Carrots as a Model for Defining Critical Period of Weed Management, Biofertilization, and Market Opportunities for Great Plains Vegetables Producers

Objective 1. Determine the length of the critical period of weed control in organically grown carrots 

As expected, weed competition reduced carrot yields, with carrot biomass decreasing on average by 31.2% ± 10.5 (mean±SE) in plots subjected to season-long weed competition compared to weed-free plots. All weedy and weed-free treatments up to the pre-determined growth stages also caused some level of yield loss compared to the season-long weed-free treatment. Based on the fitted model and with a 5% acceptable yield loss, the CPWC was determined for early and late-planted carrots in each growing season (Figure 2). Notably, variability was observed between the different trials in both the timing of the CPWC and its duration. While GDD was used to reduce variability across growing seasons, planting dates, and weather patterns by relying on temperature accumulation, differences in the CPWC may still arise due to environmental conditions, variations in weed communities, or pre-sowing weed management¹.

Figure 2.   Predicted CPWC for organic carrots in 2021, 2022, and 2023 at varying planting dates based on an acceptable yield loss of 5%.

To help explain the observed variability, we evaluated the relationship between the length of CPWC and planting date, especially since planting timing has been found to influence the CPWC in conventional carrot production².  In agreement, we observed an increase in the length of the CPWC with later planting dates (linear regression, R²= 0.5, p-value= 0.09) (Figure 3). This suggests that the later carrots are planted, the longer the period during which weed management is necessary.

Figure 3. Linear regression of planting dates and length of the critical period of weed control (R²= 0.54, p-value= 0.09)

The observed trend between planting date and the CPWC may be attributed to the emergence patterns of dominant weed species. Our analysis of weed communities identified redroot pigweed (Amaranthus retroflexus) as the region's most prevalent weed on vegetable farms, especially within carrots. Redroot pigweed typically begins emerging from late spring to early summer and continues to emerge throughout the growing season³. With later planting dates, this dominant, highly competitive weed will likely have a greater emergence and density and, therefore, may explain the increased time needed for weed management during the growing season. Furthermore, due to its late-season emergence relative to other common weed species, redroot pigweed may avoid being controlled by early-season weed management practices, such as preparing a false or stale seedbed. Other dominant species observed in carrot crops included Canada thistle (Cirsium arvense), common sowthistle (Sonchus oleracus), and quackgrass (Elymus repens). Another possible explanation for the extended CPWC in later plantings is reduced labor availability for weed management before sowing carrots. Managing weeds in carrot beds before planting is crucial to minimizing weed pressure during the growing season, with farmers suggesting cultivation as frequently as every ten days until sowing (Mac Burgess, personal communication, 4 June 2024). As more crops are cultivated later in the season, fewer resources may be available for weed management before planting, potentially extending the CPWC with those carrots that are sown later in the season.

In addition to evaluating how the CPWC can be used to enhance yields with less weeding requirements, we also determined that it can be used to optimize the marketability of carrots. Across all trials, the average CPWC to maintain carrot quality/marketability with a 5% acceptable loss in marketability score ranged from 648.9 ± 82.7 GDD (mean±SE) and 1971.0 ± 214.6 GDD (mean±SE). Compared to the CPWC for optimizing yield, the CPWC for marketability occurs slightly later in the season, requiring more GDD accumulation. Nevertheless, there is substantial overlap between the two, suggesting that the CPWC can achieve both desired yields and marketability. Notably, carrot biomass and quality rating were positively correlated (R2 = 0.32, p-value < 0.001), indicating that larger carrots tended to have higher quality. This correlation may contribute to the observed overlap in the CPWC for marketability and yield.

The on-farm plots provided valuable firsthand experiences demonstrating how the timing of the CPWC can vary year-to-year and farm-to-farm, especially given the variability in weed densities and management practices. Across all on-farm plots, weed-free carrots consistently had the highest biomass compared to all other treatments, including farmer-managed plots, while the weedy season-long had the lowest. At Amaltheia, farmer-managed carrots exhibited greater biomass than weedy and weed-free up to the 4^th leaf stage, likely due to the relatively low weed pressure observed on the farm. In contrast, at Chance Farm, farmer-managed carrots had lower biomass than both weed-free and weedy carrots at the 4^th leaf stage weed and weedy carrots. This can be attributed to the abandonment of cultivation in the carrot bed for the season due to high weed pressure.  At Bodhi Farms, in both years, the farmer-managed carrots were smaller than weed-free up to the 4th leaf stage and smaller than the weedy up to the 4^th leaf stage. This may be due to high weed pressure or the high-density planting of carrots on the farm.

