Crop-weed interference is a broad spectrum term that explains the possible interactions among plant species when they grow in proximity. This interaction can be positive, negative, or neutral. Positive interactions take place when stimulant substance produce by one plant species has beneficial impact on other species, whereas negative interference may be due to toxin produced by the plant or consumption of shared resourced (Burkholder, 1952). According to Vandermeer (1989), crop-weed interference is a double transformation process, where a plant and their environment transform and affects each other. A plant could influence its neighbors by changing their environment. Sometime it is very difficult to decide that whether those changes are due to addition or by subtraction of the available resource or some other indirect effects. The indirect effect may be due to change in temperature or wind velocity and other factors that severely affect one plant species over another (Harper 1977).
There is lack of consensus among weed scientists in describing negative interference among plant. For example, competition, this is a type of negative interference (Burkholder 1952), where two or more plants seek the same pool of limited resources, such as light, water or nutrients within a limited space. In that case competition will take place among individuals (Falloon and White 1980). But under competition it assumed that both the species occupy the same niche so that each one is equally influenced by limited availability of resource. Thus, it is differential abilities of plants to use the environmental resources that decide the fate. There are two types of competition, intraspecific (within same species) and interspecific (between or among individuals of different species). The two primary factors that regulate crop-weed competition are competition for light and moisture (Nelson and Nyland 1962). Most of the crop-weed interference studies focus on negative aspect of the association, but there are other types of plant to plant interaction that is either positive or neutral. Thus to understand the crop-weed interference, it is important to take all types of crop-weed association into account. Better understanding of these aspects will provide more options to control weeds and optimize the yield while reducing the cost of cultivation. For example, under some circumstances, negative effect of some plant upon its neighbor is so profound that competition for common resource pool is not sufficient to explain the outcome. In this case, the effect is more prominent in one species but not in other species. This type of condition is described under allelopathy.
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Allelopathy is a biological processes in which chemical produced by one species of plants have inhibitory effect on another species (Rice 1984). Allelopathy is a broad term that also includes the activities mediated by microorganism and lower plants through different biochemical which is called as allelochemicals (Putman 1986; Rice 1984). Allelochemical affects the growth survival, and reproduction of target plants by releasing plant produced secondary metabolites. Allelochemicals that have inhibitory effect can be present in almost any part of the plant including roots, stems, leaves, flower, bark, and buds. Considering the diversity of this phenomenon, allelopathy can be defined as: a complex plant interference process mediated by secondary metabolites to the rhizosphere. Whereas a competition is also a plant inference but it is different from allelopathy. Competition described as process by which plants interfere with the growth
of neighboring plant by utilizing growth limiting resources, such as space, light, nutrient, and water.
It is very difficult to separate the allelopathic effect from other form of plant interference such as competition under field condition. According to Putman and Duke 1978, there are several reasons that create the difficulties in differentiating general plant competition from allelopathy. These problems include a lack of appropriate design of laboratory bioassay to understand the allelopathic effect, failure to identify the effect of microenvironmental factors and lack of well-defined nomenclature of plant interference and many others. This entire study was conducted to understand especially competition and allelopathy in field pea under organic cropping system.
The main focus of this thesis is to understand different types of crop-weed interference to choose the best field pea cultivars that can better suppress the weeds to prevent the yield loss under organic cropping system. The knowledge gained from this research will help the organic producers who rely mainly on crop competitive abilities due to limited options to control the weeds.
2.0 Literature Review
2.1 General information about field pea
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Field pea (Pisum sativum L.), is a native of southwest Asia, was among the first cultivated crops by man (Zohary and Hopf 2002). Now field peaâ€™s total coverage is grown over 10 million hectares worldwide (FAO 2012). The production area is mostly confined to temperate regions and cooler high altitudes in the tropics due to field pea sensitivity to extreme climate (Baigorri et al. 1999). Field peas are commonly used for direct human consumption or livestock food (Cousin 1997). Field pea has relative advantage over many other crops due to free nitrogen fixation from the atmosphere which makes it useful as an alternate and rotational crop. Field pea production in the United States was initially concentrated in three states: Washington, Oregon, and Idaho. However, production of field pea in the US has increased in recent years, particularly in North Dakota and Montana. Total acreage of field pea in North Dakota increased from 810 ha in 1991 to 175,000 ha in 2010 (NASS/USDA 2011).
North Dakota ranks second for total acreage under organic crop production nationally. North Dakota is the leading organic producer for several crops such as lentil, and potato, along with many other forage, grain and seed crops. Considering the lucrative market conditions and potential gaps in production, field pea acreage could be increased in the United States under conventional as well as organic management systems. According to the Economic Research Service report of 2008, 1.9 million ha in the US were under organic production, of which 87,642 ha were in North Dakota. Currently, organic food production and consumption is rapidly gaining momentum worldwide. Many farmers are considering ecofriendly and sustainable farming practices such as organic agriculture, where use of most synthetic chemicals is prohibited (Bruinsma 2003). However, the US does not contribute significantly to organic field pea production, possibly due to some drawbacks in this cropping system, such as increased weed pressure and deficiencies of soil nutrients such as nitrogen and phosphorous that can cause the potential yield reductions under organic management (Ryan et al. 2004).
