What's Cropping Up? Blog

Articles from the bi-monthly Cornell Field Crops newsletter

June 7, 2016
by Cornell Field Crops
Comments Off on What’s Cropping Up? Volume 26 Issue No. 3 – May/June 2016

What’s Cropping Up? Volume 26 Issue No. 3 – May/June 2016

The full version of What’s Cropping Up? Volume 26 No. 3 is available as a downloadable PDF and on issuu.  Individual articles are available below:

Save

May 26, 2016
by Cornell Field Crops
Comments Off on Weed Seedbanks on Local Organic Farms

Weed Seedbanks on Local Organic Farms

Sandra Wayman, Brian Caldwell, Chris Pelzer, and Matthew Ryan
Sustainable Systems Cropping Lab, Soil and Crop Sciences Section, School of Integrative Plant Science, Cornell University

The goal of this project was to quantify soil weed seedbanks in the fields of four local organic farms. In particular, we have heard from organic farmers that red clover (Trifolium pratense L.) often volunteers profusely in some fields after tillage. This may be due to the common practice of harvesting red clover for seed on these farms, during which a substantial number of seeds shatter and are dropped on the field. Under these conditions, red clover could act either as a valuable nitrogen-fixing resource or perhaps as a weed.

We asked four organic grain farmers to identify “weedy”, “clean”, and “high clover” fields on their farms. We took soil samples from these fields, did greenhouse germination bioassays to allow weed seeds to germinate from the soil samples, and then counted the seedlings of each species. This report details the results from the first year of sampling, which is being repeated this year.

Sampling Protocol
Fields were sampled in the spring of 2015, before spring weed germination. Our sampling protocol was to divide each field into four evenly sized quadrants and take 30 soil cores from inside each quadrant. We avoided sampling from headlands. We used a 5/8th inch diameter soil probe and sampled to a depth of 8 inches. Field conditions generally made for easy soil sampling. Soil samples were kept in a cooler until greenhouse weed seedbank bioassay. We also dried separate subsamples of the soil to calculate gravimetric water content and estimate bulk density.

Greenhouse Germination Bioassay

Figure 1. Flats of germinating weed seedlings in the greenhouse for the weed germination bioassay.

Figure 1. Flats of germinating weed seedlings in the greenhouse for the weed germination bioassay.

In the greenhouse, 1 kg of soil from each quadrant was spread out in flats (10 x 10 inches) over a thin layer of vermiculite and watered daily (Figure 1). Weed seedlings were identified to species (or at least genus, if unknown), counted, and removed from the flat. After all weed seedlings were removed, the flats were left to dry for a few weeks, soil from each flat was mixed, and then watering began again to encourage a second flush of weeds. Data presented are from both flushes of weeds.

Red Clover Seedbank
Red clover was the 6th most common species accounting for approximately 5% of all emerging seedlings. Our results show that these organic farmers do in fact have red clover in their seedbanks, bolstering their observations that the red clover seedbank likely increased via intentional planting and losses from seed harvest. The average density of red clover ranged from 0 to 23 seedlings per kg soil across all 12 fields (Figure 2). Half of the fields had over 4 seedlings per kg of soil, which is quite a lot. For example, one seedling per kg of soil equates to over 100,000 seeds per acre in the top inch of soil—enough for a good stand of red clover.

Figure 2. Average red clover seedling counts standardized per kg of soil from three fields on four organic farms. Error bars indicate standard error.

Figure 2. Average red clover seedling counts standardized per kg of soil from three fields on four organic farms. Error bars indicate standard error.

Interestingly, across all four farms, red clover populations were greater in the field chosen as “weedy” compared with the field chosen as “high clover.” This suggests that these organic farmers might not be as good at identifying which fields actually have the highest red clover populations. The red clover seedbank was densest in the fields from Farm 3 (Figure 2). This farm has been under organic management for over 30 years, and occasionally they harvest red clover for seed. Compared with Farm 3, red clover seedling emergence was much lower in the “high clover” fields on the other farms. In the spring of 2016 on Farm 2 we observed an abundant stand of red clover in the “high clover” field, even though red clover was not planted the previous year (Figure 3). Oats had been the previous crop in this field in 2015.

