Industrial Hemp (Cannabis sativa L.) Germination Temperatures and Herbicide Tolerance Screening

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Chapter 3: Herbicide tolerance screening of industrial hemp (Cannabis sativa L.)
Abstract
Industrial hemp (Cannabis sativa L.) is a multi-purpose crop that can be used in industries as varied as construction and health. This potential, coupled with substantial reductions in the restrictions on research and production, has spurred renewed interest in hemp in the US over the last 10 years. However, much remains to be investigated to make industrial hemp a sound economic alternative to other crops. At present, little information has been generated regarding suitable pre- and post-emergent herbicides for hemp production, particularly in the eastern US. Thus, the objective of this study was to assess response to various herbicides to identify suitable options for industrial hemp grain or dual-purpose (fiber and grain) production. Preliminary greenhouse experiments with preemergent (PRE) and postemergent (POST) herbicides were conducted to inform herbicide choices for subsequent field trials. The PRE field screening resulted in no differences in grain yields, which ranged from 0.37 to 0.76 Mg ha-1, despite >50% injury 60 days after treatment from chlorimuron, linuron, and pendimethalin. Pendimethalin, and linuron herbicides appear to be suitable PRE options for industrial hemp production as measured by low phytoxicity and acceptable plant growth in greenhouse conditions. In the POST field study, no differences in grain yield were detected relative to the nontreated plots. Yields ranged from 0.28 to 0.74 Mg ha-1. Halosulfuron was the only POST treatment to cause visible injury (70%) relative to the nontreated 30 days after treatment. Among POST treatments, sethoxydim, clopyralid, bromoxynil, and quizalofop applications caused the least injury and resulted in favorable yields (> 0.7 Mg ha-1) that were similar to the nontreated check. Key words: crop safety, injury, stand reduction, weed management.
Introduction
A reawakening to the versatility and usefulness of hemp for products ranging from engineering fibers and textiles to food and health products has developed in the US over the last 30 years. However, until 2014, use of the plant was limited by Federal restrictions. As of 2016, approximately 30 countries produced hemp as a commodity. This number is expected to rise as the industry continues to grow. In the US, farmers now can grow hemp after passage of the 2018 Farm Bill, but they must make production and management decisions with little basic information, since most hemp agronomic research has been conducted in Europe, Canada, and Asia. Hemp productivity may be affected by several factors, one of which is weed pressure. Weeds compete for nutrients reducing yield (Peters and Linscott, 1988), crop quality (Oerke, 2006), and harvest ability (Smith et al., 2000). Few studies have focused on weed management strategies for hemp production, and best practices in hemp cropping systems are mostly unknown. Cultural practices that are well established in other crops – including planting date, planting density and spatial arrangement, and crop rotation – are likely to affect stand success and productivity. E.g., wider row spacing used for grain production may support greater crop productivity, but this also may allow for greater weed competition. Little information is available on hemp’s tolerance to herbicides, and currently no herbicides are labeled for use in hemp production in the United States. Currently only a few herbicides are labeled for use with hemp in Canada. Ethalfluralin (Edge® Granular) has added hemp to its label, which is registered for all of Eastern and Western Canada (Gowan Canada, 2018). Quizalofop-p-ethyl (Assure® II Herbicide), is labeled for use in hemp production in Canada (Workflow-Process-Service, n.d.). Few data have been published regarding hemp’s tolerance to different herbicides. Maxwell (2016) applied preemergent (PRE) and postemergent (POST) herbicides to industrial hemp at two sites in Kentucky. Pendimethalin applied PRE caused limited (5%) injury to hemp, while POST herbicides bromoxynil and monosodium methyl arsonate (MSMA) caused only minor (6%) injury. The objective for this research was to test hemp tolerance to PRE and POST herbicides in greenhouse and field studies. Experiments were designed to test the null hypothesis that industrial hemp would not differ in plant injury, growth, and yield responses to various herbicide treatments. Determining hemp’s response to PRE and POST herbicides serves a broader objective of developing best management practices to support the development and growth of a potential industrial hemp industry.
