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

Across the 2021-2024 growing seasons, a total of 322 grower-cooperators participated in the Cultivar Check Program and Midwestern Hemp Database Projects.  Of those who participated, 51 grower cooperators (89 site-years) evaluated 52 different hemp cultivars (Chemotype III and Chemotype IV) for agronomic performance and cannabinoid development via coordinated on-farm trials. 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). 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, 28 (57%) of the 49 cultivars in the program exceeded the regulatory limit at some point during the flowering period.

Data from the 2021-2024 growing seasons has been utilized to develop estimated compliance harvest/ sampling schedules based on Total THC (%). Cultivars are subsequently broken down into the following categories, indicated by color, based on the findings:

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

For CBD-dominant (Chemotype III) cultivars, both Total THC and Total CBD increased throughout the flowering period. Total THC and Total CBD were both significantly impacted by cultivar and sampling period (P<2.00e-16); however the interaction between cultivar and sampling period for both cannabinoids was not significant. Individual cultivar performance varied, but average values for Total THC and Total CBD at Week 7 post-terminal flower initiation were 0.33% and 8.26%, respectively. Most CBD-dominant hemp cultivars currently on the market will go “hot” (Total THC >0.3%) if not monitored during flowering. To illustrate, 28 (57%) of the 49 CBD-dominant cultivars evaluated via the CCP exceeded the regulatory limit at some point during the flowering period. CBD:THC was impacted by sample timing. Most CBD-dominant cultivars did not exceed a CBD:THC of ~27:1 and are thus not able to achieve greater than ~8% Total CBD without exceeding the threshold of 0.3% Total THC. 

For CBG-dominant (Chemotype IV) cultivars, both Total THC and Total CBG increased throughout the flowering period. Total THC and Total CBG were impacted by cultivar (P<.003 and P<2.21e-16, respectively) and sampling period (P<.001 and P<1.29e-7); however, the interaction between cultivar and sampling period for both cannabinoids was not significant. CBG:THC was slightly impacted by sample timing, but remained relatively consistent throughout the flowering period. None of the three CBG-dominant cultivars exceeded the THC threshold for compliant hemp by the week 7 sampling period. CBG-dominant cultivars may therefore provide an alternative cropping option for those looking to reduce risk of non- compliance compared to production of CBD-dominant cultivars.

Differences in photoperiod and heat unit accumulation can have an impact on flowering dates. Considering these factors, and the large geographic range involved these trials, agronomic performance ratings (flowering date, germination, uniformity, overall performance etc.) will not be discussed in
this report. Additionally, not all cultivars have been evaluated equally across sites, resulting in an imbalanced data set. Rather, interested parties are encouraged to visit the MHD, which will allow users to select various parameters (state of production, latitude, cannabinoid of interest, etc.) to refine the dataset to answer critical agronomic performance questions. 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. 

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.