Introduction

The corn or maize (Zea mays L.) inbred lines CO477, CO478, CO479, and CO480 were developed at the Ottawa Research and Development Centre (ORDC), Agriculture and Agri-Food Canada (AAFC), Ottawa, ON. These lines are best suited for mid- to late-season growing regions of North America with up to 3000 crop heat units (CHU) (Brown and Bootsma 1993) and the primary intent of growing is biofuel, silage, and (or) sugar production.

Climate change and the rise of atmospheric carbon dioxide concentration stirred the need to reduce the use of non-renewable fossil fuel resources and substitute with crop-based renewable energy sources. Several crops, including oil palm, sugarcane and sweet sorghum, are recognized for ethanol production (Sims et al. 2006). Despite the lower efficiency (cost per liter of ethanol production), corn grain is dominant as a biofuel source in North America because sugarcane and other more efficient crop sources are restricted to growth within tropical and sub-tropical regions. Juice from corn stalks has been pilot tested and proven for biofuel production with promising ethanol yield comparable to sugarcane and sweet sorghum (Gomez-Flores et al. 2018).

Corn with high stalk sugar, commonly referred as sugarcorn (Reid et al. 2015) is a fast-growing biofuel energy crop suited for Canadian environments with readily fermentable sugars and a potential to save on energy and enzyme cost, as compared with corn grain-based biofuel production systems. High stalk sugar is a heritable trait in corn with mostly additive gene effects and acceptable selection gains have been achieved using pedigree breeding methods at AAFC (Reid et al. 2016). These four sugarcorn inbred lines are among the first to be developed from the ORDC, AAFC maize breeding program.

Pedigree and Breeding Methods

CO477, CO478, CO479 and CO480 were developed from four crosses CO384 × C103, CO388 × C103, CO442 ×C103 and CO444 × C103, respectively. The common parent in the four crosses, C103, was developed at the Connecticut Agricultural Experiment Station, New Haven, CT, USA (Singleton 1948) and it is a high stalk sugar containing inbred line belonging to the Lancaster heterotic group. CO384, CO388, CO442 and CO444 were developed and released by AAFC for their high grain yield and other desirable traits when in hybrid combinations (Reid et al. 2001, 2008). CO384 is an early European flint type inbred line. CO388 is a medium maturing stiff stalk (BSSS) type inbred line with intermediate resistance to gibberella ear rot caused by Fusarium graminearum. CO442 is an iodent type inbred line and CO444 is an early European flint type inbred line.

Initial crosses for the development of CO477, CO478, CO479 and CO480 were made in 2007. Successful ears from self-pollination in 2008 were advanced 10 more generations using a modified pedigree method with selection for high levels of sugar in the stalk juice as well as other desirable corn traits such as low stalk breakage, lodging resistant and overall good agronomic performance. Sugar in the stalk juice were measured following the protocol described by Reid et al. (2015). Briefly, the above ground biomass of corn plants were harvested 30 days after silking, which is 10–15 d earlier than a standard silage harvest time. Juice from the stalks was extracted in the field immediately after harvest using a custom diesel sugarcane crusher/juicer machine with 4 ton capacity. A digital brix refractometer (PR-32, Atago Co. Ltd, Bellevue, WA, USA) calibrated with known sucrose solutions from a composite juice sample obtained from 5 cm of stalk tissue was used to estimate the sucrose concentration as degree brix (°B_x). One degree Brix is 1 g of sucrose in 100 g of solution.

Juice weight harvested per hectare was calculated as:

Sugar harvested per hectare was calculated as:

Combining ability with testers of different heterotic patterns was conducted at the S4, S6 and S9 generations. At S8 and S9 generations, resistance screening for 6 different diseases namely, gibberella ear rot, common smut, fusarium stalk rot, Northern Corn Leaf Blight, Eyespot, and common rust were conducted. All experiments from the initial crossing to the final inbred line testcross performance evaluation for grain yield and high stalk sugar content/silage were done at the ORDC experimental field station in Ottawa, ON.

The data reported here were from sets of field experiments conducted using S10–S12 generations. Testcross performance evaluations of CO477, CO478, CO479 and CO480 for grain yield were conducted in 2017, 2018 and 2019 and for high stalk sugar content/silage in 2017 and 2019. Two sets of testcross performance evaluation experiments were conducted in 2017 and 2019. Set 1 was for evaluating grain yield and Set 2 was for evaluating high stalk sugar content/silage. Set 3, which was conducted only in 2017, was for evaluation of the inbred lines for high stalk sugar content/silage on per se basis. Testcross performance evaluation experiment for high stalk sugar content/silage was not conducted in 2018 due to limited testcross seed availability for that year.