2.1.1 Selection of highly competitive cultivars
Choosing a highly competitive pea cultivar along with refining other management practices could help decrease pea yield loss due to weeds. Weeds are usually one of the major factors limiting yield and causing management problems in organic production systems However, the lack of research on highly competitive field pea cultivars contributes to weeds remaining a serious problem in organic farming (Murphy et al. 2007). There are few published research articles evaluating the competitive ability of different pea cultivars in organic agriculture. There is a knowledge gap on the physiological basis of competition, i.e., an understanding of the influence of total radiation interception, crop canopy architecture, and water and nutrient use efficiency on crop yield. Research aimed at evaluating field pea varieties for competitive ability against weeds in certified organic fields in the Great Plains, including North Dakota, will provide growers with highly competitive varieties of field peas that are adapted to high endemic weed pressure under organic management systems.
2.1.2 Concept of competitive ability
Competitive ability of a cultivar can be evaluated on the basis of morphological, physiological, and biochemical traits (Lemerle et al. 2001). These traits are controlled by genetic as well as environmental factors. Several other morphological factors that determine the competitive ability of a crop are early vigor, growth rate, biomass, leaf area, leaf angle, crop density and tillering capacity (Grace 1990). For instance, Lemerle et al. (1996) reported that tillering capacity confers greater competitive ability in wheat in addition to many other competitive traits such as height and canopy structure. In the same manner, field pea competitive ability also depends on genetically controlled plant traits.
Considering these facts, field pea early vigor promotes early emergence which results in quick canopy closure and greater interception of incoming solar radiation (Uzun and Acgoz 1998). Dry biomass measured on a per-area basis of field pea is positively dependent on density (Bakry et al. 1984). For instance, a previous study showed that, dry mass per plant was greater at low densities than at higher densities (Townley-Smith and Wright 1994). Larger normal leaf field pea leaves receive more intercepted light radiation; therefore, normal leaved cultivars could suppress the weeds more than the tendrilled type because greater pea LAI would limit light penetration through the pea canopy to the weeds.
2.1.3 Approaches to evaluate crop competitive ability
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Morphological and physiological differences can contribute substantially to the competitive ability of a crop cultivar. Plant height plays an important role in competition of wheat with weeds (Huel and Hucl 1996); the shortest wheat cultivars are associated with the largest reduction in yield under organic systems because the shorter canopy allows vigorous weed growth. Under a conventional management system, short cultivars have an advantage over taller cultivars because of resource partitioning in favor of grain yield over vegetative parts (Oâ€™ Donovan et al. 1997). Thus it is evident that many crop cultivars have been bred to have a dwarf stature, which usually increases yield in conventional production systems, but not necessarily in organic systems, where different constraints to yield are present. Hence, the evaluation of the different morphological traits involved in competition can provide an understanding to increase the yield of crops grown in organic systems, because, as with many crops, competitive ability is largely controlled by genotypic constitution and associated morphological traits (Caton et al. 2003; McDonald et al. 2003).
Choice of crop cultivar can; therefore, be an important aspect of minimizing yield loss due to weeds in organic systems. The process of identifying highly competitive cultivars should be based on high weed suppression ability and competition tolerance. Weed suppression ability of a cultivar is its ability to reduce weed growth, seed production, and seedling establishment (Hoad et al., 2008). Tolerance of a cultivar to weed pressure is its ability to gain a higher yield consistently when weeds are present in the system (Goldberg1990).
Four different types of experiments can be conducted to evaluate plant competition in mixed stands: additive, replacement, Nelder, and neighborhood (1991). In an additive experiment, Crop density remains constant, while the density of the weeds varies. This approach is most common and relevant to agricultural settings. Under field conditions, crop density is fixed and effect of increasing weed density will have an additive effect on yield loss. The major problems of this experiment are difficulties in differentiating between intra- and inter- specific effects and this result a crude picture of competition (Harper 1977).
Most of the criticism against additive experiments can be overcome by using a replacement series design. In this type of design, two species are grown together at constant densities in varying proportions and within monocultures. The yield of the monoculture is compared with mixture. Even the replacement series is not appropriate for agronomic settings where understanding the effect of varying densities on the competition is the key concern. The Nelder design is often referred as a systematic method because the plant density and spatial arrangement changes systematically. However, interference can be evaluated only among individuals of single species.
