“Weed seed size comes into play in particular with no-till and strip-till,” points out University of Wisconsin weed scientist Chris Boerboom. “With those systems, large-size weed seeds don’t get incorporated and therefore don’t germinate well. For example, velvetleaf, a medium- to larger-seeded weed, becomes much less of a problem in no-till farming.”
Germination and growth patterns of medium-seeded types, says Stachler, fall between the large and small.
Weeds And Their Seed Sizes
As a rule, large-seeded weeds are more difficult to control with pre-emergence herbicides than are small-seeded weeds, notes Wisconsin’s Boerboom. “That’s partly because they emerge from a greater depth and don’t take up as much herbicide as do those weeds that germinate near the surface. We can usually control large-seeded weeds more effectively with post products.”
“When the seeds of large-seeded weeds lie on the surface, as they do with no-till, they make only one flush and are easier to control with a single herbicide application,” explains Jeff Stachler, weed scientist at Ohio State University. “Also, since they are on the surface, they are more likely to be eaten by rodents, insects and birds. Over time, because of predation, good weed control, and lack of incorporation, the number of large-seeded weeds declines in no-till fields.”
Medium-seeded broadleaf weeds: common ragweed, velvetleaf, jimsonweed, smartweed, Canada thistle, kochia and common dandelion.
The intensity of seed predation is highly heterogeneous in the agricultural ecosystems based on management practices (Mittelbach & Gross 1984 ). Differential effects of seed burial (Hulme 1994 ; Hulme & Borelli 1999 ) and tillage (Cromar et al. 1999 ) have been reported to affect the intensity of seed predation. Studies have shown that factors such as soil cultivation pattern (Tyler & Ellis 1979 ; Holliday & Hagley 1984 ; Carcamo et al. 1995 ), cropping system (Dritschilo & Wanner 1980 ; Kromp 1989 , 1990 ; Booij & Noorlander 1992 ; Carcamo et al. 1995 ; Carmona & Landis 1999 ; Blubaugh et al. 2011 ), and the use of pesticides (Lee et al. 2001 ; Marko & Kadar 2005 ) can influence seed predator activity.
Department of Agronomy and Plant Breeding, Faculty of Agriculture, Azarbaijan Shahid Madani University, Tabriz, Iran
Ground beetles and other ground-dwelling invertebrates have a much lower dispersal rate than rodents (Westerman et al. 2003a ). However, carabid beetles (Coleoptera: Carabidae) are significant generalist predators in annual row-crop agricultural systems (Lee et al. 2001 ; Harrison et al. 2003 ; Gallandt et al. 2005 ). Invertebrates like beetles feed on weed seeds close to the center of the fields in addition to field boundaries (Cromar et al. 1999 ; Tooley et al. 1999 ), although Westerman et al. ( 2003a ) observed no apparent pattern of predation by vertebrates and invertebrates related to field boundaries. Several carabid species eat remarkable numbers of weed seeds under laboratory and field conditions (Lund & Turpin 1977 ; Harrison et al. 2003 ). Holland ( 2002 ) reported that six species of carabid beetles known to consume weed seeds (i.e., Harpalus pensylvanicus, Bembidion quadrimaculatum oppositum, Pterostichus melanarius, Chlaenius tricolor, Harpalus herbivagus, and Bembidion rapidum). Harpalus rufipes also is a polyphagous ground beetle that has been identified as a potentially important weed seed predator (Zhang et al. 1997 ). Its activity–density was greater in vegetated treatments than in treatments lacking vegetation and varied among the vegetated treatments. Its greater activity–density was in oat–red clover and red clover (Trifolium pretense L.) sod than winter squash (Cucurbita maxima Duchesne)(38 vs. 26 trap −1 48 h −1 , respectively; P < 0.001)(Gallandt et al. 2005 ). Honek et al. ( 2009 ) reported that carabids (Amara, Anisodactylus, Harpalus, Pseudophonus rufipes, and Ophonus azureus) are the most important predators of dandelion (Taraxacum officinale L.) seeds with consumption of 1–2 orders of magnitude greater than that of other predators. Saska et al. ( 2010 ) indicated the larger Pseudoophonus rufipes consumed more seeds (29.0 seeds day −1 ) than the smaller Harpalus affinis (average 12.2 seeds day −1 ) and within species, females consumed more seeds than males due to their larger bodies. They also reported that seed consumption by both sexes of H. affinis increased over a range of temperature or no response has been seen above the optimum temperature (20°C) by P. rufipes. In addition to adult carabids consuming seeds, carabid larvae are known to consume and cache a variety of weed seeds (Kirk 1972 ; Hartke et al. 1998 ). First, second and third instar larvae of members of the genus Harpalus have been shown in a laboratory study to eat all species of the most common weed seeds found in maize, Zea mays L. (Hartke et al. 1998 ).
