Post-dispersal weed seed predation in contrasting herbaceous crop systemsF. D. Menalled, A. H. Heggenstaller, and M. Liebman Department of Agronomy, Iowa State University, Ames, Iowa 50011-1010Menalled@iastate.edu

IntroductionIntegrated weed management programs combine judicious use of herbicides with multiple control tactics including crop rotation, cover crops, crop variety, soil fertilization, and tillage. The goal of these practices is to combine multiple weed mortality sources that could be individually weak, but cumulatively strong.

In several natural ecosystems, seed predation is an important mortality factor affecting plant population and community dynamics. Previous studies have reported variable rates of weed seed predation in herbaceous annual crop fields (Menalled et al., in press). However, little is known on the importance of this source of mortality in the development of multiple-tactics weed management programs.

Objectives• 1. Compare post-dispersal weed seed predation rates across a range of annual cropping systems that differ in rotation length and management practices.

• 2. Assess the importance of weed seed predation in determining weed population dynamics.

Materials and MethodsThree cropping systems were established in Boone, Iowa in 2002

• 2-year rotation: corn–soybean

• 3-year rotation: corn–soybean–triticale + red clover green manure

• 4-year rotation: corn–soybean–triticale + alfalfa–alfalfa hay

The experimental design followed a randomized block-design with four blocks and each crop present every year (Fig. 1).

Figure 1: Aerial photo of the experimental site, Boone, Iowa. Codes refer to crops present in 2003.

C2 = corn, 2-year rotation; C3 = corn, 3-year rotation; C4 = corn, 4-year rotation;

S2 = soybean, 2-year rotation; S3 = soybean, 3-year rotation; S4 = soybean, 4-year rotation;

T3 = triticale + red clover green manure, 3-year rotation; T4= triticale + alfalfa, 4-year rotation;

A4 = alfalfa hay, 4-year rotation.

Management practices differed among cropping systems with the 2-year rotation receiving conventional management practices and the 3- and 4-year rotations receiving less fertilizer and herbicide (Table 1).

Nitrogen Fertility Management and Tillage Practices

Weed Management Practices

Crop and system Mechanical controls Herbicides

Corn 2 rotary hoe (1x) PPI: metolachlor, isoxaflutole;

POST, broadcast: nicosulfuron+rimsulfuron, mesotrione

Corn 3 and 4 rotary hoe (1x),

interrow cultivation (2x)

POST, banded: nicosulfuron+rimsulfuron, mesotrione

Soybean 2 ––– PPI: metolachlor;

POST, broadcast: bentazon+clethodim,

flumiclorac

Soybean 3 and 4 rotary hoe (1x),

interrow cultivation (1x)

PPI: metolachlor;

POST, banded: flumiclorac

Triticale/clover 3 and Triticale/alfalfa 4

stubble mowing (1x) –––

Alfalfa 4 hay removal (3x) –––

Table 1. Summary of management practices employed at the experimental site.

Post-dispersal seed predation was assessed for two weed species commonly found in the Midwest USA: Setaria faberi (giant foxtail) and Abutilon theophrasti (velvetleaf). These species were selected because of their different growing habitats and seed characteristics (Fig. 2).

Figure 2. Weed species utilized in this study.

Photos: WSSA Web site and Davis, L.W.  1993.  Weed Seeds of the Great Plains

Field trials were conducted using two treatments:

• 1) Total exclosures that prevented vertebrates and invertebrates from removing weed seeds and were used to assess the experimental error inherent in seed recovery

• 2) No exclosures that allowed both vertebrates and invertebrates to consume weed seeds

Fifty velvetleaf or giant foxtail seeds were placed on individual 12 x 10 cm cards. Thirteen times between May and October 2003, cards were left in the field for 48 hs, recovered, and number of seeds remaining on cards was determined in the laboratory (Fig. 3).

Figure 3. 1) Total exclosure and 2) no exclosure treatments with detail of the seed card utilized to quantify post-dispersal weed seed predation.

A simulation model examined the importance of weed seed mortality, including post-dispersal seed predation, in determining weed population dynamics (Liebman et al., 2003).

References Liebman, M., P. Westerman, F. Menalled, and A. Heggenstaller. 2003. Weed responses to diversified cropping systems. Symposium on Beyond Thresholds: Applying Multiple Control Tactics in Integrated Weed Management. Proceedings of the North-Central Weed Science Society meeting. Louisville, Kentucky.

Menalled, F, M. Liebman, and K. Renner. In press. The ecology of weed seed predation in herbaceous crop systems. In Handbook of Sustainable Weed Management. D. Batish, editor. The Haworth Press, Inc. Binghamton, NY.

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Results

An ANOVA test on arc-sine transformed data showed that percentage velvetleaf and giant foxtail seed removal was affected by crop phase. Rotation length modified velvetleaf seed removal, but had no influence on giant foxtail removal (Table 2).

Conclusions•Weed seed predation is mainly influenced by crop phase and crop canopy closure.

•Because the 2-yr rotation achieves high levels of efficacy from direct weed control tactics, it is less reliant on natural weed mortality factors such as seed predation.

•Equivalent weed seedbank densities can be achieved between the 2- and the 4-year rotations provided that weed seed predation reaches high enough rates in the reduced input and diversified cropping system.

Figure 6: Seedbank abundance in 2- and 4 year rotation systems under different seed predation modeling scenarios

2002 crop 2003 crop 2003 Nitrogen inputs Tillage practices

Soybean 2 Corn 2 110 kg N/ha + 40 kg N/ha Surface cultivation

Triticale 3 Corn 3 composted manure +

55 kg N/ha + 40 kg N/ha

Moldboard plow

Alfalfa 4 Corn 4 composted manure + 55 kg N/ha

Moldboard plow

Corn 2 Soybean 2 ––– Chisel plow, surface cultivation

Corn 3 Soybean 3 ––– Chisel plow, surface cultivation

Corn 4 Soybean 4 ––– Chisel plow, surface cultivation

Soybean 3 Triticale 3 30 kg N/ha No tillage

Soybean 4 Triticale 4 30 kg N/ha No tillage

Triticale 4 Alfalfa 4 ––– No tillage

Multiple comparisons indicated that more velvetleaf and giant foxtail seeds were eaten in alfalfa than in the other crops (Fig 4a). Crop rotation did not affect foxtail seed removal, but more velvetleaf seeds were eaten in the 4- and 3year rotation systems than in the 2-year rotation system (Fig 4b).

Figure 4. Weed seed removal averaged across a) crops and b) cropping systems. Different letters denote significant differences, P < 0.05. Capitalized letters compare velvetleaf seed predation. Lower case letters compare giant foxtail seed predation.

Giant foxtail Velvetleaf

Crop <0.0001 <0.0001

Rotation 0.8908 0.0006

Crop*Rotation 0.4878 0.8517

Block 0.1313 <0.0001

Crop*Block 0.0045 0.8658

Rotation*Block 0.9477 0.7999

Crop*Rotation*Block 0.4817 0.0003

Table 2. Probability values of an ANOVA test on the effect of crop phase, rotation length, and block on overall percentage weed seed predation.

The two species showed similar temporal patterns of weed seed removal. However, seed predation rates were crop-specific and closely associated with canopy closure. While weed seed removal in corn and soybean plots increased during the growing season as a crop canopy closed, weed seed removal in alfalfa showed periodic peaks associated with dense canopies prior to mowing. Weed seed removal in triticale showed a maximum in July 2003 prior to harvest and a secondary peak associated with the development of a red clover canopy (Fig 5).

Figure 5. Giant foxtail and velvetleaf seed predation through the 2003 growing season.

A population dynamics model indicates that weed seed predation, as a component of seed mortality, represents an important variable determining weed seedbank abundance in reduced inputs and diversified crop systems (Fig. 6).

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Abutilon theophrastiVelvetleaf850 mg / 100 seeds

Setaria faberiGiant foxtail40 mg / 100 seeds

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