Objective 2. Evaluate the impact of elevated CO₂ on radish growth and weed competition

Distinct differences were observed between elevated and ambient conditions in the growth and competitive effect of radish, buckwheat, and cheatgrass. In all three species, greater biomass was observed in plants grown in elevated CO₂ conditions. Notably, radish yields were higher in both weedy and weed-free season-long conditions for both weedy species when grown in elevated CO₂  (Figure 4). We also evaluated the impact of CO₂ on plant emergence (Table 1). MET was not impacted by changing CO₂ conditions for radishes and only marginally shifted for weedy species. EI% between the ambient and elevated CO₂ conditions had greater differences across species. Radish emergence was inhibited by elevated CO_2, while the opposite was observed in the weedy species. Therefore, although the timing of emergence was similar, the number of seedlings emerging varied considerably between ambient and elevated CO₂, favoring weedy species.

Figure 4. Comparison of radish biomass with no weed competition throughout the experiment (weed-free season-long treatment) and weed competition for the entirety of the experiment (weedy season-long treatment) for cheatgrass or buckwheat under ambient and elevated conditions.

Table 1. Average Mean Emergence Time (days) and Percentage Emergence Inhibition (%) of radish, cheatgrass, and buckwheat in ambient and elevated CO₂ conditions.

Results assessing the impact of CO₂ on the CPWC varied by weed species (Table 2). In competition with cheatgrass, the radish CPWC had inconsistent results, with an increase in Trial 1 by 350.7 GDD and a decrease in Trial 2 by 146.3 GDD (Figure 5). Even though cheatgrass seeds were sourced from the same population, differences between trials may be explained by the high plasticity and genetic variability within cheatgrass populations⁴. In contrast, the radish CPWC in competition with buckwheat had more consistent results. Unlike cheatgrass, buckwheat is a volunteer weed commonly used as a cover crop. As such, buckwheat populations are less likely to have extensive genetic variability and plasticity compared to an invasive grass like cheatgrass. Our results show that in competition with buckwheat, the radish CPWC decreased in elevated CO₂ conditions by 105.8 GDD in Trial 1 and 95.3 GDD in Trial 2 (Figure 6). Specifically, the CPWC ended earlier in both trials under elevated CO₂ conditions, indicating that weeding could be stopped earlier in the season without resulting in significant yield loss. The increased competitive ability of radish in elevated CO₂ may be due to its enhanced aboveground competitive ability. In the buckwheat CPWC trials, radish aboveground biomass increased by 47.1% ± 4.8 (mean ±SE), while buckwheat aboveground biomass only increased by 35.7 % ± 4.4 (mean ± SE). This may explain the shift to an earlier end of the CPWC if the larger amount of aboveground biomass enabled the radish to outcompete the buckwheat.

Table 2. Differences in the range and length of the radish critical period of weed control between species under ambient and elevated CO₂ conditions.

Figure 5. The critical period of weed control of radish under cheatgrass competition in elevated and ambient conditions

Figure 6. The critical period of weed control of radish under buckwheat competition in elevated and ambient conditions

Objective 3: Investigate value-added products for vegetables in the Great Plains, specifically those impacted by weed competition

When collecting data for CPWC, the sugar content of different carrot qualities (1-5) was measured to evaluate the impact of competition on taste. We found weak evidence (linear regression, R² = 0.017, p-value= 0.0013) for a correlation between sugar content and marketability. This suggests that the marketability/looks of a carrot does not impact its taste. Due to this, unmarketable carrots provide an excellent opportunity to explore developing value-added products with unmarketable produce. We worked with two local food processors on the carrot value-added product they produce: Farmented Foods and their “Spicy Carrots” and Roots Kitchen & Cannery and their “Curried Carrot Pickles” (Figure 7). Farmented Foods “Spicy Carrots” consists of sliced carrots chips pickled with spices.  Roots Kitchen & Cannery’s  “Curried Carrot Pickles” use carrot sticks pickled with spices. Despite the similarities in products, there were distinct differences in the companies’ ability to use unmarketable carrots.

Figure 7. Value-added products created with unmarketable carrots: Farmented Foods “Spicy Carrots,” and Roots Kitchen & Cannery, “Curried Carrot Pickle.”

According to our survey, Roots Kitchen & Canney was unwilling to use lower-quality carrots. The company expressed interest in only purchasing carrots of the high and highest quality (categories 4 & 5) as they were unable to use unmarketable carrots due to inconsistencies in their length and thickness that would make them difficult to process. Even from a marketability score of 5 to 4, Roots Kitchen and Cannery estimated labor demands would be increased by 20 lbs. an hour.  For the highest quality, they were willing to pay $1 and for high-quality carrots, $1.10 (4), a 12% reduction in price paid (Figure 8).

