Document Type

Dissertation - University Access Only

Award Date

2004

Degree Name

Doctor of Philosophy (PhD)

Department / School

Plant Science

First Advisor

Sharon A. Clay

Abstract

Amaranthus rudis (Sauer) and A. retroflexus (L.) are two common species that infest crops in the midwestern United States. They have become two of the most problematic weeds due to their extended period of germination during the growing season and the presence of biotypes resistant to triazines, sulfonylureas, and protoporphyrinogen oxidase-inhibiting herbicides. Previously, A. rudis and A. retroflexus were controlled with broadcast applications of various soil-applied herbicides, followed by one or more postemergence herbicides and/or cultivations in row crops. However, minimum tillage coupled with glyphosate [N-(phosphonomethyl)-glycine] applications in glyphosate tolerant (GT) crop cultivars has favored the expansion of these species. These weeds can emerge early in the growing season and are large enough to tolerate the herbicide, or germinate after the last herbicide application and escape control. These species are adaptively flexible and can germinate, grow, and produce seed below crop canopies. Understanding weed seed production in specific cropping systems may allow design of better management strategies. The objective of this study were to determine the differences in growth and seed production of A. rudis and A. retroflexus at four simulated emergence dates when grown with soybean, corn, or in monoculture.

The field study was conducted at the Swan Lake Research Farm of the USDA-ARS facility in Morris, MN, in the summers of 2001 and 2002. Eight experiments were established each year. Experiments were: A. rudis frown in association with corn; A. rudis grown with soybean; A rudis grown in monoculture with the same fertilization rate used in corn (high fertilization); and A. rudis grown in monoculture with the same fertilization rate used in soybean (low fertilization). The other four experiments were A. retroflexus grown in the same combinations as A. rudis. Treatments for each experiment were four transplanting dates of these species arranged in a randomized complete block design with four replications. Corn and soybean were planted in 76 cm row width on May 29 of each year. Amaranthus seeds were planted into peat pots and germinated under greenhouse conditions on May 29, June 12 and 26, and July 10 in 2001 and June 5 and 19, and July 3 and 17 in 2002. When plants reached the first true leaf state, they were thinned to one seedling per pot and transplanted into corn, soybean or non-crop plots at a density of 5.2 plants m-2 . Plants grown in association with crops were transplanted in the middle of two crop rows. The distance between Amaranthus plants was 25 cm. Amaranthus grown in monoculture had the same plant density as plants grown in association with crops. Crpwm grpwjt states pm dates at wjocj trams[;amtatopm pccired were VE, V2, V5, and V9 in 2001 and V1, V4, V8, and V10 in 2002. Soybean growth states on dates at which transplantation occurred were Ve, V3, V5, and V8 in 2001 and V1, V4, V6, and V11 in 2002. At 3, 6, and 9 weeks after transplanting height and canopy diameter were measured on four randomly selected plants for each replication. After the first frost of the season, the bagged plants were harvested. Plants were divided into vegetative and reproductive organs. Vegetative organs were dried and weighed. Seeds were cleaned using a commercial seed cleaner. The seed was weighed and the total seed number per plant was estimated from the weight of 100 seeds. Data for dry weights and seeds within treatments were pooled across years, resulting in eight values for statistical analysis for each variable and species.

Transplanting time affected plant height, canopy diameter, dry matter, and seed number. In 2001, heights of A. rudis transplanted into com at V2, V5, and V9 were reduced by 58, 85, and 85%, respectively, compared with those transplanted at VE. In 200 2, plant heights ofV4, V8 and Vl0 transplants were reduced 56, 71, and 71 %, respectively, compared to Vl transplants. In 2001, canopy diameters ofV2, VS, and V9 transplants were about 32, 71, and 71 % smaller, respectively than those of VE transplants. In 2002, V 4 and V8 transplants had about 40 and 60% smaller canopies, respectively, compared with the canopy ofVl. Dry matter decreased with respect to that of transplants at VE by about 77, 93, 96, 98, 100, 100, and 100% with each successive delay in transplanting, whereas seed number decreased 51, 93, 93, 97, 100, 100, and 100% with each successive delay in transplanting (i.e., Vl, V2, V4, V5, V8, V9, and VI0, respectively).