Figure 3. Red clover volunteers in the “high clover” field at Farm 2 on March 25, 2016. Oats were grown in 2015 and no red clover seed was planted in 2015.

Figure 3. Red clover volunteers in the “high clover” field at Farm 2 on March 25, 2016. Oats were grown in 2015 and no red clover seed was planted in 2015.

Weed Seedbank Density
On Farm 3, total weed seedlings were comparable to the other farms and similar among the three different fields chosen by the farmers (Figure 4). Specifically, the “weedy” field on Farm 3 was no weedier than either the “clean” field or the “high clover” field.

Total weed seed density in the “weedy” fields on farms 1, 2, and 4 tended to be higher than those in the “clean” fields. This indicates that the farmers were pretty good at knowing what fields were weedy and what fields were not. Although the total seedbank size might seem large, these densities are comparable to other studies in organic and non-organic fields. More importantly, experienced organic farmers are typically able to manage competition from weeds and grow high yielding crops, so even high weed seedbank densities are not necessarily a problem.

Figure 4. Total seedling counts from four organic farms by field, standardized per kg soil, from greenhouse bioassay.

Figure 4. Total seedling counts from four organic farms by field, standardized per kg soil, from greenhouse bioassay.

Weed Species Diversity
Most weed species observed across the four farms were summer and winter annuals and there were fewer perennials (Figure 5). The top 12 species accounted for over 75% of all seedlings counted in the greenhouse bioassay. Pigweed (redroot and Powell), giant foxtail, common ragweed, and common lambsquarters dominated weed counts and are warm-season species. The winter annuals purslane speedwell and wild mustard also occurred at high density. Path rush, yellow woodsorrel, and broadleaf plantain are low-growing, relatively non-competitive perennials. Fleabanes (Erigeron spp.) include both annuals and perennials.

Because of the relatively diverse rotations on these farms, which include warm season grains, winter cereals, and perennial legumes, a high diversity of weed species was expected. The weed community that we observed would persist by reproducing and replenishing the soil weed seedbank when conditions allowed.

Figure 5. Total counts of the top 12 most frequently occurring weed species on all four farms.

Figure 5. Total counts of the top 12 most frequently occurring weed species on all four farms.

Average weed species richness (i.e. number of species per kg soil) ranged from 6 species per kg soil in the “high clover” field from farm 1 to 27 species per kg soil in “weedy” field from farm 4 (Figure 6). As some weed species might provide benefits similar to cover crops, and greater biodiversity is typically assumed to be beneficial, high species richness can be good, especially when combined with low weed seedbank density. However, as the number of weed species in a field increases, so does the probability that a highly competitive/problematic weed species will be present in the community.

Figure 6. Total weed species richness (i.e., number of species) standardized per kg of soil in three fields on four farms, from greenhouse bioassay.

Figure 6. Total weed species richness (i.e., number of species) standardized per kg of soil in three fields on four farms, from greenhouse bioassay.

As not all weed species are equal in terms of their potential to reduce crop yields, we list the top species accounting over 50% of all individual seedlings counted on each farm below. Species are listed in order of abundance.Weed seedbank  - Table 1

Although no single weed was part of the top 50% across all farms, common ragweed was dominant at three farms and several other species were dominant at two farms. This indicates that these weeds are likely common on other organic farms in the region. We are currently repeating this study and will be evaluating the correlation between weed abundance and weed community composition between 2015 and 2016. We also analyzed a subsample of soil for soil health properties and will be testing the relationship between soil properties and weeds.

This work was supported by grant proposal titled Agroecological Strategies for Balancing Tradeoffs in Organic Corn and Soybean Production from the Organic Transitions Program (ORG) National Institutes for Food and Agriculture (NIFA) U.S. Department of Agriculture (Project: 2014-51106-22080). Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the view of the U.S. Department of Agriculture.

Special thanks goes to Scott Morris for weed identification assistance and to the four anonymous farmers for the use of their fields.