The neighborhood approach considers a target plant in association with other plants within a fixed radial area around the target plant. The fixed area is called a neighborhood radius, which has the greatest influence on target plant overall performance. Therefore, distance between the neighbors and overall spatial configuration are an important factor in this type of crop-weed competition study (Stoll and Weiner, 2000) and the local environment decides the fate of a plant to larger degree. This type of study can be conducted in a glass house (Pacala and Silander, 1987; Lindquist et. al. 1994) or in the field (Bussler et. al. 1995).
The classical methods of studying crop-weed competition discussed above do not explicitly account for weed suppressive ability and tolerance of competition. Also, these approaches require establishment of many experimental plots with precise content and spatial arrangement of competiting plants. This requirement is not too onerous when evaluating the competitive ability of one crop variety against one weed species. However, for studies aimed at evaluating competitive abilities of many crop cultivars against a wide range of species, these types of designs are much too difficult to establish and time-consuming to manage. Moreover, these more complicated designs often donâ€™t reflect realistic field production conditions. An alternative, more manageable, and more realistic approach to evaluating crop cultivar competitive ability was therefore developed by Hoad et al. (2008), who used their approach to evaluate the competitive ability of several wheat cultivars in an organically managed cropping system.
2.1.4 Concept of weed suppressive ability and sensitivity of suppressive ability
According to Hoad et al. (2008), the sensitivity of crop cultivar weed suppressive ability in response to changes in weed pressure or density provides a way to gauge relative competitive ability among various cultivars. The evaluation of both weed suppressive ability and sensitivity of this ability across different levels of weed growth/weed populations has considerable potential for selecting new suitable cultivars for organic pea cultivation. The most competitive varieties will be those with both high weed suppression ability and low sensitivity (high stability in suppressive ability over a range of weed densities).
As explained by Hoad et al. (2008), weed suppressive ability can be determined by measuring the amount to which presence of the crop reduces the endemic weed cover. To calculate weed suppressive ability, weed growth is determined at each critical crop growth stage by evaluating the percentage ground cover of the endemic weed population when viewed from directly above. Two measurements are required: first the percentage of weed ground cover when grown with a crop cultivar (Wvar) and second the weed percentage ground cover in a weedy check composed of endemic weeds only. The weedy check provides an estimate for unrestricted weed growth (Wmax). Weed suppression ability of each cultivar (Svar) is calculated as the percentage reduction in weed cover in crop plots compared to unrestricted weed growth in the weedy check,Wmax .
Another important component of competitive ability is the sensitivity of cultivar weed suppressive ability in relation to changing levels of weed pressure or weed density. Hoad et al. (2008) determined this sensitivity as the slope of a linear regression of Svar against Wmax. A large positive or negative slope signifies low stability in weed suppressive ability of a particular cultivar (Svar) and vice-versa. With this approach, two cultivars having the same weed suppressive ability could differ in sensitivity to changes in weed growth. The rationale behind evaluating the sensitivity across different growth stages of weeds and different weed pressure is to provide a clear picture of competition across different locations with different weed populations. These two measures of competitive ability provide a valuable potential tool for cultivar evaluation in organic agriculture across a range of favorable and unfavorable conditions. There could be a possibility that a cultivar is able to maintain greater yield at low weed pressure, but if we increase the weed pressure slightly, there is a great loss in yield. This means that a particular cultivar is highly sensitive to change in weed pressure, which undesirable, especially for organic agriculture. This method is particularly suited to evaluating cultivars under organic management because of the use of weed-free checks, which would be extremely difficult to achieve in organic plots, is not required.
Field experiments were conducted by Hoad et al. (2008), for three consecutive years to evaluate weed suppressive abilities of 15 wheat cultivars against endemic weed populations under organic management. During each year, weed natural populations were allowed to grow without any agronomic or chemical intervention. Initially the weed pressure in the trial areas was low but increased in subsequent years. The experiment utilized a randomized complete block design with 15 different cultivars in each block of 12 m X 2 m along with a crop-free weedy check in each block. Several measurements of crop and weed vegetative covers were taken at different growth stages along with final yield of the crop. Visual score of percentage ground cover of weeds growing among each cultivar and within the weedy check were taken at regular intervals. These measurements were used to calculate weed suppression ability and the sensitivity of suppressive ability to weed density for each wheat cultivar. Cultivar and growth stages were chosen as treatment effects whereas seasonal effects were random components.
As the Wmax for each block for different seasons provided different levels of weed pressure that have been linearly regressed with weed suppression ability of the cultivar (Svar). A large regression coefficient signifies the higher sensitivity (lower stability) which means that Svar is poor at high weed pressure. Small regression coefficients for sensitivity indicated low sensitivity (high stability), which means weed suppression ability is greater with greater weed pressure. Results from this experiment indicated that choosing the most competitive cultivar could be more risky if that cultivar is highly sensitive to changes in weed pressure. In organic agriculture, where weed pressure is often high, the best cultivar will be one with high weed suppression ability but also high stability of weed suppression ability over a wide range of weed densities.
