Adoption of cover crop interseeding within sugarbeet in the Red River Valley

The Red River Valley of North Dakota and Minnesota is a major sugarbeet production region in the United States. In the Northern Great Plains, wind erosion causes significant soil loss from agricultural fields. Due to conventional tillage practices, soils have less residue cover after sugarbeet harvest in the fall (Figure 1a), and the soil is exposed to wind and water erosion. In the spring, severe damage due to wind blast of soil particles can cause sugarbeet stand loss. Cover crops have the potential to reduce the impacts of soil erosion and improve nutrient use efficiency by reducing nutrient loss, fixing nitrogen, and improving soil quality. Interseeding cover crops before harvest increases the likelihood of cover crop establishment and growth (Figure 1b). Cover crops that are interseeded can accumulate more biomass than fall-seeded cover crops and thus have greater potential to reduce soil losses (Figure 1c,d). However, the success of cover crop stand establishment depends on the growing season and cultivar characteristics.

The most commonly grown cover crops fall into three main botanical families: grasses (Poaceae), legumes (Fabaceae), and plants in the Brassicaceae family (henceforth referred to as brassicas) with different benefits and management considerations for each family. It is crucial to select a cover crop that provides benefits without a negative effect on the cash crop yield. Brassicas can scavenge nutrients and suppress weed, whereas grasses such as oat and rye are known for soil building and scavenging soil. Legume cover crops are most often selected for biological N₂ fixation, which may reduce N inputs required for the subsequent crop. Therefore, it is critical to compare cereals, brassicas, and legumes in terms of sugarbeet root yield, sugar content, and economic profitability.

Planting time of cover crops plays a major role in determining the performance of the following crop. Some cover crop species require an early planting time for fall growth, whereas winter annual species focus on spring growth the following season and can be planted later in the fall. The choice of the cover crop may largely depend on the planting time and the desired benefit and cost. For example, rye is chosen for its large quantities of biomass production and the ability to establish well with a later planting time. Therefore, this study’s goal was to determine the effect of interseeding on cover crop biomass and sugarbeet yield and quality. It was hypothesized that the interseeding of cover crops would improve sugarbeet yield and quality. Two interseeding times, early vs. late, were compared along with four cover crop species: (i) winter rye, (ii) winter camelina, (iii) winter Austrian pea, and (iv) brown mustard. Influence of planting time and cover crop species were examined based on (i) cover crop biomass production, (ii) sugarbeet yield, (iii) sugar content, (iv) recoverable sugar, and (v) economic profitability, after planting and before harvest.

Field Trials and Data Analyzed

Field trials were conducted at Ada, MN during the 2018–2020 growing seasons (Table 1). The experiment was laid out in a randomized complete block design (RCBD) with factorial arrangement of four different cover crop species and two interseeding times and check (no cover crop) treatments. Four cover crops species were winter rye, winter camelina, winter Austrian pea, and brown mustard. Nine treatments were pseudo-replicated four times. Individual treatment plots measured 11 ft wide and 30 ft long. Each plot contained six sugarbeet rows spaced 22 inches apart. Crystal 093, a glyphosate-tolerant sugarbeet cultivar, was planted at the rate of 59,975 plants/ac. Sugarbeet seed was planted to a 1.96-inch depth with a six-row John Deere row crop planter.

Cover crops were interseeded at two different times, i.e., early (first interseeding) and late (second interseeding) (Table 1). Pea and rye were interseeded at 20 lb/ac, mustard at 10 lb/ac, and camelina at 6 lb/ac. Cover crop seeds were planted using a V-shaped hoe with two blades 5.9 inches apart to make a parallel furrow to simulate planting with a commercial interseeder.

For weed control, glyphosate at a rate of 2.5% and ammonium sulfate at 1.0% were sprayed on the third week of May to the second week of July each year. Similarly, to control Cercospora leaf spot in sugarbeet, three fungicides (difenoconazole, thiophanate methyl, and pyraclostrobin) were applied on the first and second week of August at rates of 51.2, 55.5, and 73.0%, respectively.

Information regarding price, revenue from agricultural products, and operation cost of the respective growing season was collected from personal communication with American Crystal Sugar Inc. personnel (Joe Hastings, agronomist).

Data were analyzed statistically with nine treatments (four cover crop species × two interseeding times and control) and factorial RCBD excluding control with four replications. The effect of cover crop interseeding on yield was analyzed using RCBD. Cover crop biomass and sugarbeet production parameters of three growing seasons were analyzed using the general linear model (GLM) of the Statistical Analysis System 9.4. Probabilities equal to or less than .05 were considered significant for main effects and interactions. The least significant difference (LSD) test was used to separate differences between treatment means if analysis of variance indicated the presence of such differences.