April 6, 2015
by Cornell Field Crops
Comments Off on What’s Cropping Up? Vol. 25, No. 2 – March/April – Full Version

What’s Cropping Up? Vol. 25, No. 2 – March/April – Full Version

The full version of What’s Cropping Up? Volume 25, No. 2 is available as a downloadable PDF and on issuu.  Individual articles are available below:

April 6, 2015
by Cornell Field Crops
Comments Off on Control Glyphosate-Resistant Horseweed in Zone/No-Tillage Corn and Soybeans

Control Glyphosate-Resistant Horseweed in Zone/No-Tillage Corn and Soybeans

Russell R. Hahn and R.J. Richtmyer III
Soil and Crop Sciences Section, School of Integrative Plant Science, Cornell University

Fig. 1: Over-wintering horseweed rosette.  From:  Weeds of the Northeast.  Photo by J. Neal.

Fig. 1: Over-wintering horseweed rosette. From: Weeds of the Northeast. Photo by J. Neal.

Horseweed, also known as marestail, is a winter or summer annual weed which reproduces by seed that germinates in spring or late summer.  Seed that germinate in late summer over-winter as rosettes or basal clusters of leaves not separated by stem elongation (Figure 1).  These over-wintering rosettes rapidly elongate (bolt) to produce erect flowering stems in the spring and early summer.  Mature plants are unbranched at the base and may be 6 feet tall with many small flowering branches near the top as shown in Figure 2.  Seeds are about 1/16 inch long with many white bristles on the end.  These bristles allow for wind dispersal of the seed.

Fig. 2: Horseweed plant showing the lower part of the leafy stem, upper part of the stem with flowers, and seed with slender bristles on one end. From: Weeds of the North Central States, North Central Regional Research Publication No. 281.

Fig. 2: Horseweed plant showing the lower part of the leafy stem, upper part of the stem with flowers, and seed with slender bristles on one end. From: Weeds of the North Central States, North Central Regional Research Publication No. 281.

Zone/No-Tillage Problem
Horseweed is native to North America and is commonly found in fallow fields, pastures, roadsides, and wasteland.  Although not common in conventionally tilled and planted fields, it is common where zone/no-tillage cropping is practiced.  Many of the states reporting glyphosate-resistant (GR) horseweed have a long history of no-tillage cropping.  In these areas, over-wintering horseweed rosettes have likely been subjected to glyphosate selection pressure since the 1970s when growers started using Roundup (glyphosate) for burndown in no-tillage fields.  With repeated glyphosate use over the years, susceptible horseweed plants were likely controlled while glyphosate tolerant plants flowered and set seed. This resulted in a shift to a horseweed population dominated by the resistant biotypes.

GR Horseweed is Widespread in U.S.

Fig. 3: Horseweed population that survived burndown and postemergence glyphosate applications.  Photo by R.J. Richtmyer III.

Fig. 3: Horseweed population that survived burndown and postemergence glyphosate applications. Photo by R.J. Richtmyer III.

The International Survey of Herbicide Resistant Weeds, http://www.weedscience.org, shows that 24 states in the U.S. have documented GR horseweed populations.  It appears that the increasing popularity of zone/no-tillage cropping in NY, along with the widespread use of GR crops and repeated use of glyphosate herbicides has led to the development of GR horseweed populations here as well.  Greenhouse trials with horseweed seed from two locations in Central NY are being conducted to confirm this.  In one case, glyphosate was applied for burndown prior to planting no-tillage soybeans and again for postemergence weed control.  As can be seen in Figure 3, horseweed survived both glyphosate applications.  Although field experiments to evaluate control programs for GR horseweed have not been conducted in NY State, conversations with weed science colleagues on the Delmarva, where GR horseweed was first confirmed in 2000, have been helpful in formulating control recommendations for GR horseweed in zone/no-tillage corn and soybeans.  Effective control programs target the rosette stage shown in Figure 1 as part of the burndown herbicide application prior to planting zone/no-tillage crops.  Once stem elongation begins, horseweed becomes increasingly difficult to control.  The other key element of an effective control program is to incorporate herbicides with sites of action that are different from glyphosate, which is a Group 9 herbicide.

