The Cultivar Check Program: Utilizing the Midwestern Hemp Database (MHD) and Grower-Cooperators to Assess Variety Performance of High Cannabinoid Hemp

Across the 2021, 2022, and 2023 growing seasons, a total of 322 grower-cooperators participated in the Cultivar Check Program and Midwestern Hemp Database Projects.  Of those who participated, 36 grower cooperators (58 site-years) evaluated 39 different hemp cultivars for agronomic performance and cannabinoid development. As a result, the information synthesized from these trials marks a significant increase in regional hemp knowledge and is an important step towards successful adaptation of hemp as a viable option for Midwestern farmers.

Total THC and Total CBD were impacted by cultivar and sampling period (P < 0.05). Cannabinoid data is presented as averages across all locations at each time point. THC and CBD increase as flowering progresses, with cultivars exhibiting varying optimal harvest intervals for both compliance (THC) and profit potential (CBD). Many, if not most, CBD dominant hemp cultivars currently on the market will go “hot” (Total THC >0.3%) if not monitored appropriately during flowering. To illustrate, 25 (66%) of the 38 cultivars in the check program exceeded the regulatory limit at some point during the flowering period.

Data from the 2021, 2022, and 2023 growing seasons has been utilized to develop estimated compliance harvest/ sampling schedules based on 95% confidence intervals for Total THC (%). Cultivars are subsequently broken down into the following categories based on the findings:

• Compliant Prior to Week 3 (Red)
• Compliant Through Week 3 (Orange)
• Compliant Through Week 5 (Yellow)
• Compliant Through Week 7 (Green)

The ratio of CBD to THC concentration (CBD:THC) is impacted by cultivar and sampling period (P<0.05). As such, cultivars were evaluated individually and CBD:THC is presented in terms of averages across all locations at each time point. CBD:THC of many of the hemp cultivars were unaffected by sample timing, remaining consistent throughout flowering. Similarly, CBD:THC was unaffected by grower location, remaining consistent across environments. This supports previous work by researchers from Cornell University showing that CBD:THC remains stable throughout flowering for uniform cultivars. Total CBD (%) infrequently exceeds ~8% without exceeding the regulatory threshold of 0.3% Total THC, resulting in a CBD:THC of ~27:1. Considering this, cultivars with a stable CBD:THC  throughout flowering will help to maximize profitability while maintaining compliance.

For each cultivar, cannabinoid development was impacted by sampling period and location/environment (P<0.05). As such, cannabinoid data is presented in the following manner for each cultivar: Total THC (%) and Total CBG (%) are presented in terms of averages across all locations at each time point. THC and CBG increased over time, with cultivars exhibiting varying optimal harvest intervals for both compliance (THC) and profit potential (CBG). None of the three CBG dominant cultivars exceeded the THC threshold for compliant hemp by the week 7 sampling period. Similarly, across the entire MHD data set, average Total THC (%) of CBD dominant cultivars was 0.258 compared to 0.075 for CBG dominant cultivars. CBG dominant cultivars may provide an alternative cropping option for those looking to reduce risk of non-compliance compared to production of CBD dominant cultivars.

Flowering data are presented as the Julian Calendar Date at which a cultivar was deemed to be flowering.  Results of the ANOVA show that flowering date was significantly impacted by cultivar and location (P > 0.05). Across all cultivars, the mean 50% flowering date was day 226 or August 15th. Cultivars were subsequently grouped into maturity groups (early and late) based on mean expected flowering day. Agronomic performance ratings (germination, uniformity, overall performance etc.) are given as averages across all environments for each cultivar. These ratings will not be analyzed for statistical significance given the subjective nature of the qualitative ratings and are meant to guide future research trials and cultivar selections only. University station trials may be more useful/accurate sources of information for yield metrics and will not be discussed here.

Growers will want to consider the following factors when making variety selections in their region:

• Seed Quality (Germination, Uniformity, etc.)
• Maturity Group (Photoperiod)
• Agronomic Performance (Yield and Quality)
• Cannabinoid Development (Compliance Potential)

Seed certification standards in the hemp industry are still being developed. Growers are encouraged to develop relationships with seed providers and to look to university published resources to guide their selections. Seed providers should provide seed testing data (germination, dormancy, noxious weed presence, etc.) but growers may also wish to look to local seed certifying agencies (such as crop improvement centers or departments of agriculture) to find cultivars which have either been certified or are in the process of becoming so. AOSCA provides an updated list of cultivars eligible for certification, which can be found on their website (AOSCA.org).

Growers will want to consider maturity group when making variety selections. For example, growers in northern latitudes may want to plant earlier maturing cultivars to maximize the shorter growing season compared to their southern counterparts. It should be noted that some cultivars exhibit heterogeneity across and within cultivars which can make agronomic performance and cannabinoid development less predictable. Due to the non-uniformity of the flowering process, unstable/non-uniform cultivars could reach maturity at different points in the growing season, which could have adverse impacts on testing and harvesting strategies at the field level. As such, growers may want to consider uniformity of growth and development of plants within a cultivar when making selections to avoid compliance concerns.  

Importantly, cultivars with a history of certification or compliance may not be agronomically suited to a region while some cultivars with a history of high performance may not be reliably compliant. To better understand cultivar agronomic performance in a similar region, growers are encouraged to access local university cultivar trials for the most accurate regional information. Similarly, growers are encouraged to access the MHD for the best information available regarding compliance potential of evaluated cultivars. Using the information from both of these sources will allow growers to make informed decisions.

As cannabinoids do not begin to develop rapidly until flowering has initiated, growers are encouraged to delay sampling until after terminal flowering to eliminate unnecessary testing costs. Compliance with state, federal, or tribal regulations is determined by showing that each hemp lot produces Total THC <0.3%. Under the current final rule, no more than 30 days prior to the anticipated harvest of cannabis plants, a “sampling agent” must collect samples for compliance testing. If producers do not harvest within 30 days of sampling, the lot must be retested prior to harvest, and the plants will likely have a higher THC level at harvest than the initial sample. Growers will want to consider their cultivar’s cannabinoid development following flower initiation in conjunction with this 30-day window from sampling to harvest to maximize profitability while maintaining compliance. Lastly, there is currently a great deal of variation across sampling, sample handling, laboratory sample preparation and analytical methods. This disparity between current field and laboratory procedures makes cannabinoid results difficult to compare. As such, using USDA/state approved sampling methods and submitting samples to an approved, accredited laboratory is recommended.