location. Spring emergence ranged from 5 to 32% of total emergence, with greater spring emergence at Rosemount than at Ames. Emergence occurred as late as early June at Rosemount and late May at Ames. Results indicated that horseweed may emerge well into the growing season, and spring emergence should be considered in weed management systems for no-tillage crop production.

Field experiments were conducted in 1996, 1997, and 1998 to evaluate weed control and corn tolerance from soil-applied RPA 201772. Treatments alone and in tank mixtures with other corn herbicides were applied preemergence (PRE) at two locations with conventional tillage and at early preplant (EPP) and PRE application timings in no-tillage corn. RPA 201772 was applied alone and in tank mixtures with one-half the typical field rates of other PRE corn herbicides. In conventional tillage experiments in 1996, 1997, and at one location in 1998, all treatments containing RPA 201772 provided > 90% control of common lambsquarters, redroot pigweed, common ragweed, and velvetleaf. In two no-tillage experiments, common lambsquarters and velvetleaf control was > 90%, regardless of application timing. However, control of redroot pigweed and common ragweed varied among years and application timings. Weed control was more variable from herbicide treatments applied EPP compared with the PRE application timing. Giant foxtail control in both tillage systems was rate, timing, and year dependent. RPA 201772 rates higher than 79 g/ha controlled giant foxtail > 85% at three of five locations. At one location, tank mixtures with RPA 201772 increased giant foxtail control. Corn injury occurred in one of two conventional tillage locations and at the no-tillage location in both 1996 and 1997. Injury was most commonly observed in coarse-textured soils with low clay and organic matter and was more severe with higher rates of RPA 201772. Increased corn injury was also observed when RPA 201772 was combined with acetochlor plus dicloramid or BAYFOE 5043 plus metribuzin. Corn injury from RPA 201772 occurred at application rates above the proposed rate for use on corn. In some cases, severe injury to corn reduced corn yield. Injury to corn from RPA 201772 was not unique to any tillage system and was site, year, and rate dependent.

Field studies were conducted in 1991 and 1992 to determine potential soybean injury due to 2,4-D ester at 560, 1120, and 1680 g ai/ha applied 0, 7, and 14 d before planting (DBP). Delayed emergence and increased injury of soybean were observed as 2,4-D rate increased and interval between application and planting decreased. Delay in emergence ranged from 0 to 67% and 0 to 50% in 1991 and 1992, respectively. Injury at 15 days after planting (DAP) combined over years ranged from 0 to 61%. Minimal (0 to 8%) delay in emergence and minimal (0 to 5%) injury at 15, 30, 45, and 60 DAP occurred when 2,4-D at 560 or 1120 g/ha was applied 7 or 14 DBP. No significant yield reduction occurred due to 2,4-D rate, application timing, or their interaction.