Influence of Growing Year, Cover Crop Species, and Interseeding Time on Sugarbeet Production

The growing season had a significant effect on sugarbeet root yield, sugar concentration, RSY, and economic return (Table 2). Cumulative growing season (April–September) rainfall was 13.5, 17.1, and 15.4 inches for 2018, 2019, and 2020, respectively. In 2018, it started out warmer and drier than the 30-year average in May (+7˚F) and June (+2˚F). Precipitation was below average for the majority of the growing season. In 2019, planting was delayed for almost two weeks compared with the normal growing season due to wet soil conditions from spring snowmelt. However, rainfall was below average in May (–0.8 inches) and June (–1.8 inches) but above average from July (+0.4 inches) throughout the growing season. A similar trend was observed in 2020, but it received higher rainfall in August than the other two years. Field trial outcomes are highly dependent on weather and environmental conditions, which is similar to the findings of Chatterjee et al. (2018) where they found a significant effect of growing condition and site characteristics on root yield and sugar concentration in the Red River Valley of North Dakota and Minnesota.

The effect of growing season, interseeding time, and species on cover crop biomass, sugarbeet root yield, sugar concentration, and recoverable sugar yield are presented in Table 2. Interseeding time and species showed no significant effect on cover crop biomass. However, the results shows that the early interseeding produced three times more biomass than late interseeding. Higher biomass with early interseeding can attributed to less competition for sunlight and less canopy closure than at the time late interseeding (Crowley et al., 2018). Sugarbeet suppressed cover crop growth during the late growing season (July–August) by decreasing the amount of light available to the cover crops beneath the sugarbeet canopy. In our study, the mean cover crop biomass production was greater for winter pea followed by rye, brown mustard, and winter camelina. Inconsistency in biomass for cover crop species each year in our study was likely due to differences in cover crop physiology and the suitability of different plants to received rainfall.

The analysis of variance was significant only for the interaction between year and the interseeding time for sugarbeet root yield and recoverable sucrose yield. This significant interaction is the result of higher sugarbeet root yield and recoverable sucrose yield with the cover crops interseeded late for each growing season. We found no significant effect of the interseeding time and cover crop species in sugarbeet root yield and recoverable sucrose yield (Table 2). None of the interseeded cover crop species used in this study jeopardized the sugarbeet root yield and recoverable sugar yield compared with those grown without interseeded cover crops. The mean sugarbeet root yield was more than 13% higher for late compared with early interseeding, but no significant difference was observed between two interseeding times. Such yield reduction among the early interseeded cover crop species can be associated with less resource distribution followed by competition with cover crops (Romaneckas et al., 2018).

For sugar concentration, a significant interaction was observed between the growing year, interseeding time, and cover crop species (Table 2). Significantly lower sugar concentration was observed in the treatment with no cover crop in the 2018 growing season and late-interseeded winter pea in the 2019 growing season compared with early interseeded rye and winter camelina. Early interseeding had higher sugar concentration but was not significantly different to late interseeding. It is possible that the removal of soil N by the cover crops might be responsible for increased sugar content in the cover crop treatments. This shows cover crop interseeding under sugarbeet can be a potential strategy to reduce excess deep soil nitrate and increase recoverable sugar yield and profit.

Economic return for the cover crop interseeding treatment compared with the control is presented in Table 2. Interseeding time and species had no effect on economic return. In 2018, all of the treatments with interseeded cover crops had the higher economic return compared with the control but were not significantly different. Interseeding had a significant effect on return in 2019 when compared with control with no cover crops. In 2019, late interseeding with mustard had the highest return, and early interseeded mustard had the lowest return. These results indicate the same species could have a positive or negative effect on return depending on interseeding time. Early interseeded rye and pea also reduced the economic return compared with no cover crops. In 2020, the treatment without a cover crop had the highest economic return but was not significantly different to the interseeding treatments.

Conclusion

This study showed that cover crops interseeded into sugarbeet provide some late-fall benefits, including ground cover, forage, and nutrient cycling without reducing sugarbeet productivity. Cover crops could be interseeded within sugarbeet with careful consideration of species selection and planting time. We observed that interseeding cover crops had minimal to no negative effects on sugarbeet yield, quality parameters, and economic return. We did not find a single species that works best in terms of the effect on root yield or sugar concentration. Another important consideration when choosing species and time to plant cover crop is the choice of herbicides. It is critical that both pre- and post-emergence herbicides are carefully chosen because some herbicidal metabolites will limit cover crop growth and establishment. Growers should assess their needs and long-term goals before adopting a cover crop management strategy in sugarbeet.