Canola (Brassica napussubsp. napus) is an important oil crop for countries with cool growing seasons especially in East Asia, Europe, and North America [1]. It was originally bred from three species, Brassicanapus, B. rapaandB. juncea, with the first of these being the most important. In North America and Europe, canola can be divided into 1) spring/summer varieties grown in Canada and northern United States, with primary production being in North Dakota, Minnesota, and the Prairie Provinces [2]; 2) winter varieties grown primarily in states from Virginia to Texas. Other temperate climates of the European Union such as in France, Germany, Poland, and the United Kingdom also have production of winter canola [3]. Meanwhile, in Australia, canola is grown in areas with winter rains [4]; while in China, it is widely grown in fall to spring season along coasts and valleys [5]. A similar, mid-latitude growing area for winter canola is found in the Southeastern parts of the U.S including Gulf States of Alabama, Louisiana and Mississippi plus East Coast states of Georgia, North and South Carolina, and Virginia [6][7]. Inland states of Arkansas, Kansas, Kentucky, Missouri, Oklahoma, and Tennessee can also produce canola [1][8].

Previous studies indicate significant potential for winter canola in North America [9] with an average yield of 2500 Kg∙ha⁻¹ and maximum attainable yield projected to be 5000 to 7000 Kg∙ha⁻¹. Given fluctuating temperatures, especially due to climate instability and winter cold fronts in the interior states of the mid-South, two primary concerns are the date of planting in the fall and date of harvest in the spring. Early fall establishment of canola root system and growing tips are key to survival across the winter months [10]. Winter canola is often double-cropped after a summer crop making planting in September impractical [11]. Thus, winter canola is planted in September or early October in the Mid-South and October or early November in the Southeast. Fertility is also key to good harvests, with nitrogen applied at rates of 100 to 200 Kg∙ha⁻¹ recommended for Tennessee [12].

Inevitably, for early or late fall plantings, some leaf dieback and seedling death occur with winter canola and the impacts will depend on the severity of freezes that occur sporadically across the cooler months [1][8][13]. Extremely cold conditions in winter and/or severe freezes at flowering can lead to low yields in the Mid-South [12]. However, cold weather does not always reduce yields as canola can fill in for neighboring plants, and therefore some plant thinning by freeze damage is tolerated in crop fields [8]. Drought at planting and delayed germination may obviate any benefits from early fall planting [1], and September to October tend to be the driest months in Tennessee. Therefore, the November planting of canola is not out of the question. One advantage of this is that the canola’s growing point is more dormant during the cold weather of winter.

Stem elongation, known as bolting, is a serious problem when canola is planted too early, especially for vigorously developing varieties [1]. In this case, the growing point is overexposed to winter temperatures by producing too much biomass in the fall. Plants of open- pollinated (OP) cultivars tend to be smaller in aerial biomass, and their growing tips are less susceptible to freezing temperatures. As a result, the flowering spikes and racemes emerge in early spring with less yield loss due to freeze or extremely cold weather, however with more limited yield potential than the hybrids.

Climate fluctuations have exacerbated the temperature extremes during winter months, especially for continental climates of the United States, with very bitter cold followed by warming to unprecedented highs [14]. Warm spring and early summer weather in the Mid-South have caused higher than average temperatures during certain critical weeks of canola production. Therefore, the latest dates desirable for winter canola to mature in the region are May and June [13]. This is especially true when winter canola precedes a summer row crops like corn, cotton, sorghum, and soybeans that must be planted before June.

Despite these issues, winter canola has a large potential in the Southeast especially as a new commercial oilseed crop, but also as a locally produced biodiesel [15], or in the future even as a sustainable aviation fuel source under demand by airlines facing the high costs of jet fuel [16].

For this study, we tested germplasm in Tennessee that was developed at Alabama A&M University (Huntsville in Northern Alabama) and Virginia State University (Petersburg, central Virgina). The region is at the heart of the mid-South; mid-way between the deep South and the mid-West. The growing conditions in this area require canola varieties that are locally adapted and winter-hardy, although in some seasons, snow protects growing plants from low-temperature damage [12][17][18]. Crops of canola can also be used as food plots for wildlife [19], and after oil is pressed the canola residues are a high protein feed cake for cattle [12][20]. Winter canola is also an effective cover crop with nematocidal and weed control properties [18].

Our goal for this study was to compare the publicly bred, early-maturing winter canola germplasm from Alabama A&M University (AAMU) to other check varieties of open-pollinated (OP) canola from further north (Virginia State University, VSU and Kansas State University, KSU) both for yield potential and oil and protein content. Early maturing canola should be sufficiently high yielding to be of interest for double cropping with some summer crops in the South. Such lines would be very valuable for producers interested in diversifying their row crop agriculture in the Southeast and Mid-South regions and producing a valuable winter crop to make use of, cover, and protect their fields.

In this study, we found that certain early maturing winter canola lines from AAMU and VSU as well as later maturing ones from KSU were well adapted to middle Tennessee, similar to what was found by other researchers testing only KSU lines [10][12]. This is not surprising as other Brassica species, especially leafy vegetable greens such as collards, mustards, and kale are adapted to fall planting in the mid-South [21][31][32].

However, like other Brassicas, the main abiotic factors for winter canola to overcome are cold stresses [33]. Sudden freezes can severely damage the growing tip of the plants even if the stem and roots of canola are well established [34]. With the similar planting dates of mid-October we used for both seasons, the differences between years were likely due to winter weather conditions, with several early winter freezes affecting survival and drought in fall affecting establishment. This may also have affected earliness to flower. For example, an early spring thaw in the first season could have sped up the flowering compared to freezes in the second season.