Materials and Methods
Experiments were conducted to test industrial hemp tolerance to PRE and POST herbicides in greenhouse and field settings in Blacksburg, VA. Herbicides were chosen for the study based on options commonly used in corn and soybean production and guided in part by previous research conducted in Kentucky. Weed control spectrum is well characterized for these herbicides and therefore was not evaluated in this research. Greenhouse studies were conducted to preliminarily screen herbicides for further testing in the field trials.
Preemergence greenhouse study
Hemp tolerance to 14 different PRE herbicides (Table 3) was tested using a randomized complete block design with eight replications. A nontreated check was also included. A monoecious cultivar (‘Felina 32’, a dual-purpose French variety) was used. Hemp was grown in pots with Ross silt loam (Fine-loamy, mixed, superactive, mesic Cumulic Hapludoll; NRCS, 2018). Routine soil analyses were conducted prior to study initiation and amended based on recommendations for corn production (Brann et al., 2009). Experimental units (3.78 L plastic pots) were lined with plastic bags to prevent water drainage and possible herbicide leaching, as well as to maintain uniform soil moisture. Plants were watered every three days. Pots were filled by volume with soil and 10 seeds were sown by hand into each pot to a 1 cm depth. Immediately after planting, PRE herbicides were applied in a spray chamber at a rate of 140 L ha-1 spray volume with a TeeJet VS8002E nozzle (TeeJet Technologies, Springfield, IL) at 206 kPa. The study was conducted in a greenhouse during periods of increasing day length in the summer of 2017 and repeated in time in 2018. Following herbicide application, plants were assessed for response to treatments. Response variables included visible injury, total number of live plants (count), plant height, and aboveground dry biomass. Plants were scored for visible injury (plant injury or phytotoxicity) on a 0 (no injury) to 100% (complete plant necrosis) scale (Fehr et al., 1971). Plant heights were measured from the soil surface to the top of each plant in every pot. Average height of living plants within each pot then was calculated. Injury measurements were taken every two wk over an eight wk period. Above ground biomass and height measurements were collected at the final assessment (eight wk after treatment) by cutting plants (dead or alive) 1 cm above the soil surface. Fresh weights of all plants were measured with a field balance and subsamples weighed, dried at 60°C for 48 hr using a forced air oven, and reweighed to determine dry matter concentration.
Postemergence greenhouse study
Individual plants were used to test hemp response to each of 13 POST herbicides (Table in addition to a nontreated check. Plants were grown in 3.8 cm diameter containers (Cone-tainers™; Stuewe and Sons, Inc., Tangent, OR) lined with plastic bags for reasons previously described. Location, soil, soil amendments, and hemp variety were the same as the PRE greenhouse study, previously described. Seeds (one per container) were planted at 1 cm depth into each of 120 containers.Herbicide treatments were applied as previously described when plants reached 20 to 28 cm in height. Visible plant injury (%), aboveground biomass (g), plant height (cm) were measured as previously described.Field studies To test the effects of PRE and POST herbicides (Table 3) on hemp in a field setting, a third set of experiments was conducted. Due to limited seed availability and therefore space, herbicides selected for the field study were mostly based on top performing treatments in the initial greenhouse work, but limited to one herbicide per site of action or chemical family. The studies were conducted with ‘Helena’ in 2017; ‘Joey’ was used for the studies in 2018 because sufficient Helena seed were not available. Both varieties are monoecious, dual-purpose cultivars. Helena was developed in the former Yugoslavia and provided by Schiavi Seeds (Louisville, KY). Joey was developed in Canada and purchased from Parkland Industrial Hemp Growers Co-op, Ltd. (Manitoba, Canada). Each year, hemp was planted into a tilled seedbed to a depth of 1 cm and with 19-cm row spacing using a drill. In 2017, planting occurred June 5 at a seeding rate of 22.5 kg ha-1. In 2018, planting occurred on June 7, with a 33.7 kg ha-1 seeding rate, adjusted to reflect results of separate seeding rate studies. Each year, nitrogen (N) was applied as urea (46-0-0) at a rate of 67 kg ha-1. No additional fertilizers were applied as soil P, K, Ca, and Mg levels were moderate or high in both years. Each year, experimental plots (1.83 by 3.66 m) were established within areas of the stands that were the most uniformity. Separate experiments were conducted for PRE and POST studies. For the PRE studies, herbicides were applied June 8 each year prior to hemp emergence (three days post planting in 2017 and one day post planting in 2018). POST herbicide applications occurred on July 10, 2017 and July 3, 2018, when hemp was approximately 30 and 25 cm tall, respectively. Data collected in the PRE study included visible injury at 30 and 60 days after treatment application in both years as previously described. Stand counts were taken 60 days after application. In the POST studies, visible injury data were collected 10, 20, and 30 days after herbicide application. PRE and POST field experiments were harvested September 15, 2017 and September 7, 2018. Grain yields were obtained using a small-plot combine (Wintersteiger, Ried im Innkreis, Austria). Grain was dried to 8% moisture at 55°C using a forced air dryer to determine final yield values.
