2.1. Data
Plots were established at two University of Tennessee AgResearch and Education centers: Plant Sciences Unit of the East Tennessee AgResearch and Education Center (ETREC) in Knoxville, Tennessee (35.892495, −83.959613) and Middle Tennessee AgResearch and Education Center (MTREC) in Spring Hill, Tennessee (35.721164, −86.964894). The soil type at ETREC is Sequatchie loam (fine-loamy, siliceous, semiactive, thermic Humic Hapludult) and at MTREC is Maury silt loam (fine, mixed, active, mesic Typic Paleudalf). Treatments were a factorial design with fifteen cover crop species and a no-cover control evaluated under two management systems. One management system was defined as single-use, where the cover crop was chemically terminated and left as residue before cash-crop planting. The other system is defined as dual-use, where cover-crop residue was mechanically harvested then chemically terminated prior to cash crop planting. This was repeated under two different cover-crop planting/harvesting times, resulting from different cash-crop rotations, corn/cover crop–soybean and soybean/cover crop–corn, and was repeated over two growing seasons (fall 2017 through spring 2019).
Cover crops were seeded at rates recommended by University of Tennessee Extension for forage production (see Reference [
24];
Table 1). Cover crops included arrowleaf clover (
Trifolium vesiculosum Savi), crimson clover (
T. incarnatum), red clover (
T. pratense), common vetch (
Vicia sativa), hairy vetch (
V. villosa), woolypod vetch (V. villosa), winter pea (
Pisum sativum), barley (
Hordeum vulgare), oats (
Avena sativa), triticale (
X Triticosecale), cereal rye (
Secale cereale), winter wheat (
Triticum aestivum), canola (
Brassica napus), forage radish (
Raphanus sativus), and turnip (
B. rapa).
Table 1 also shows the average cost of the cover crop seeds, which were collected from local seed providers.
Cover crop plots planted before corn were established in late October of each year. Forage harvest occurred in mid-April, and plots were terminated shortly after. Corn was planted in these plots in mid-April and harvested in mid-September. Cover-crop plots preceding soybeans were established in late-September to early-October. Forage harvest occurred in mid-April, and plots were terminated shortly after. Soybean was planted in these plots in mid-May and harvested in late September to late October. The planting date, harvest dates, and input (e.g., fertilizer and chemicals) application followed University of Tennessee Extension recommendations and were consistent across treatments. Exact planting and harvest dates, as well as crop varieties, can be found in Bracey [
25,
26]. Plots were arranged in a randomized complete block design with three replicates. Plots were planted 3.1 m × 9.1 m and were trimmed to 7.5 m length at ETREC and 8.2 m at MTREC.
Table 2 shows the average corn and soybean yields as affected by cover crop harvest across all years and locations. The average corn yield for Tennessee is 11,432 kg ha
−1, which is 115 kg ha
−1 less than what the average was in these data [
27]. The average soybean yield in these data was 205 kg ha
−1 higher than the Tennessee average of 3232 kg ha
−1 [
27].
Forage harvest for ETREC and MTREC occurred in late-March, before corn planting; and in late-April, before soybean planting. Cover crop biomass from one randomly selected 0.09 m
2 area of each plot was sampled at a height of 5 cm. These samples were weighed, dried at 55 °C for 72 h, and reweighed to determine total forage dry biomass.
Table 3 shows the average total forage yield when harvested prior to corn and soybean planting across all years and locations. For more details of the experiment, exact dates, cover crop varieties, forage quality data, and soil-quality data, please refer to the data in Bracey [
25,
26]. Perception was average relative to the last 25 years within the timeframe of the study [
28].
The calculated cost for corn production per acre is
$1554 ha
−1. Soybean production calculated cost was
$1111.5 ha
−1 [
29]. The cost associated with planting the cover crop was
$35.6 ha
−1, and forage harvest cost was
$82.4 ha
−1 [
30]. Cover crop seed cost is shown in
Table 1 for each cover crop species. The estimated price for corn was
$107.95 Mg
−1, and the soybean estimated price was
$299.37 Mg
−1. All prices were adjusted to 2021 dollars, using the Implicit Gross Domestic Product Price Deflator [
31].
2.2. Net Returns
Net returns were calculated for corn and soybean production, using enterprise budgets. Net returns are the revenue from the cash crop (corn or soybeans) minus the production expenses. Previous studies have used enterprise budgets to analyze the profitability of cover crops [
14,
15,
16,
17,
18,
19]. We assume that the producers are profit maximizers and would select to plant cover crops and the preferred cover crop species with the highest net returns. As discussed above, producers choosing to adopt cover crops have additional costs of planting and terminating the cover crops; thus, higher revenue through increased yields is needed to pay for these additional costs. In this calculation, we assume the traditional approach of cover crop systems that cover crops are not harvested for revenue before planting the cash crop. Mathematically, the net returns are calculated as follows:
where
is the net returns (
$ ha
−1) for crop
j (
j = corn or soybeans) and cover crop species
i (
i = 1,…,15);
is the price (
$ kg
−1) for crop
j;
is the crop yield in kg ha
−1;
is the total production cost of planting the cash crop
j;
is an indicator variable equal to one if crop cover is planted, and it is zero if otherwise;
is the cost of seed for cover crop in
$ kg
−1;
is the seeding rate for the cover crop; and
is the is the cost of planting the cover crop. Therefore, a profit maximizing producer would choose to plant a cover crop (
) and the cover crop species (
i) that maximizes net returns.
2.3. Breakeven Price of Cover Crop Hay
Next, we set up a different economic framework, assuming a corn or soybean producer planted a cover crop. This framework was established to determine the breakeven price of hay a producer would need to harvest their cover crop for hay. That is, a producer has a cover crop planted and determines if he/she should terminate it and plant the cash crop or harvest the cover crop for hay. Profitability is determined by the value of the cover crop hay in a given year. Thus, we investigated this breakeven price. In this scenario, a producer has a cost of planting the cover crop that is shown in Equation (1) and will terminate the cover crop regardless of whether it is harvested for hay or terminated by other means. Thus, a producer would have the added costs of harvesting the cover crop for hay. This means the breakeven price of harvesting the cover crop for hay would be the total cost of harvesting divided by the hay yield or , where is the breakeven price in $ kg−1, is the cost of harvesting the cover crop for hay in $ ha−1, and is the forage yield from the cover crop in kg ha−1.