Burndown/Control Recommendations
The recommendations shown in Table 1 are a first attempt to make written recommendations for GR horseweed in zone/no-tillage corn and soybeans.  They have not been incorporated into the Cornell Guide for Integrated Field Crop Management. They emphasize the importance of controlling horseweed early in the season when the plants are still in the rosette stage, and they incorporate herbicides with sites of action that are different from glyphosate (Group 9).  For both corn and soybeans, 2,4-D LVE (synthetic auxin or growth regulator Group 4 herbicide) makes a significant contribution to horseweed burndown/control.  For corn, residual herbicides like atrazine (photosynthesis inhibitor Group 5 herbicide), or Verdict a premix of Kixor (cell membrane disrupter Group 14 herbicide) and Outlook (seedling shoot inhibitor Group 15 herbicide), have proven helpful in controlling horseweed, and provide residual for control of summer annual weeds.  In the soybean recommendations, OpTill or Valor XLT help control the horseweed and provide residual weed control.  OpTill is a premix of Kixor and Pursuit (ALS inhibitor Group 2 herbicide), while Valor XLT combines Classic (ALS inhibitor Group 2 herbicide) with Valor, (cell membrane disrupter Group 14 herbicide).

Table 1.  Burndown/control recommendations for GR horseweed in zone/no-tillage corn and soybeans.

Table 1. Burndown/control recommendations for GR horseweed in zone/no-tillage corn and soybeans.

At this time, it seems prudent that NY farmers scout zone/no-tillage acreage for horseweed that is not readily controlled with glyphosate, and that they employ an aggressive herbicide resistance management plan.  Key elements of such a plan involve rotating herbicides with different sites of action, and using tank mixes/premixes or sequential applications that include herbicides with different sites of action.

December 5, 2014
by Cornell Field Crops
Comments Off on What’s Cropping Up? Vol. 24, No. 6 – November/December – Full Version

What’s Cropping Up? Vol. 24, No. 6 – November/December – Full Version

WCUVol24No6The full version of What’s Cropping Up? Volume 24, No. 6 is available as a downloadable PDF.  Individual articles are available below:

December 5, 2014
by Cornell Field Crops
Comments Off on Glyphosate-Resistant Weeds Likely in NY

Glyphosate-Resistant Weeds Likely in NY

Russell R. Hahn, School of Integrative Plant Science, Soil and Crop Sciences Section, Cornell University

The number of herbicide resistant weed biotypes has increased from 404 to 437 in the past 12 months.  A summary of resistant biotypes for various herbicide site-of-action groups is shown in Table 1.  There have been 13 new cases of ALS (acetolactate synthase) inhibitor resistance (Group 2 herbicides) and 7 new cases of glyphosate (EPSP inhibitor) resistance (Group 9 herbicides) around the World during the past year.  Along with these newly documented cases of herbicide resistance, there continues to be much media attention to this problem, especially related to glyphosate-resistant (GR) weeds.

Table 1.  A summary of resistant weeds by site-of-action herbicide group as of December 1, 2014 is shown below with information from http://www.weedscience.org

Table 1. A summary of resistant weeds by site-of-action herbicide group as of December 1, 2014 is shown below with information from http://www.weedscience.org

WSSA Takes Action
In response to the growing concern about herbicide resistance, the Weed Science Society of America (WSSA) sponsored a national scientific summit on this topic September 10, 2014 in Washington D.C.  This summit built on the insights and perspectives developed at a similar event in 2012.  Dr. David Shaw, a past president of WSSA and Chair of the WSSA Herbicide Resistance Education Committee said “We want everyone to walk away with a clear understanding of specific actions they can take to help minimize the devastating impact of herbicide resistance on agricultural productivity”.  In Addition, WSSA issued a new fact sheet to address the media attention/hysteria about herbicide resistance on October 8, 2014.  The fact sheet discusses the truth behind two common misconceptions about “superweeds”.  According to WSSA, the first misconception is that “superweeds” are the product of rampant gene transfer from genetically modified crops creating herbicide resistant weeds.  The second misconception is that “superweeds” have supercharged abilities to muscle out competing plants in new and more aggressive ways”.  The WSSA fact sheet is posted online at http://wssa.net/weed/wssa-fact-sheets.

Glyphosate-Resistant Weeds
While ALS inhibitor-resistant weeds account for one-third of the documented cases, GR weeds get more attention because of the connection to the vast acreages of GR crops and because of the rapid spread of GR Palmer amaranth across the U.S.  A summary of GR weeds in the U.S. is shown in Table 2.