For the high sugar content/silage testcross performance evaluation, five testers were used in 2017. One tester, CL30 was developed by AAFC and has a European flint type heterotic pattern. Four of the five testers were commercially sourced and belong to the stiff stalk B14 (MBS Genetics MBS1130GT, Thurston Genetics TR2040 RMQZ) and iodent (MBS8148, TR1995) heterotic groups. In 2019, the testers were MBS1130GT and TR2040 RR2 from the stiff stalk B14 type heterotic group while MBS8148 and TR1633 were from iodent. Most of the commercially available testers including the ones used in evaluating the combining ability of the current inbred lines aim to test grain yield performance. These testers were used in both grain yield and high stalk sugar content/silage experiments because there were no commercially available testers to evaluate the combining ability for high stalk sugar content. In addition, the aim of developing these inbred lines was to combine high grain yield with high stalk sugar content.

For Set 1 and 2, field trials were conducted with three replications in two-row plots of 8 m length, 0.16 m and 0.76 m within and between row spacing. Sample for estimating juice, sugar, and biomass of the stalks in Set 2 experiments were collected from plants growing within 1 m of the middle of each row. Set 3 was conducted with two replications and single-row plot of 3.7 m length, 0.16 m and 0.76 m within and between row spacing. In Set 3, juice, sugar, and biomass data were collected from 5 plants in the middle of each row. Check hybrids used in Set 1 and 2 varied based on their end use, grain yield or silage. In Set 1, check hybrids for grain yield namely Dekalb DKC38-03RIB, MAIZEX MZ 395x, Pioneer P9188AM and Pride A6015 were used. In Set 2, check hybrids known for silage production Pioneer P9644AMX, Pioneer P9789AMXT and Pride A5892G3 EDF RIB were used in 2017 and Brevant B96R17SX, Pioneer P0242AMXT and Pioneer P9789AMXT were used in 2019.

Performance

When compared with the high-stalk-sugar parent C103, CO477 and CO478 had significantly higher (P < 0.05) stalk juice and sugar yield on a per se basis while CO479 had only significantly higher (P < 0.05) sugar yield (Table 1). In terms of biomass yield, the inbred lines CO479 and CO480 had significantly lower biomass than C103 but CO477 and CO478 were not significantly different. CO480 was an exception, having the lowest biomass, stalk juice and sugar yield of the four sugarcorn inbred lines and had a sugar yield that was not significantly greater than C103.

Table 1.

Inbred line per se performance evaluations for stalk sugar content and biomass in 2017.

In sugarcorn hybrids, the amount of stalk juice harvested and the proportion of sugar in the juice are equally important factors determining the final sugar yield per unit area of production. Unlike sugarcorn, some silage corn cultivars are developed with traits that promote easy digestion and nutrient acquisition by ruminants. The brown mid-rib trait is responsible for the easy digestion (Cherney et al. 1991) and it is branded by some private corn breeding companies as effective digestion fiber (EDF). EDF branded hybrids have high moisture in the stalks and ears. One of the check hybrids used in the current study, Pride A5892G3 EDF RIB had substantially high juice (13028.6 kg·ha⁻¹) and sugar yield (1107.6 kg·ha⁻¹) than any of the other check hybrids tested although the degree brix (8.6%) is lower than most of the testcrosses of the sugarcorn inbred lines (Table 2). This can be a confounding factor affecting the total sugar yield per hectare given Pride A5892G3 EDF RIB is not the ideal sugarcorn hybrid.

Table 2.

Testcross performance evaluations for stalk sugar content and biomass in 2017.

The testcross performances of CO477, CO478, CO479 and CO480 in 2017 for stalk juice, sugar content and biomass yield are shown in Table 2. CO477 had the highest amount of sugar yield (1186.1 kg·ha⁻¹) when crossed with the tester MBS8148 although the proportion of sugar in stalk juice or degrees brix was the highest (14.1%) when crossed with a different tester CL30. CO477 also combined well with CL30 and had the second highest amount of sugar yield (1116.7 kg·ha⁻¹) despite the low juice yield. For biomass yield, CO477 combined the best with TR1995 (wet biomass = 31803.5 kg·ha⁻¹, dry biomass = 11131.2 kg·ha⁻¹) followed by TR2040 RMQZ (wet biomass = 30778.7 kg·ha⁻¹, dry biomass =10772.5 kg·ha⁻¹). CO478 and CO479 combined the best with CL30 for degree brix, at 12.1% and 11.6%, respectively, but total juice harvested was lower than some of the other testers resulting in low sugar yield per hectare by comparison. CO478 had the highest juice (9434.5 kg·ha⁻¹), sugar (941.3 kg·ha⁻¹) and biomass yield (wet biomass = 31267.0 kg·ha⁻¹, dry biomass = 10943.4 kg·ha⁻¹) when crossed with TR1995. CO479 performed the best when crossed with MBS1130GT yielding 12878.8 kg·ha⁻¹ of juice, 1286.8 kg·ha⁻¹ of sugar and 35480.8 kg·ha⁻¹ of wet biomass and 12418.3 kg·ha⁻¹ of dry biomass. CO480 combined the best with TR2040 RMQZ for sugar yield (842.2 kg·ha⁻¹) and biomass (wet biomass = 35099.7 kg·ha⁻¹, dry biomass =12284.9 kg·ha⁻¹).