Alternatively, Farmented Foods was willing to purchase the carrots at any quality stage. At the highest quality (5), high quality (4), and medium quality (3) categories, Farmented Foods was willing to purchase the carrots for $0.75/lb. For low carrot quality (2), Farmented Foods was willing to pay $0.65/lb, a 13% reduction in price, and for the lowest quality (1), $0.55/lb, a 27% reduction in price (Figure 8). The main limitation identified by Farmented Foods for using unmarketable carrots was additional labor time. Specifically, many of the low and lowest quality carrots had forking and would require more time to clean and ensure all dirt was removed. As a result of the increased labor demand, Farmented Foods would not pay as much for these higher-quality carrots but still would be willing to use them.

Figure 8. Price food processors are willing to pay for 1lb of each carrot quality category (1-5). 

The difference in unmarketable carrot quality usability in value-added products between Farmented Foods and Roots Kitchen & Cannery was largely due to product type. Because Farmented Food’s product uses sliced carrots, they are less limited by uniformity among them.  Alternatively, Roots Kitchen & Cannery’s product uses carrot sticks, which require more consistency with their carrots in length and thickness. Therefore, food processors are more likely to purchase unmarketable produce if the product being created does not require distinct shapes of the carrots, for example, juices or pulps. Surveys also found for both companies that the additional labor required for processing lower-quality carrots was a significant barrier to their adoption. Rather than the look itself, a more substantial concern of food processors is the added labor time cleaning or processing them would take. These results suggest that to create more market opportunities for value-added products using “ugly” produce for farmers, products must not require uniform carrot sizes and ideally have low labor requirements for processing.

Despite these limitations, results show that “ugly” vegetables impacted by weed competition still have the potential to be used in value-added products. Growers can sell this otherwise unmarketable produce to food processors of value-added products or make them their own, resulting in additional revenue from vegetables that would otherwise be composted (See Objective 4).

Objective 4. Estimate enterprise budgets for carrot production that include the different weed management scenarios  

The budget was developed using diverse resources to attain the most realistic estimations, including data collected directly from this project and others at Montana State University, as well as direct consultation with farmers Dr. Mac Burgess, Sophie Lattes, and Nate Brown. Additionally, on-farm data was collected on farm operations at Towne Harvest Garden by Dr. Mac Burgess, Sophia Lattes, and students enrolled in the Towne's Harvest Practicum (SFBS 296, Burgess) course. Lastly, budgets developed by extension and universities were referenced.

Based on these sources, we estimated that the profitability from an acre of carrots is $44,074.91, considering all fixed and variable costs aside from marketing (Table 3). In a weed-free scenario, we estimated higher profitability and increased carrot size and marketability, bringing the total revenue to $51,360.39. However, achieving and maintaining weed-free conditions is an enormous and, most often, unrealistic feat. Therefore, using the CPWC and the sale of “ugly” produce to companies creating value-added products provides a more practical approach for farmers to increase profitability. Using the CPWC, labor requirements can be reduced by over 1/3 when weeding weekly to maintain weed-free conditions. As a result, we determined that the CPWC can save farmers $1,026.58 in labor costs annually. Additionally, it is estimated that in farmer-managed carrots, ~8% are deemed “unmarketable” due to their appearance and size based on yield results. Incorporating the results from Objective 3, even if all these carrots were in the lowest quality rating of category 1, they still have the potential to be sold for a minimum of $0.55/lb. Therefore, farmers can potentially increase their gross revenue by $1,290.00. Combined, both weed management based on the CPWC and the sale of unmarketable produce have the potential to result in a net income of $46,365.11 (Table 4).

Table 3. Enterprise budgets for 1 acre of organic carrots. 

Table 4. Enterprise budget for 1 acre of organic carrots with the incorporation of the critical period of weed control and sale of unmarketable produce for value-added products.

References – Research Results 

1. Ramesh K, Vijaya Kumar S, Upadhyay PK, Chauhan BS. 2021. Revisiting the concept of the critical period of weed control. J Agric Sci. 159(9–10):636–642.
2. Swanton CJ, O’Sullivan J, Robinson DE. 2010. The Critical Weed-Free Period in Carrot. Weed sci. 58(3):229–233.
3. Mohler CL, Teasdale JR, DiTommaso A. 2021. Manage weeds on your farm: a guide to ecological strategies. College Park: Sustainable Agriculture Research & Education (SARE).
4. Hufft RA, Zelikova TJ. 2016. Ecological Genetics, Local Adaptation, and Phenotypic Plasticity in Bromus tectorum in the Context of a Changing Climate. In: Germino MJ, Chambers JC, Brown CS, editors. Exotic Brome-Grasses in Arid and Semiarid Ecosystems of the Western US [Internet]. Cham: Springer International Publishing; [accessed 2023 Aug 4]; p. 133–154.