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- Materials and methods
Weed management is a major cost and constraint in organic arable farming. For example, Schotveld & Kloen (1996) found that hand weeding on Dutch organic farms in the early 1990s required about 1500 h of labour per farm (approximately 60 ha). In rotations of alternating combine harvested crops and root crops, a major input into weed seed banks is often made in cereal crops (Mertens 1998). Weed management options in cereals are limited, while the direct need for control to prevent damage is relatively low, because cereals can competitively suppress weeds. However, because weed seed production is not completely prevented in cereals, there is an increased risk of high population densities of weeds in subsequent, less competitive crops.
A total of 70–99% of the weed seeds that are produced annually in cereals cannot be retrieved from the seed bank or do not emerge as seedlings (Mitze 1992; Cardina & Norquay 1997). Predation may be responsible for the larger part of these losses. It has been shown in non-agricultural ecosystems that seed predation is an important factor in limiting the population expansion of plant species. (e.g. Reichman 1979). In agroecosystems, high consumption rates of weed seeds by seed-eating animals over short periods have also been recorded (e.g. Table 1). Whether this is high enough to stabilize or at least slow down the build-up of the weed seed bank depends on the annual seed predation rates. Weed population dynamics are strongly affected by seed mortality, and an annual loss of 25–50% seems to be enough to slow down weed population growth substantially (e.g. Firbank & Watkinson 1986; Medd & Ridings 1989). However, annual losses due to predation have as yet not been assessed. If it significantly impacts on seed bank dynamics, the loss of seed predator populations could be detrimental to weed control and should be prevented.
The annual seed loss due to epigaeic predation is determined by the duration of the exposure period and the rate of predation during that period. Exposure starts with seed shed, although some seed predators will consume ripe unshed seeds from the plant (Kjellsson 1985). The timing of seed shed is species, crop and climate specific and is documented for a number of weed species (e.g. Rauber & Koch 1975; Leguizamón & Roberts 1982). Exposure ends with burial (Thompson 1987; Hulme 1994), or germination of the seeds. Seed burial is usually accomplished by tillage, especially stubble cultivation and ploughing after crop harvest (Cousens & Moss 1990). However, some seed burial will occur during the cropping season due to natural causes. The rate of seed burial seems to depend largely on weather, soil type and seed characteristics (e.g. Peart 1979; Chambers, McMahon & Haefner 1991). Preliminary trials under Dutch conditions indicated that 50% of small-sized seeds of Chenopodium album L. disappeared from the soil surface in about 2 weeks (Seguer Millàs 2002). Burial rate of large-sized species, such as Avena fatua L. and Polygonum convolvulus L., was considerably lower. Germination of newly produced weed seeds prior to crop harvest is limited due to reduced light intensity and quality under a dense canopy (Pons 2000), unfavourable conditions for germination on the soil surface during summer (Grundy & Mead 1998), and primary dormancy in many weed species (e.g. Vleeshouwers 1997). The exposure and vulnerability of weed seeds to seed predation is therefore mainly affected by variation in the timing of seed shed and seed burial. Differences in predation probability among weed species may also result from differences in attractiveness of the seeds to predators, caused for example by physical features, palatability and nutritional value (e.g. Borchert & Jain 1978; Jørgensen & Toft 1997).
We investigated the potential role of epigaeic seed predation in limiting the expansion of weed populations in organic cereal fields in the Netherlands. We estimate the magnitude of annual seed losses due to predation by determining:
the magnitude and seasonal variability in epigaeic seed predation;
differences in seed predation among weed species differing in seed size and temporal pattern of seed shed; and
the magnitude and seasonal variability in weed seed production.
Because seed burial rate was not included in this study, we used a simple model to explore ranges of seed predation per weed species and per farm under different scenarios of seed burial.
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- Materials and methods
Estimates of M̄, annual weed seed losses due to predation in cereals, ranged from 32% to 70% per farm, when assuming continuous exposure to seed predators (Table 3). When the exposure period of seeds was limited to 4 or 2 weeks after seed shed, M̄ still reached levels of 28–67% and 18–57% per farm, respectively. These figures indicate that epigaeic seed predation is responsible for a substantial part of the unaccounted seeds in cereals (Mitze 1992; Cardina & Norquay 1997). In fact, seed predation seems to be a more important loss factor than mortality in the seed bank due to physiological ageing and microbial activity (e.g. Roberts & Feast 1972). It is clear from this study that seed predation contributes substantially to the containment of weed population growth on at least three of the four farms participating in this study. From a population dynamics point of view, seed predation appears to be as effective as mechanical methods of weed control (e.g. Wilson, Wright & Butler 1993).
