Potentials of crops residues – A case study for the province Vojvodina

Wheat, soybean and corn, in the period 2011 to 2016, were used to conduct the investigation. The samples of aboveground biomass were collected from plots of the advanced farms that practiced common up-to-date technology, at three to five locations in the province of Vojvodina, which is located in the north of Serbia, at least four to eight mostly used varieties or hybrids of each species were used. From the selected plots, five samples were taken, for wheat and soybean from one 1 m², and for corn from 1.4 m². As common seasons, those with average precipitations during vegetation periods (about 280 mm m⁻²) were considered, and dry with a reduction of precipitation of more than 20%. There were three dry seasons for wheat and two for soybean and corn.

Each sample was randomly taken from the field, avoiding the outskirts. Samples consisted of the total aboveground mass. The samples were processed as follows. Wheat was divided into: grain, chaff and stalks+leaves. Soybean was divided into: grain, stalks, branches+leaves and husks. Furthermore, stalks of both wheat and soybean straw were split into segments to determine the dry mass distribution in stems’ height (Figure 1).

Figure 1

The amount of harvestable wheat straw, that is, the technical potential, included harvested stalks or mass, which is obtained by subtracting the amount that remains in the stubble and 30% of the harvested leaves (other remained on the field). The amount of harvestable soybean straw included stalks, without those which, as in the previous case, remains in the stubble and 30% losses of branches/leaves (it is assumed that 70% of the leaves or branches/leaves are collectable). It is assumed that the overall baler losses (pickup device) for both crops were 10%.

The mass of each plant part was measured using the balance with an accuracy of 0.1 g. For the determination of moisture content, grains were dried using the standard procedure defined by ASABE (2008) and crop residue fractions according to the procedure defined by ASABE (2012).

Based on the determined values of moisture content for each fraction, yields expressed to dry matter were determined. For the stalks fraction, diagrams of cumulated mass were made, starting from the ground. They were used to determine the remaining mass of the stubble on field, depending on the height of cutting bar.

Each corn plant was divided into: grain, cob, husk, and stalk with leaves and tassels. The lowest 0.2 m of stalks was treated as unusable (not harvestable) (Figure 2).

Figure 2

The amount of harvested stover was calculated based on the following harvest procedures data:

Two-pass harvest: Grain harvest by combine with snapper–head and integrated shredder-cornrower described in Straeter (2011) and Shinners et al. (2012). According to these authors, the stover is picked up from the wind-row by a round or big rectangular baler. Cutting height was 0.2 m. Percentages of harvested fractions were 70, 80 and 90%, for stalks+leaves, cobs and husks respectively, with additional baling losses of 20%.

Common and overlooked impact on defining sustainable potentials of crop residues’ removal were identified, based on own experiences and practice, and statements of other researchers.

Figure 3 shows the cumulated relative mass of wheat stalks depending on the height for the years 2011 and 2012, as examples for a common (2011) and dry (2012) season. Based on this, the amount of stalks, which remained on the field, that is, stubble, depending on the cutting height, can be determined. Examples of cutting heights 10, 15 and 20 cm are indicated. On stubble remained mass of stalks was in 2011 about 16 and 26%, and in 2012 about 22 and 33%, for cutting heights 10 and 15 cm respectively. The average grain yields were 6.85 and 5.11 Mg ha⁻¹, with a harvest index (HI) of 0.48 and 0.49. The total average yield of the aboveground crop residues were 7.60 and 5.19 Mg ha⁻¹, respectively, for selected representative seasons (Table 1).

Figure 3

Average on field remained residual mass makes at least 50%. This should be respected by considering the impact of straw harvest on soil organic matter and preservation of soil potential. This is because many considerations of straw removal were overlooked. Harvestable mass, technical potential, in dry seasons was compared to the common values, reduced for 45%, and on field remained mass 18%, in average.

The cumulated percentage of soybean stalks, average for all samples, is given in Figure 4, and on stubble remained mass for cutting bar heights 7.5 and 10 cm, here presented examples for common (2011) and dry season (2012). On stubble remained mass of stalks was in 2011 about 19% and 24%, and in 2012, about 25% and 32% for cutting heights 7.5 and 10 cm, respectively.

Figure 4

The all relevant data, the average values for all measurements are given in Table 2. The inconvenient weather conditions (drought) caused the reduction of harvestable mass of 38%, and on field remained mass of 22%, on an average.

Here is a percentage of on field remained mass even higher, 60% and more. Harvestable mass, technical potential, in dry seasons was, compared to common values, reduced for 38%, and on field remained mass 22%, on an average.

The obtained data on the yields of fractions are also calculated as relative, related to grain yield, and presented in Figure 5 for the selected representative seasons. Usable mass of aboveground residues makes about 85% of mass of harvested grain in 2011, and about 132% in 2012, common and dry seasons respectively. This significant change was the consequence of drought impact on grain yield. At the same time, the yield of aboveground mass was considerably reduced.

Figure 5

The results are presented in Table 3. In the common seasons, the HI was slightly lower than 0.5, but for dry ones, it was 0.43. Average grain yield in the common seasons was 10.3 Mg ha⁻¹, and 5.3 Mg ha⁻¹ in the dry ones. For both types of seasons, the percentage of usable residual aboveground mass related to the total for the harvest procedures 1 and 2 was the same, 53 and 43% respectively, but the harvestable mass was considerably lower with procedure 2 than with procedure 1 in both seasons.

On field remained mass makes, depending on the harvest procedure, 43 to 53%, and this is not different for common and dry season. Harvestable residual mass, that is, the technical potential was in dry seasons, compared to common ones, reduced for 31%, and on field remained mass 30%, on an average.

Summarized results of the investigation are presented in Table 4. Here, the potential reduction caused by drought are given, and this has a considerable impact on the supply security. Here, the presented data can be used for the planning of crop residues’ potential, availability, considering also the impact of weather conditions. Presented data can be used for the evaluation of impact of crop residues removal, nearby others, on the preservation of soil fertility.

Impact of crop residues’ removal on soil fertility has been investigated and reported frequently. However, it is still missing a clear approach for its quantification and evaluation. Here, some not elaborated or only tackled impacts are overviewed:

Impact of erosion, primarily wind erosion. This issue was partly elaborated related to the assessment of land surface coverage, ASABE standard (2012). Instruction for evaluation was given by extension service in USA (Hickman and Schoenberger, 1989). Some evaluations have been performed for wheat and soybean (Golub et al., 2013) and corn (Golub et al., 2016). Of course, the result depends on the applied soil tillage. This issue has environmental implications.

Technical potential of crop residues fluctuate, depending on weather conditions, and other impacts (e.g., diseases, insect infestations). This means some reserves of biomass should be planned and this has an impact on the feedstock overall costs. All above-mentioned make defining of sustainable potential a complex task, but it is not impossible.