3.1. Cover Crop Biomass
ANOVA revealed significant effects for cover crop, termination treatment, and termination timing. ANOVA also showed that the year by species interactions were significant (
p < 0.10). Therefore, each species is discussed separately by year. In 2014, a maximum May biomass of 8838 kg ha
−1 was attained in cereal rye plots, followed by Austrian winter peas (7177 kg ha
−1), hairy vetch (6213 kg ha
−1), and crimson clover (5124 kg ha
−1) (
Table 2). Rape provided the least biomass (1657 kg ha
−1). Terminating covers in April resulted in approximately 50% less biomass in all comparisons, while terminating covers in March resulted in less than 1000 kg ha
−1 regardless of cover crop. These results are similar to previously reported biomass and are indicative of proper management and environmental conditions (
Figure 2) [
19,
29]. Conservation agriculture specialists consider cover crop biomass amounts exceeding 4000 kg ha
−1 as ‘high biomass’ systems [
25]. However, for more effective weed suppression, weed scientists recommend >6000 kg ha
−1 [
20,
21,
22,
29].
In 2015, a relatively warm winter and higher early spring temperatures (
Figure 2) resulted in quicker spring cover crop growth compared to 2014 and covers attained maximum biomass at the April (2nd) termination date (
Table 2). Similar to 2014, cereal rye attained the highest biomass at 5828 kg ha
−1 followed by crimson clover and hairy vetch (3995 and 3787 kg ha
−1, respectively). Austrian winter pea provided 1623 kg ha
−1 while rape again resulted in the least biomass (558 kg ha
−1) when terminated in April. Again, terminating covers in March again reduced cereal rye and rape biomass by half and decreased the legumes’ biomass by a third. Early termination due to planting schedules for early cash crops is a challenge to producing high biomass, especially as you move to more northern latitudes [
20].
3.2. Cover Crop Termination
As previously stated, cover termination ratings were estimated by visually accessing greenness over the whole plot and recorded on a scale from 0% (no affect compared to non-treated control) to 100% (complete termination). ANOVA revealed significant effects for cover crop, termination treatment, and termination timing. ANOVA again showed year by cover crop species interactions were significant (p < 0.10). Therefore, each species is discussed separately by year. To facilitate discussion, the 7 (DAT) % termination estimation will be referred to as ‘quick’ or ‘quickly’, whereas the 21 DAT % termination estimation will be referred to as ‘waiting longer’. As previously stated, rolling/crimping was accomplished on all plots, therefore rolling/crimping alone will be discussed first, followed by the remaining treatments applied immediately following this practice.
Hairy Vetch. In March 2014, rolling/crimping alone provided no quick termination or after waiting longer (
Table 3). This result shows additional/alternative practices are required in March to attain termination. The highest (90%) quick termination was attained following flaming; however, regrowth occurred, and termination decreased to 23% after waiting longer. Similarly, significant regrowth was observed in effective quick (90%) glyphosate non-organic comparison treatment after waiting longer, further highlighting hairy vetch resiliency. In March, 95% termination was attained after covering with clear plastic sheeting after waiting longer, and regrowth did not occur following plastic removal (data not shown). In March, other termination practices were not effective quickly or after waiting longer. In April, rolling/crimping alone again provided no termination quickly or after waiting longer (
Table 3). Flaming in April resulted in 93% termination quickly, and again following regrowth, only 68% termination was observed after waiting longer. In May, rolling/crimping alone provided 90% termination after waiting longer (
Table 3). Cinnamon/clove oil provided a quick 28% increase in termination in April, compared to the ineffective rolled/crimped alone treatment; while vinegar failed to injure hairy vetch. However, regrowth again occurred and negated the control attained with oil. In 2015, likely due to advanced maturity in March, rolling/crimping alone provided 32% termination quickly but did not increase to adequate termination after waiting longer. The addition of flaming provided 93% termination quickly and near complete termination after waiting longer (
Table 4). Clear plastic provided 85% termination after waiting longer, with no regrowth after removal (data not shown). In April, hairy vetch termination was 87% following rolling/crimping alone quickly and increased to 99% after waiting longer (
Table 4). Similar to 2014, cinnamon/clove oil and vinegar showed little to no substantial activity on hairy vetch in 2015 (
Table 4). Thus, when selecting a cover crop proceeding a March- or April-planted cash crop, hairy vetch might not be the best choice if quick termination is needed due to both resiliency and regrowth concerns. If slower termination can be utilized due to subsequent cash crop planting timing, clear plastic will likely be effective in terminating hairy vetch. Overall, hairy vetch termination rates increased, and regrowth was reduced by delaying termination. These results are similar to those reported by Mirsky et al. [
42].
Crimson Clover. In 2014, crimson clover response to treatments was similar to hairy vetch in most comparisons (
Table 3). In March and April, rolling/crimping alone provided no termination quickly or after waiting longer (
Table 3). Flaming clover was less effective in March compared to hairy vetch likely due to the lack of biomass to injure. Clear plastic was also moderately effective in March after waiting longer, and was highly effective in April and May quickly. In April, clover was terminated 42% with cinnamon/clove oil quickly, and this was the second highest susceptibility observed compared to the non-treated, albeit still inadequate, for any species evaluated in this experiment terminated with cinnamon/clove oil. Rolling/crimping alone was 99% effective after waiting 14 d when terminated in May. Vinegar resulted in no termination quickly, or after waiting longer, for any application timing when compared to the rolled/crimped non-treated. In 2015, March clover termination was similar to April 2014 (
Table 4). In April, rolling/crimping alone resulted in 87% control quickly and 99% termination after waiting longer (
Table 4). Clear plastic was again highly effective in April and May quickly. Similar to 2014, cinnamon/clove oil and vinegar were non-effective at enhancing clover termination quickly or after waiting longer, at any application timing. Thus, similar to hairy vetch, selecting a cover crop proceeding a March-planted cash crop, crimson clover might not be the best choice if quick termination is needed, due to resiliency. If slower termination can be utilized due to subsequent cash crop planting timing, or use in April or later, clear plastic will likely be effective in terminating crimson clover. These results agree with those of Evans and Bellinder [
30] that reported termination rates increased with clover maturation.