Table 2.  Documented cases of glyphosate resistance in the U.S. as of December 1, 2014.

Table 2. Documented cases of glyphosate resistance in the U.S. as of December 1, 2014.

Although there are no documented cases of GR weeds in NY, it’s likely that there are isolated GR weed populations in the state.  Several years ago, there was a situation in western NY where a grower noticed giant ragweed that had not been controlled with a normal glyphosate application in soybeans on newly purchased land.  The previous landowner had purchased a combine from Ohio where there have been documented cases of GR giant ragweed.  Seed from the surviving giant ragweed were grown in the greenhouse and treated at 3 or 6 inches in height with from 22 to 88 fl oz/A of Roundup PowerMax. Some of the 3- and 6-inch giant ragweed survived up to 88 fl oz/A of Roundup PowerMax. There are also reports of horseweed (marestail) that is not controlled with normal glyphosate applications, usually in zone/no-tillage fields.  See the accompanying photo of a no-tillage soybean field that shows surviving horseweed plants following burndown and postemergence applications of glyphosate.

Figure 1. Horseweed that survived burndown and postemergence applications of glyphosate in no-tillage soybeans.

Figure 1. Horseweed that survived burndown and postemergence applications of glyphosate in no-tillage soybeans.

Several Midwest states believe that GR Palmer amaranth was introduced on contaminated cotton seed imported for dairy rations.  This was cause for alarm last summer when an astute crop advisor noticed an unfamiliar pigweed that was not controlled with a normal glyphosate application in Wayne County.  According to Anna Stalter, Associate Curator and Extension Botanist of the L. H. Bailey Hortorium Herbarium, Palmer amaranth (Amaranthus palmeri) is not native in NY.  However, there are two Palmer amaranth specimens from NY in the herbarium collection.  One was from Corona on Long Island in 1936 and the other from Albany in 1949.  It is believed that the strange pigweed was tall waterhemp (Amaranthus tuberculatus).  Stalter says that tall waterhemp is considered native throughout NY, having spread from the Midwest.  There are 17 specimens of tall waterhemp from NY in the herbarium collection dating from 1891 near Fort Ann in Washington County to 2005 near DeKalb in St Lawrence County.  None are from west of Cayuga and Tompkins.  Ten of the Wayne County tall waterhemp plants were sprayed with a normal rate of glyphosate when they were 12 inches tall and actively growing.  Nine of these plants died, but one survived, making one believe there may be GR tall waterhemp plants in this population.

Herbicide Resistance Management
Effective herbicide resistance management, to avoid or control herbicide resistant weed populations, involves engagement of all involved in weed management decisions.  Primary responsibility falls on the grower or crop consultant who must scout fields to determine if weed control practices are working and to identify and determine the reason(s) for weed escapes.  Key elements of an effective grower/crop consultant weed management plan includes some, or all of the following practices;

1) Crop rotation and the use of hybrids/varieties with different genetic traits for herbicide resistance.
2) Cultivation of row crops to control escaped weeds.
3) Rotate or use herbicides with different sites-of-action over the course of the crop rotation.
4) Use tank mixes/premixes or sequential herbicide applications with different sites-of-action.
5) Scout fields to identify weeds that survive herbicide application and then determine why.

Chemical and seed companies, which are often one and the same, provide information and products that reinforce management practices for those who are on the front lines in this battle.  Among these are: 1) including site-of-action group numbers on all herbicide containers, 2) developing and marketing premixes of herbicides with different sites-of-action, and 3) developing and marketing crops with multiple types of herbicide resistance/tolerance.  It is this last item that is receiving much attention in this battle against herbicide resistant weeds.  There are examples of crops with multiple types of herbicide resistance in the marketplace. Most everyone is familiar with SmartStax corn hybrids with resistance to glyphosate (Roundup etc.) and glufosinate (Liberty 280 SL) as well genetic traits for resistance to insects.  In addition, there are recently deregulated herbicide resistant crops with new combinations of herbicide resistance/tolerance traits and others under development.

Subscribe

Follow this blog

Get a weekly email of all new posts.

Skip to toolbar