Based on this ability, the main purpose of this review was highlighting the potential of predispersal and postdispersal weed seed predators in agricultural ecosystems and the factors that influence the level of weed seed predation under field conditions.
Correspondence to: Vahid Sarabi, Department of Agronomy and Plant Breeding, Faculty of Agriculture, Azarbaijan Shahid Madani University, 35 Km of Tabriz- Maragheh Road, Tabriz, East Azarbaijan Province, Iran. P.O. Box: 53714-161.
Studies of seed predation in several crops have been relatively consistent in identifying the greater importance of invertebrate compared to vertebrate seed predators (Gallandt et al. 2005 ). Cromar et al. ( 1999 ) inferred that invertebrates account for 80–90% of the seed predation in long-term tillage and crop rotation studies in maize, soybean (Glycine max L.), and wheat fields. They attributed 10–22% of total seed consumption to vertebrates. Similarly, Cardina et al. ( 1996 ) reported that vertebrates accounted for 13–15% of the velvetleaf (Abutilon theophrasti Medik.) predation in continuous maize fields. O’Rourke et al. ( 2006 ) found that within a field, seed predation rates varied depending on the crop type such as maize, soybean, or triticale-alfalfa (Medicago sativa L.) suggesting that differences in crop plant structure and management practice influence invertebrate activity-density and predation services (Labruyere et al. 2016a ). Fox et al. ( 2013 ) reported that crop type had a significant effect on weed seed removal and invertebrate seed predator activity-density, so that Heggenstaller et al. ( 2006 ) and Meiss et al. ( 2010 ) showed that weed seed removal rates and activity-density of native invertebrate seed predators were lower following harvest in maize fields than in soybean and hay fields. Labruyere et al. ( 2016b ) reported that oilseed rape field is of prime importance for the two spring breeders Poecilus cupreus (L.) and Amara similata (Gyll.) in terms of activity-density and nutritional state. Davis et al. ( 2013 ) also indicated that cumulative predation rates for the maize-soybean sequence were lower than in the soybean-wheat and wheat/red clover-maize sequence. In another research study, Westerman et al. ( 2005 ) estimated seed predation using models and concluded that weed control efficacy in soybean was 86% without allowing increases in velvetleaf density in 4-year rotation (maize-soybean-triticale + alfalfa-alfalfa) that received 82% less herbicides, whereas without predation, more than 97% control was required to prevent increases in velvetleaf density in 2-year rotation system (maize-soybean) managed with conventional rates of herbicides. Heggenstaller et al. ( 2006 ) indicated that seed predation in triticale intercrops with clover or alfalfa increased to moderate in autumn because of re-establishment of canopy cover by under seeded-legumes following triticale harvest. They stated that this strategy could be useful for enhancing autumn seed losses by predators. These observations support the theory that increased cover from crop residues after harvest such as in a hay field increases weed seed predation compared with a crop such as maize, which leaves a higher proportion of bare soil following harvest.
Postdispersal seed predation occurs at the soil surface after weed seeds have been shed and contribute to weed suppression. In contrast to predispersal predators, postdispersal predators include a wide range of vertebrates and invertebrates (Chambers & MacMahon 1994 ). Understanding the variety of taxa that contribute to seed removal is important because they may be locally important (Koprdova et al. 2010 ). Birds, rodents, and many invertebrates, including ground beetles (Zhang et al. 1997 ), ants (Mittelbach & Gross 1984 ), and crickets, have been classified as granivores and thus potential postdispersal weed seed predators. In agricultural ecosystems; mice, ground beetles, or ants are usually the main groups of organisms responsible for the destruction of weed seeds resulting from their soil surface activities (Cardina et al. 1996 ; Cromar et al. 1999 ). These and other organisms have different mobility, food and habitat preferences, and population dynamics and may, therefore, respond differently to farming practices (Westerman et al. 2003a ). Hence, we evaluated the potential of these vertebrates and invertebrates that have been known as proper seed predators: