D.L. Tanaka and R.L. Anderson1

1Soil Scientist, USDA-ARS, Mandan, ND, and Research Agronomist, USDA-ARS, Akron, CO.

Every production cultural practice leads to its own complement of weed species (1, 5). This association of weeds with a cultural practice develops when that cultural practice is imposed continuously on the weed community (7, 11, 12). For example1 reducing the level of tillage usually leads to a proliferation of small-seeded weed species such as kochia (Kochia scoparia L.Schrad.) and Russian thistle (Salsola iberica Sennen & Pau) (8, 9). Also, applying N fertilizer increases nitrophilous species such as common lambsquarter (Chenopodium album L.) (6).

Crop selection also impacts weed community dynamics (5). In the Northern Great Plains, producers have attempted continuous cropping, but deleterious weeds problems arose (4). Growing winter wheat continuously resulted in severe downy brome (Bromus tectorum L.) infestations (3), while continuous spring wheat lead to high levels of foxtail species (Setaria spp.) and wild oats (Avena fatua L.) (4). However, with the suitability of alternative crops such as sunflower (Helianthus annuus L.) for this region (2), a more varied sequencing of crops can be used to minimize the dominance of specific weeds (1, 3).

The practice of fallowing degrades the soil resource by

decreasing soil organic matter and increasing erosion (10). To counter this degradation in soil quality, producers can crop more frequently, subsequently reducing fallow acreages. A long-term rotation study at Mandan, North Dakota is demonstrating that increased crop intensity produces more grain, crop residue, and soil protection (2). This paper characterizes the weed associations occurring with selected cultural practices (rotation, tillage system, and nitrogen level) in this study.

MATERIALS AND METHODS

The study is established on a Temvik-Wilton silt loam (fine-loamy, mixed, Typic and Pachic Haploboroll) near Mandan, ND. Two rotations are being compared: spring wheat - fallow (SW - F) and spring wheat - winter wheat - sunflower (SW - WW - SUN). Main plot size is 135 m by 75 m, with the plots arranged in a completely randomized block design with three replications.

Each main rotation plot is split by three tillage systems. Management of tillage systems is based on target quantities of crop residue at planting: conventional tillage (CT) , 30% or less crop residue on the soil surface; reduced tillage (RT), 30 - 60% residue cover; and no tillage (NT) , 60% or greater crop residue cover. The CT and RT Systems do not have a fixed pattern or number of tillage/spray operations during non-crop periods, but rather a combination of operations as needed for weed control and for obtaining the target residue levels. Plots are either tilled with a sweep plow (also known as undercutter or sweeps) or sprayed with Roundup (glyphosate). The NT systems relies entirely on Roundup for weed control.

Nitrogen fertilizer is applied before planting at three levels for each rotation: 0, 22, and 44 kg N/ha for the SW - F rotation; and 34, 64, and 100 kg N/ha for the SW - WW - SUN rotation. The N level1 based on projected N needs for each rotation, is less for SW - F because fallow increases soil N availability. The N treatments split each tillage system subplot.

Weed management is based on best management practices, where appropriate herbicides are used to control in-crop weeds if needed. Spring wheat is treated with Tiller (fenoxaprop + 2,4-D + MCPA) and Buctril (bromoxynil) , while winter wheat is sprayed with 214-D and Buctril. For sunflower in the CT and RT systems, granular Sonalan (ethalfiuralin) is incorporated with two tillage passes. For the NT sunflower system, Surf lan (oryzalin) is applied without soil incorporation. Roundup controls weeds present at planting for all crops.

After ten years of cropping (1994), we counted and identified weed species in the spring wheat plots of both rotations. Eight 0.25 m2 quadrats were arranged in a W pattern across each plot and weeds assessed in early June, before in-crop herbicides were applied. Precipitation during 1994 was near normal, thus weed species were indicative of soil seed bank.

RESULTS AND DISCUSSION

Weed Community Response to Rotation

Weed community density. A rotation by tillage system interaction occurred. With both rotations, weed density was greatest with NT. For SW - F, weed density increased from 29 plants/m(2)2 in CT to 280 plants/m(2)2 for the NT (Table 1). A similar trend occurred with SW - WW - SUN, where weed density was highest with NT. However, the magnitude of change was only 2-fold with this rotation, compared to the 10-fold change in SW - F.