Cold hardiness and adaptation to temperature fluctuations is required for canola in middle TN where very cold night temperatures in spring are common and where snow cover protection is rare. Secchi et al. [8] concluded that winter conditions can be very challenging for canola to grow between 35˚N and 40˚N latitude, as we found especially when continental weather is variable. Breeding of winter canola certainly requires selection of cold tolerance [20]. In addition, plant phenology in terms of dates of early flowering and maturity is important to winter survival and overall productivity.

Early in our experimentation with the Southeastern-adapted lines, we noticed differences in earliness, especially for AAMU lines which were early flowering when compared to KSU lines. Their earliness was noted by Sangireddy et al. [22] in Northern Alabama. We also found that the early lines generally yielded less than later maturing ones. Flower evaluations showed that AAMU lines were early both in bud and pod raceme production. This made them more susceptible to cold snaps than KSU lines in season one. VSU, which flowered moderately early, yielded well. KSU lines being later, yielded even better than AAMU or VSU genotypes in season 1. The missing AAMU18 entry and absent KSU checks in season 2 affected year-to-year comparisons, but overall, we had enough entries to draw the conclusion that earliness affected yield, despite different weather each year.

Other factors affecting yield in canola can be date of planting [9][10][35][36], fertilization [37]-[39] plot biomass and soil coverage [12], as well as how much flower abortion, pod drop, and shattering the variety presents [40], or how long the plant flowers in the springtime [17]. Top yields in winter canola can be over 3500 Kg∙ha⁻¹ using hybrid and GMO cultivars under seasons of favorable weather with dedicated agronomic management [9][20][36]. In this trial, AAMU lines were lower yielding due to their earliness but could be useful as low-input crop or as a rotation for double-crops, summer species due to their speedy harvest, allowing a full season of growth for the second crop in the rotation.

Oil content of canola is among the most important traits for canola and usually is in the range of 40% for the summer crop grown in North Dakota [40] and similar but more variable for the winter crop in Virginia [24][29][41] or Tennessee [10]. In the lower yielding production environment of the Southeast compared to Northern Plains, the oil percentage has ranged as low as 37% and as high as 48%. Meanwhile, the protein content of winter canola in Tennessee has been found to be around 21% [12], like our results. We also found negative correlations between oil and protein first observed in Tennessee by [10]. We also found tentative but not strict oil and protein relationships to yield. Another result we can highlight was the utility of a UAV drone canola for monitoring early flowering in the second season. With that application we expand the use of UAV technology for agricultural monitoring as described before for our region of the United States [42].

In terms of agricultural productivity, the maximum yields for winter canola we observed for both years of 2000 plus kg/ha showed promise for local agriculture in Tennessee as it confirmed yields found in other OP varieties tested by [12]; although our yield was somewhat lower than those of winter canola hybrids used by Tetteh et al. [10] perhaps due to the genetics but also the weather and site conditions.

The information found for early canola in this study could be useful in various other parts of the mid-South of the United States, namely in the Mississippi watershed and then more generally across the Eastern U.S. seaboard to the Mid-Atlantic states. However, the single-location design and two-season scope of our study mean that validation by on-farm and additional experimental station sites would be useful before extending our results to the broader Southeast region. One method to do this would be to include some of the best AAMU lines in further testing of the NWCVT organized by Kansas State University or by placing them in cross-species nurseries of the National Cover Crop Variety Testing system organized by the USDA-CAP project on winter cover crops.

Southeastern states with humid climates, such as Georgia, have recommended protocols for production of winter canola [6], but would benefit from testing of early maturing lines that would allow a wider range of crop rotations. Kentucky has studied double cropping soybean after canola [43]. These states have similar climates to the ones represented by the germplasm developed for this study and all could benefit from the lines tested. Finally, the germplasm described here are non-genetically modified organisms (non-GMO) and could fit into organic production systems before or after vegetables in innovative small farm rotations.

In conclusion, the early maturing germplasm from this study could be useful across a broader region of similar latitudes in the various US regions. Overall, the AAMU lines were variable in yield but due to their earliness are promising for production in double cropping systems for similar regions, and this monitoring could be done by a UAV as we proved in this study. The VSU line had good yield but could be outdone by the KSU varieties used as checks, but this was a tentative conclusion based on the lines tested in the first-year check set that were not available for the second year. All the AAMU genotypes tested were open pollinated (OP) and could be distributed without intellectual property constraints and therefore could be used by farmers for seed saving, although this can introduce some variability into the cultivar compared to pure hybrids. The principal advantage to the AAMU lines as well as the VSU variety, therefore, would be their earliness to maturity which would allow rotation with other crops.

We appreciate the field program in Tennessee State University (TSU), including Eddie Williams for farm management at the Nashville Agricultural Research Education Center (AREC). We acknowledge check controls from Harbans Bhardwaj at Virginia State University and Michael Stamm at Kansas State University; as well as funding from the NIFA–USDA for an AFRI competitive grant to AAMU and for Evans Allen Funding (TEN-X07) and a subgrant from the Sustainable Agricultural Systems (SAS) program housed at the Center for Regenerative Agriculture at University of Missouri-Columbia (Grant No. 2023-68012-38993) for TSU. Advice from Rob Myers and Etienne Sutton as part of that project is also acknowledged. This work is dedicated to Dr. Ann Marie Thro for being a pioneer at USDA in the encouragement of canola research in the 1890s land grant university system.