As the trend in seed demand is very consistent over farms and years, differences in the calculated estimates of M̄ between farms are mainly caused by variation in the temporal patterns of exposure, which in turn depends on the timing of seed shed and seed burial. Environmental conditions and agricultural practices that advance weed phenology, cause early seed shed and postpone seed burial, should result in higher proportions of seed loss in cereals. For example, estimates of M̄ were appreciably higher for crops sown in winter than for the crop sown in spring (Table 3). However, because total seed production was much higher in the winter-sown crops, seed return was ultimately higher in winter than in spring cereals. Exceptionally warm and dry weather conditions, causing early ripening and seed shed, may have been responsible for the high seed losses reported by Mitze (1992). Furthermore, the composition of the weed vegetation influences the timing of seed shed, because usually only one or a few species are responsible for the majority of the seeds produced on a farm (Jones & Naylor 1992). For example, in this study seeds of A. spica-venti, the dominant weed on farm 5, were shed late, while seeds of S. media, the dominant weed in farm 2, were shed relatively early. Total weed seed production was comparable to that usually found in cereals (e.g. Jones & Naylor 1992; Moorcroft et al. 2002).
In this study, observed differences in seed preference between weed species were small, although larger differences have been reported in the literature (e.g. Borchert & Jain 1978). Calculated differences in M̄w can therefore largely be ascribed to differences in the timing of seed shed. The early maturing C. bursa-pastoris was predicted to have suffered the highest seed losses due to predation on all farms. Species that started to disseminate seeds early but continued to do so during most of the growing season, such as S. media, V. arvensis, and in this study also C. album, suffered substantial losses as well. Although seed burial rate was assumed to be identical for all weed species, preliminary data indicate that small-sized seeds are more quickly incorporated into the soil and thus exposed to predators for a shorter period of time than large-sized seeds (Seguer Millàs 2002). In this study, all weed species with large-sized seeds, such as P. convolvulus and V. cracca, were late maturing weed species, shedding seeds in late summer and autumn, when predation pressure is lowest. The production of large seeds late in the season may be an adaptation for predation avoidance (Brown & Venable 1991). Differential seed predation in cereals may influence the composition of the weed flora in the long term. However, in the Netherlands, cereals are grown in rotation with other crops such as potatoes and sugar beet, which may exert a different selection pressure and counteract any shift in weed composition induced in cereals.
Invertebrates, mainly granivorous ground beetles, were the dominant seed predator on farm 4 in 1999, while vertebrates, presumably wood mice Apodemus sylvaticus (L.), were the main seed predator on the remaining fields (Westerman 2001; Westerman et al. 2003). We do not know why seed predation decreased during the cropping season. We anticipated an increase in the numbers of seed predators, as a result of reproduction in spring and summer (Zhang et al. 1997; Ouin et al. 2000), and thus a gradual increase in seed demand, as observed in maize fields in Ohio by Cardina et al. (1996). Other studies on seed predation in arable fields are too fragmentary in time to detect seasonal patterns (Table 1). Seed demand decreased when seed availability increased and it is possible that seed predators simply became satiated (e.g. Cardina et al. 1996).
Density-dependent responses to local differences in seed supply may be involved in seed predation (e.g. Reichman 1979). Further studies are required to elucidate which density dependent mechanisms may be involved, and whether they might have biased our estimates of annual seed loss. Our experimental set-up may have been sensitive to density-dependent effects, as we created areas of high and low food availability, via (1) seeds on cards, e.g. 10, 30 or 50 seeds per card; (2) the uneven distribution of transects over the field; and (3) the different sizes of the observation areas, viz. 1 or 1·5 ha. On average, we introduced 0·24 seeds per m2 or 0·00017 g m−2 every 2 weeks. These numbers are negligible compared with the natural seed production per 2 weeks, which was at least 100 times higher than our maximum. In addition, the distribution of naturally produced seeds was highly variable spatially (data not shown). We therefore conclude that the error introduced by our experimental set-up is insignificant.
The protection and encouragement of naturally occurring seed predators must be worthwhile, although at this stage we do not know how to achieve this goal. Despite big differences between fields, in for instance the level of seed production, weed species composition, type of cereal, sowing date and density, field size, soil type, row spacing or type of field edge vegetation, none of these factors resulted in a noticeable change in trend or level of seed predation. Landis & Marino (1999) predicted that seed predation would be higher in landscapes with a higher abundance of noncrop habitats such as the Veluwe area included in this study, because these should support a more abundant and diverse fauna of seed predators. However, their prediction was not confirmed in this study. Apparently, the seed predator populations are robust and relatively insensitive to changes in management or environments. They may therefore already be at their maximum. If they are not, we need to know which resources are essential for their survival and reproduction. Only then can habitat management be directed to maximize weed control by seed predators.