Table l. Weed density in spring wheat as affected by rotation and tillage system.
Tillage	SW - F	SW - WW - SUN
	plants/m(2)
CT	29	32
RT	21	25
NT	280	65
LSD	72	26

Weed community diversity. Increasing crop diversity in rotation increased weed species diversity. With SW - F1 two foxtail spp. (Setaria glauca (L.)Beauv. and Setaria viridis (L.)Beauv.) and fairy candelabra (Androsace occidentalis Lunell) comprised 99% of the weed community. In contrast, eight species, icochia, both foxtails, Russian thistle, wild mustard (Brassica kaber (DC.)L.C. Wheeler) , annual sowthistle (Sonchus oleraceus L.) , redroot pigweed (Amaranthus retroflexus L.) , and common lambsquarter made up 95% of the weed community in SW - WW - SUN.

Weed Associations with Cultural Practice

SW - F tillage Foxtails and fairy candelabra differed in response to tillage system in SW - F. Foxtails were present in the CT and RT systems, but not in the NT (Table 2). In contrast, fairy candelabra only appeared in NT, where density was 278 plants/m(2).

Table 2. Foxtails and fairy candelabra response to tillage system in the SW - F rotation.
Tillage	Foxtails	Fairy candelabra
	plants/m(2)
CT	28	0
RT	20	0
NT	0	278
LSD	10	72

SW - F nitrooen level. Applying N to spring wheat increased the density of fairy candelabra in NT compared to the 0 N level (Figure 1) Spring wheat yields were not increased by additional N (2) , thus N in the 22 and 44 kg/ha treatments exceeded the needs of the crop and stimulated fairy candelabra growth and subsequent population. Foxtail density was not affected by N level.Figure 1. Foxtails (CT and RT) and fairy candelabra (NT) response to N rate in SW - F. An * indicates N treatment differs from N at 0 kg/ha.

SW - WW - SUN tillage Weed response to tillage in the SW - WW - SUN rotation was species-s~ecif ic. With foxtails, kochia, and Russian thistle, densit~es increased in NT (Table 3). However1 tillage did not affect densities of wild mustard, annual sowthistle, redroot pigweed, and common lambsquarters (data not shown).

Foxtails Kochia ~ussian thistle

(plants/m2)

Table 3. Response of three weeds in SW - WW - SUN to tillage system
	Foxtails	Kochia	Russian thistle
	plants/m(2)
CT	4	3	0
RT	1	0	0
NT	31	8	16
LSD	15	6	9

SW - - SUN : N level

With all species, densities decreased at higher N levels (Figure 2). Weed community density also decreased at higher N levels. Spring wheat with N at 33 kg/ha had 79 weeds/rn2, averaged over all tillage systems, but only 12 weeds/m(2)2 were present when N was applied at 100 kg/ha (data not shown).

Higher N levels increased yields of all crops in this rotation (2), indicating that N was needed to maximize yields. Adding N also increased crop competitiveness with weeds, subsequently reducing weed growth and seed production.

Management Implications

This study demonstrates the ecological tendency for weeds to associate with cultural practices, as shown by fairy candelabra proliferating in NT with SW - F. A second trend was increased weed community densities in NT for both rotations.Figure. 2. Response of four weed species to N rate in a SW -WW - SUN rotation. For each species, data in tillage system with highest densities were analyzed. An * indicates N treatment differs from N at 33 kg/ha.

To counter this trend of increased weed density with NT, producers could rotate tillage systems to broaden their diversity of cultural practices. This diversity in tillaqe would aid in preventing dominance of individual species. Also, by diversifying crop sequence in NT, producers can minimize weed density, as shown by comparing weed densities of SW - F to SW -WW - SUN (Table 1).

Producers should be judicious in their N management1 a. applying N beyond the needs for crops will favor weeds and lead to higher weed populations in future crops. Fairy candelabra (Androsac occidentalis Lunell) was well adapted to the NT system of the SW - F rotation. This is the first observation of this species infesting crop lands. Fairy candelabra is a annual broadleaf plant common on Northern Plains rangelands. Its leaves form a rosette, are green until maturity, then the leaves turn a bright red (Figure 3). Its white flowers are arranged in an umbel structure, and are generally five to eight inches tall.

PAIRY CANDELABRA DESCRIPTION

Figure 3. Plant characteristics of fairy candelabra.

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