Statistical analysis
Analyses of variance (ANOVA) were conducted on all data types in all experiments using JMP software (SAS Institute, Cary NC). Treatment and location (where applicable) were considered fixed effects while replication was considered random. For all studies, means separations for all response variables were conducted using Tukey’s HSD for all response variables with  = 0.1. No treatment by year interactions were observed for any data type for the POST greenhouse experiments; data were pooled accordingly. PRE herbicide effects on count data were generally apparent from the first measurement (Table 4). Clomazone and norflurazon caused substantial decreases in the number of plants present, with only a single plant observed 2 wk after treatment and low counts (3 and 2 plants) at the remaining rating dates. At all rating events, sulfentrazone, metribuzin, and flumioxazin decreased the number of plants per pot relative to the nontreated controls. Dimethenamid, fomesafen, and pyroxasulfone all reduced the number of plants relative to the nontreated check at one or more rating dates. Chlorimuron, S-metolachlor, diuron, linuron, pendimethalin, and acetochlor had similar stand counts as the nontreated pots at all rating dates. All herbicides caused at least 15% visible injury at some point during the study (Table 4). Visible injury symptoms increased 4, 6, and 8 wk after application. Clomazone, norflurazon, pyroxasulfone, fomesafen, and metribuzin were more injurious (> 48% visible injury) 4 to 8 wk after treatment than other treatments with predominant symptoms of stand loss and stunting. Chlorimuron, diuron, linuron, and pendimethalin caused <25% injury throughout the study, generally corroborating the stand count data. Effects of these treatments did not differ from flumioxazin or acetochlor 4 to 8 wk after treatment. Plant heights were not affected by diuron, linuron, dimethenamid, pendimethalin, fomesafen, sulfentrazone, flumioxazin, and acetochlor treatments (Table 5). All PRE treatments reduced biomass relative to controls (mean = 66%), but diuron, linuron, pendimethalin, sulfentrazone, and flumioxazin caused less reduction (33 to 65%) than clomazone, norflurazon, metribuzin, and pyroxasulfone, which resulted in ≥80% decrease relative to controls (Table 5). Across rating types and dates, diuron, linuron, and pendimethalin were the safest to hemp. Flumioxazin was also among the safest to hemp in terms of visible injury, height, and biomass measures, but the herbicide caused reduced stand counts.
Chapter 1: Literature Review 
Literature Cited
Chapter 2: Industrial hemp response to temperature
Abstract
Introduction
Materials and Methods
Results and Discussion
Summary and Conclusions
Literature Cited
Figures
Tables
Chapter 3: Herbicide tolerance of industrial hemp
Abstract
Introduction
Materials and Methods
Results and Discussion
Summary and Conclusions
Literature Cited
Tables
Figures
Chapter 4: Summary and Conclusions
Industrial hemp response to temperature

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