Cereal Rye. In 2014, rolling/crimping alone provided no termination and no treatment effectively terminated rye in March (
Table 3). In April, rolling/crimping alone resulted in 60% termination quickly. Flaming resulted in 97% termination; however, termination decreased to 70% following regrowth after waiting longer (
Table 3). Clear plastic covering rye resulted in 97% termination after waiting 14 d. Cinnamon/clove oil increased termination 13 percentage points over rolling/crimping alone after waiting longer. In May, rolling/crimping alone provided 94% termination quickly and increased to 99% after waiting an additional week (
Table 3). In 2015, rolling crimping in April alone provided 75% termination quickly and increased to 98% by waiting longer (
Table 3). Flaming increased the rate of termination 23% and 11% quickly and after 14 d, respectively. In May rolling/crimping alone provided 99% control quickly, with no additional treatment increasing rye termination (
Table 4). Notably, the cinnamon and clover oil mixture increased termination rates for cereal rye in one year after waiting longer. Generally, these results are similar to those reported by researchers that show termination rates following rolling/crimping increase as cereal cover crop growth stage progresses [
24,
26]. For March-planted cash crops, producers likely would benefit by not choosing cereal rye, as it is relatively hard to terminate at early growth stages. Sequential rolling/crimping or flaming operations would likely be needed.
Austrian Winter Pea. In 2014, March rolling/crimping alone resulted in no termination quickly or after waiting longer (
Table 3). Flaming provided 82% pea termination quickly, however aggressive regrowth resulted in only 10% termination after waiting longer. Clear plastic covering peas after waiting longer resulted in 58% termination, with no regrowth after removal (data not shown). Cinnamon/clove oil and vinegar provided no pea termination quickly or after waiting longer at any application timing. In April, rolling/crimping provided 25% termination alone with flaming and clear plastic treatment resulting in 98% termination after waiting 14 d (
Table 3). In May, rolling/crimping alone provided 20% termination quickly, however, termination increased to 96% after waiting longer (
Table 2). In 2015, rolling/crimping alone provided 48% termination quickly and increasing to 79% after waiting longer, again, likely due to later maturity (
Table 4). Cinnamon/clove oil increased pea termination by 23 and 11 percentage points over rolling/crimping alone after 14 d and longer, respectively. Similar to the previous cover crops, March-planted conservation tillage cash crop establishment may be challenging due to insufficient winter pea termination.
Rape. In March and April 2014, rolling/crimping provided no termination quickly or after waiting longer (
Table 3). Flaming provided 95% termination quickly, however, rape recovered completely after waiting longer (
Table 3). Clear plastic provided 58% termination after waiting longer, with no regrowth after removal (data not shown). Cinnamon/clove oil and vinegar provided no termination at any application timing quickly or after waiting longer. In April, flaming provided only 33% termination quickly and did not increase after waiting longer (
Table 3). Clear plastic provided 48% termination quickly, increasing to 78% after waiting longer. In 2015, rolling/crimping in March provided no rape termination quickly or after waiting longer (
Table 3). Clear plastic was very effective, providing 95% termination quickly increasing to 99% after waiting longer, likely due to increasing solar radiation and relatively high air temperatures. Flaming provided 99% termination quickly, however, termination decreased to 70% after waiting longer due to regrowth. Similar to 2014, cinnamon/clove oil and vinegar provided no rape termination at any application timing quickly or after waiting longer.
3.3. Cover Crop Termination Economics
Cover crop seed costs drive cover crop establishment costs. Austrian winter peas had the highest per unit seed costs, and at a seeding rate of 101 kg ha
−1, it was the highest cost cover crop. Hairy vetch and cereal rye were also more expensive due to seed costs and seeding rate, respectively. However, since one of the main objectives of cover crops is to produce adequate biomass, the cover crop seed cost per 100 kg of dry biomass was calculated to compare the production potential in terms of dollars spent (
Table 5). At the early termination date (March), rape had the highest cost per 100 kg of biomass, with biomass averaged across both years; however, at the later termination dates (April and May), there was little numerical difference between Austrian winter peas and rape. Biomass from Austrian winter peas and rape was the most expensive to produce across all three termination dates. Crimson clover and cereal rye consistently had lower seed costs per 100 kg produced biomass.
Glyphosate was the least expensive treatment option compared to non-treated; however, it obviously does not meet organic production standards (
Table 6). Of the organic methods, cinnamon/clove oil was the least expensive and one of the least effective termination methods. Vinegar, along with clear plastic, had the highest costs as driven by product costs and application rates; however, in general, vinegar was not an effective termination method, while expensive, clear plastic was an effective termination method after waiting longer. Clear plastic also requires additional labor and machinery time as compared to the other treatments. Furthermore, there may be disposal costs associated with clear plastic, which were not accounted for in this analysis. The cost of flame termination was almost four times the cost of glyphosate; however, it proved to be an effective termination option for most covers evaluated in April or May. However, there may be additional costs if regrowth occurs after termination. Furthermore, the cost to purchase the equipment and ownership costs were not included since they differ greatly between farms. From an economic perspective, organic farmers should look at their operations, goal and objectives, and potential constraints, such as labor, before deciding on the most appropriate termination methods.