In 2019, CO477 had the highest stalk juice (10856.9 kg·ha⁻¹) and sugar yield (991.9 kg·ha⁻¹) when crossed with the tester MBS1130GT but still non-significantly (P < 0.05) different from testers MBS8148 and TR1633 (Table 3). CO478 combined best with tester TR2040 RR2 for stalk juice (9360.9 kg·ha⁻¹), sugar (813.3 kg·ha⁻¹) and biomass yield (wet biomass = 38924.9 kg·ha⁻¹, dry biomass = 13623.7 kg·ha⁻¹). CO479 performed equally well with testers MBS1130GT and TR2040 RR2 for stalk juice (9259.3–9862.0 kg·ha⁻¹), sugar (960.1–1080.7 kg·ha⁻¹) and biomass yield (wet biomass = 33991.2 – 36680.5 kg·ha⁻¹, dry biomass = 11896.9–12838.2 kg·ha⁻¹). CO480 had non-significant difference in juice, sugar and biomass yield irrespective of the tester used and ranged 5650.0–8620.2 kg·ha⁻¹ stalk juice, 447.9–704.1 kg·ha⁻¹ sugar, 30344.8–36268.8 kg·ha⁻¹ wet biomass and 11423.6–13548.6 kg·ha⁻¹ dry biomass.

Table 3.

Testcross performance evaluations for biomass and stalk sugar content in 2019.

The four inbred lines had comparable grain yield with two of the check hybrids Dekalb DKC38-03RIB and Pioneer P9188AM when crossed with at least one or two of the testers but had significantly (P < 0.05) lower grain yield than the hybrid check Pride A6015 (Table 4). When comparing combining ability of the inbred lines with the different heterotic group testers, CO477 had the highest grain yield when crossed with the iodent tester MBS8148 (6.93 t·ha⁻¹). CO478 had the highest grain yield when crossed with either the iodent tester TR1995 (7.04 t·ha⁻¹) or the stiff stalk B14 tester TR2040 RMQZ (7.03 t·ha⁻¹). CO479 and CO480 had the highest grain yield when crossed with the stiff stalk B14 testers MBS1130GT and TR2040 RMQZ (6.74–6.95 t·ha⁻¹).

Table 4.

Grain yield performance averaged over three years testing (2017–2019) in Ottawa, ON.

The testcross grain yield performance of these inbred lines did not outperform the check hybrids indicating there is still room for grain yield improvement if the intention of use is for dual production of grain yield and stalk juice or sugar. On an inbred line per se basis, these four inbred lines on average appear to produce three times more sugar than their corresponding testcrosses. One reason for such a difference in stalk sugar levels could be that the hybrids are able to mobilize the photo-assimilates and store in the developing kernels more efficiently than the inbred lines. It is to be noted that the data for per se performance is from one year experiment in Ottawa, ON. It is suggestive but not conclusive. More data from multi location and year experiments will confirm the claim.

Other Characteristics

To determine the distinctness, uniformity, and stability of the newly released AAFC maize genotypes, 44 characteristics are reported of which 26 are used by the International Union for the Protection of New Varieties of Plants (UPOV) (Table 5). CO477 is 2.0 m in height with 14–16 semi-erect leaves. It takes 78–80 d for both 50% anthesis and silking (Table 5). It has a red cob with wedge shaped yellow flint kernels arranged in 16 rows. CO478 is 2.2 m in height and has 18 semi-erect leaves. It takes 79 d to 50% anthesis and 80 d to 50% silking. Cobs are red with 14–16 kernel rows and round yellow flint-like kernels. CO479 is similar in height to CO477 (2.0 m) and has 16–18 semi-erect leaves and flowers the latest from the rest i.e., 79 d to 50% anthesis and 81 d to 50% silking. It has red cob with 14–16 kernel rows and wedge-shaped yellow flint-like kernels. Of the four, CO480 is the shortest in height (1.7 m) and earliest to flower taking only 75 d to 50% anthesis and 78 d to 50% silking. The cobs are red and have 12–14 kernel rows. The inbred lines CO477, CO478, CO479 and CO480 can be used both as male and female parent when developing a hybrid. If used as female parent, seed yield would be 72, 125, 79 and 85 grams per ear, respectively. All inbred lines have moderate to intermediate resistance to common rust, eyespot, northern corn leaf blight and fusarium stalk rot but are susceptible to gibberella ear rot.

Table 5.

Characteristics of corn inbred lines CO477, CO478, CO479 and CO480; description and UPOV scores*.

Maintenance and Distribution of Pedigreed Seed Stock

Breeder line seeds of CO477, CO478, CO479 and CO480 are maintained by the Ottawa Research and Development Center, Agriculture and Agri-Food Canada, Ottawa, ON, Canada K1A 0C6. Company and university researchers who wish to receive seed will be required to enter into either a Corn Inbred Release Agreement or a Material Transfer Agreement with AAFC. Agreements can be requested from the Director, Agriculture and Agri-Food Canada, Ottawa Research and Development Centre, KW Neatby Building, 960 Carling Avenue, Ottawa, ON K1A 0C6, Canada. Further information on AAFC seed distribution and inbred lines can be obtained from the website  http://www.agr.gc.ca/ScienceandInnovation.