In 2001, heights of A. rudis transplanted into soybean at V3, V5, and V8 were reduced 27, 82, and 100%, respectively, compared with the height of VE transplants. In 2002, heights ofV4, V6, and Vl 1 transplants decreased 56, 94 and 100%, respectively, with each delay in transplanting. In both years, canopy diameter was reduced about 52, 86, and 100% with each successive delay in transplanting. Dry matter of VI, V3, V4, VS, V6, V8, and VI I transplants decreased by about 0, 72, 99, 100, 100, 100, and 100%, respectively, with each successive delay in transplanting, whereas seed number of VI, V3, V4, VS, V6, V8, and VI I decreased 0, 87, 99, 100, 100, 100, and 100%, respectively, with each successive delay in transplanting.

In 2001, heights of A. retroflexus transplanted into com at V2, VS, and V9 decreased by about 36, 83, and 83% with respect to that of VE transplants. In 2002, plants height ofV4, V8, and VI0 decreased 48, 80, and 80%, respectively, with respect to VI transplants. In 2001, canopy diameter ofV2, VS, and V9 transplants decreased 43, 71, and 81 % respectively, compared to canopy diameter of VE transplants. In 2002, canopy diameter decreased 24, 68, and 68% for V4, V8, and VIO transplants, respectively, compared to that ofVl transplants. Dry matter of VI, V2, V4, VS, V8, V9, and VI0 decreased 79, 79, 93, 99, 99, 99, and 99%, respectively, compared to that of VE transplants. Seed number of VI, V2, V4, VS, V8, V9, and VI0 decreased by about 86, 86, 86, 99, 100, 99, and 100%, respectively, with respect to seeds produced by VE transplants.

In 2001, plant heights of A. retroflexus transplanted into soybean at VE and V3 were similar and 244% taller than VS transplants. Transplants at V8 died. In 2002, transplants at VI and V4 were similar in height and 377% taller than V6 transplants. Transplants at Vl 1 died. In 2001, canopy diameter of transplants at VS was S3% smaller than canopies of transplants at VE and V3. In 200 2, canopy diameters ofVl and V4 were similar and they were 1 5 2% larger than canopy diameters ofV6 transplants. Dry matter of Vl, V3, V4, V5, V6, V8, and VI I transplants decreased 0, 4 8, 8 3, 99, 99, 100, and 100, respectively, with respect to VE transplants. Seed number of Vl, V3, V4, V5, V6, V8, and Vl 1 transplants decreased by about 0, 65, 65, 99, 99, 100, and 100%, respectively, with respect to seed produced by plants transplanted at VE.

In 2001, A. rudis transplanted at VE and grown in monoculture at high fertilization was 27, 27, and 64% taller than V2, V5, and V9 transplants respectively. In 2002, V4 transplants were taller by 1 5%, 8 5%, and 8 5% than Vl, V8, and Vl0 transplants. In 2001, canopy diameter of VE and V5 transplants were similar and 71 % larger than the canopies ofV2 and V9 transplants. In 200 2, V4 transplants had a 5 3% larger canopy than Vl, V8, and VlO transplants. In 2001, dry matter of VE and V5 transplants was similar and 146 and 290% greater than that V2 and V9 transplants, respectively. Transplants at VE produced 125% more seed than V2 and V5 transplants. Transplants of A. rudis grown in low fertilization also were affected by transplanting date. In general plants of both species transplanted late were shorter and produced less dry matter and seed than plants transplanted early.

Planting date influenced final plant size and seed numbers produced. Shading (in cropped treatments), photoperiod differences and accumulation of less heat units may have been a principal factors limiting growth and seed production of late transplants. Com was a stronger competitor with both Amaranthus species at the beginning of the growing season, whereas soybean was a stronger competitor later in the season.

Soybean eliminated seed production of very late-emerging (V8 and Vl 1) amaranths. However, V5 and V6 transplants of A. retroflexus produced many more seeds than these same transplants of A. rudis. These data indicate that more emphasis must be placed on managing late-emerging A. retroflexus than A. rudis in GT soybean. The potentially greatest contributor to overall seed production in a com/soybean rotation is A. rudis emerging prior to the V2 soybean stage growth.

Although these results are for Amaranthus, they also may be relevant to other weed species with a propensity to emerge late, particularly after late in-crop glyphosate applications. Seed production of late-emerging Amaranthus was much greater in GT com than in GT soybean. Thus, in rotations of these two crops, emphasis on late-season weed management should be placed in the com portion of the rotation. Furthermore, A. rudis appears to be the more critical species in both crops as it produces more seeds than A. retroflexus.

Publisher

South Dakota State University

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