2.1.5 Organic field pea competitive ability
Research on non-chemical weed management has been limited due to a notion that cultural weed management options are uneconomical or impractical (Upadhyaya and Blackshaw 2007). However, because weed competition is often a critical factor limiting crop yield in organic production systems and use of synthetic herbicides is prohibited, cultural weed management options are crucial to maintaining adequate yield. According to McDonald (2003), growing competitive crop cultivars can reduce reliance on herbicides. Moreover, a competitive crop cultivar should tolerate the weeds and also suppress their growth (Jordan 1993).
Many of the problems in organic field pea production are related to weed management, diseases, and lodging. Field pea has relatively slow growth during the early season, which makes it a poor competitor with weeds. For example, two wild mustard plants per square foot can reduce pea yield between 2 to 35 percent (Wall 1991). Common lambsquarters, kochia, wild buckwheat, Russian thistle and various volunteer grains are the major highly competitive weeds in field pea cropping systems in North Dakota. However, growing a pea cultivar that is highly competitive against weeds provides weed management that is both economical and practical.
Reduction in yield is often due to diseases which are difficult to control in organic agriculture. There are several fungal diseases that reduce the yield of field pea such as: Ascochyta blight, powdery mildew, root rot, Fusarium wilt and downy mildew. Ascochyta blight caused by Mycosphaerella pinodes can reduce the yield up to 30 % (Allard et al. 1993). This disease appears at flowering and mostly affects the above ground foliage (Garry et al. 1998). Field pea is prone to disease in the lower part of the plant canopy due to high humidity and excess moisture (Tivoli et al. 1996). Prostate growth habit of field pea also favors disease development in a dense canopy (Johnston et al. 2002).
Another problem that makes field pea less competitive is crop lodging due to excessive growth of vegetative parts (Stelling 1989). Lodging in field pea leads to yield reduction because of difficulties associated with harvesting. Field pea stems are very weak and the pods are fragile, creating problems in harvesting. Semi-leafless cultivars are less susceptible to lodging because plants can more easily cling to neighboring plants (Davies et al. 1985; Stelling 1989) but the twinning growth habit complicates mechanical weed removal. Lodging could be a serious problem during the initial growth stages of semi-leafless cultivar whereas with other leaf a type, lodging is a problem at the end of cropping season (Uzun et al. 2005). Based on the above mentioned problems in organic agriculture, identifying which cultivar will perform best under a given environmental condition is difficult.
Sooby et al. (2007) found that growth traits such as seedling vigor and crop canopy architecture help to explain superiority of certain adapted crop varieties under organically managed systems. Thus agronomic performance of a cultivar can be correlated with adaptive growth traits that confer a competitive advantage to some varieties over others under organically managed cropping systems.
Martin et al. (1992) found that, at equal plant densities, there was no difference in the photosynthetic efficiency of different pea phenotypes among all morphological types. Each cultivar converts intercepted radiation into dry matter with equal efficiency. Wall and Townley-Smith (1996) claimed that vine length of a pea plant primarily determines the competitive ability. Basal branching in field pea could also explain how competitive the cultivar is with weeds (Lemerle et al. 1996; Mohler 2001; Spies et al. 2010). All of these traits should be considered when evaluating the competitive ability of field peas.
Cultivars selected under conventional management are typically not ideally suitable for organic production system due to increased level of stresses in many organic systems. Evaluation of crop varieties should be conducted in certified organic fields if the goal of the experiment is to develop or identify a competitive cultivar adapted to organic farming conditions (Mason et al. 2007; Murphy et al. 2007). Cultivar performance can differ among different management systems. For example, Przystalski et al. (2008) reported that performance of small grain varieties grown in conventional agriculture was dissimilar to performance of the same varieties grown in organic cropping systems.
The number of suitable field pea varieties selected specifically under organic production conditions and available for organic growers is inadequate. Hence, organic farmers in the Great Plains either grow pea varieties selected under conventional management (using pesticides and synthetic fertilizers), or organic cultivars from different agro-ecoregions. Therefore, varietal trials conducted under certified organic management are highly supported by organic farmers and their proponents (Sooby et al. 2007). Many studies have been conducted in certified organic fields of North Dakota to evaluate the performance of small grain varieties under organic production, but field pea varieties have not been similarly evaluated. Therefore, the objective of the proposed research is to evaluate one semi-leafless and three normal-leaved pea varieties to determine which cultivar best competes against weeds in North Dakota organically-managed cropping systems.
Evaluate the competitive ability of different field pea cultivars against the endemic weed population present in an organically managed cropping system.
Quantify the competitive effect of common lambsquarters against normal-leaved and semi-leafless pea cultivars under controlled greenhouse conditions.