EFFECT OF GRAIN MOISTURE CONTENT AND THRESHING CYLINDER SPEED ON MECHANICAL GRAIN DAMAGE IN NAKED OAT CULTIVARS

Andrzej Zieliński, Maria Moś, Tomasz Wójtowicz
Department of Plant Breeding and Seed Production, University of Agriculture in Krakow, Poland

ABSTRACT

There was estimated a susceptibility of grains of the naked oat cultivars: Akt, Cacko, Polar and the line STH 4659 collected at grain moisture content from 22.9 to 10.9% to mechanical damage caused by threshing at two cylinder speeds of the threshing device: 1.6 and 2.4 m·s^-1. The control samples were made up of hand-threshed grains. Macrodamage to the embryo, damage to the endosperm and to parts of grains were distinguished. The grain endosperm (7%) and the embryo (about 5%) were most strongly exposed to the direct operation of thresher elements. Damage occurring on the embryo, in the endosperm and, at the same time, on the embryo and endosperm were distinguished in the study of microdamage, and their share ranged from 10 to 40%. Highly significant relationship between the sum of microdamage classes and the number of damaged grains (r = 0.47), total damage area (r = 0.52) and the share of damage area (r = 0.55) shows the applicability of the computer image analysis to evaluate the mechanical damage of oat grains.

Key words: computer image analysis, naked oat, mechanical damage.

INTRODUCTION

In recent years, the importance of naked oat cultivars, genetically conditioned, has been growing. Literature offers reports on the agricultural study of naked cultivars [1,3,8], their resistance to pathogens [13] and nutritive value [2,6,18]. Many grain production problems, however, call for further examination, including low grain germination capacity, which resulted in lowering the national quality standards for seeds in naked cultivars. It is commonly known that naked oat grains are more sensitive [14], the embryo is more sticking-out and hence the exposure to heavier mechanical injuries during harvesting and threshing [22,15]. Valentine and Hale [22] indicate that these factors can be the cause of reduced germination capacity and the vigour of grains.

The seed quality determined by germination capacity and, generally, by grain viability is limited by a number of injuries which, according to Strona [16], are considered one of the main causes of reduced germination capacity and field emergence, and consequently, a decrease in yield. It is hard to find agricultural practices in grain production where grains would not be exposed to injuries [11]. Each stage of technological seed treatment, from harvesting to threshing, cleaning, grading, drying, transport, storing, processing and sowing are heavily affected by external forces. The most essential cause of mechanical damage during combine harvesting, according to Gieroba et al. [7], are biological and physical properties of seeds and technological parameters of the threshing device, especially the threshing cylinder speed. To make the study accurate, it is important to present qualitative mechanical damage and so digital image analysis techniques are used more and more frequently [23]. These methods make it possible to measure many seed features simultaneously and to perform their statistical analysis. Moreover, for practical reasons, one should focus on non-invasive methods, less time-consuming and with labour-intensity reduced to minimum. Additionally, their user-friendliness and low costs of production will make those methods more and more common.

Launching naked oat cultivars in agriculture, it is interesting to find the differences between husked and naked cultivars in their susceptibility to mechanical damage resulting from variation in moisture content and different cylinder speed of threshing elements.

The aim of this study was to evaluate the effect of moisture content and the threshing cylinder speed on the occurrence of mechanical damage in naked and husked oat cultivars and the applicability of computer image analysis in cultivar assessment.

MATERIAL AND METHODS

Grains of the naked cultivars Akt, Cacko, Polar and the line STH4659 cultivated over 2002-2004 were applied to field experiments set up at the Experimental Station of the Department of Plant Breeding and Seed Production at Prusy at the vicinity of Kraków. Sixty rows 1.5 m long and at 0.20 m row spacing were sown each year.

Grains were harvested three times at 5-7-day intervals as soon as the grain moisture content reached approximately 20, 14 and 11% of water. At harvest, panicles from successive 20 rows were cut and threshed with the thresher device with two cylinder speeds: low (N) – 1.6 m·s^-1 and high (W) – 2.4 m·s^-1. The control plots (K) were threshed manually at the same grain moisture content and used for the microdamage evaluation. After threshing and cleaning with pneumatic winnower Petkus Labor Windsichter K293, the seed moisture content was determined with the drying method according to the ISTA [9], and then seeds were sieved on slotted screens 1.75 mm in width.

The grain macrodamage of the tested cultivars was determined in 2 replications 100 grains each for the samples threshed manually. Grains with visible damage to the embryo (Z), to the endosperm (B) and broken grains (P) were visually selected. The assessment of microdamage was made for samples of 50 grains in 2 replications by staining of starch using 2% Lugol’s solution. Having been immersed in Lugol’s solution for 15 minutes, the grains were rinsed with deionized water and after drying, microdamage was observed using the laboratory magnifying glass in 4-fold magnification. Microdamage of embryos (Z), endosperm (B), and both on embryo and endosperm (Z + B) were distinguished. The sum of weighed values of microdamage percentage were estimated for observations, assuming the weight value – 1 for damage to the endosperm (B), 2 – for damage to the embryo (Z) and 3 – to the embryo and endosperm (Z + B) for:

where
w_j – weight of a given type of microdamage,
x_j – value of the share of a given type of microdamage.

Additionally, the computer image analysis was applied in microdamage evaluation. 50 grains were sampled in four replications for each combination of harvest time, threshing speed, cultivars and year. After staining in Lugol’s solution and drying on filter paper, grains were put with the back side to the scan surface. Digital processing of images obtained with the scanner Agfa Snapscan e40 was carried out with Multiscan 12.07 program. In the image analysis, the total area of grain projection, projection area of grain microdamage and grain dimensions: length, width and perimeter were additionally determined.

Trifactorial analyses of variance were performed in randomized blocks design according to a constant model. The results expressed in percentages were transformed with Bliss formula: . To evaluate the percentage of the sources of variation in the total variation of the features tested, variation components were assessed [5,21] and their percentages were presented. The interaction between the features was evaluated with Pearson’s linear correlation coefficient, linear regression or square regression.

RESULTS AND DISCUSSION

The study of grains with 22.9 to 10.9% moisture content made it possible to evaluate the susceptibility of naked cultivars to mechanical injuries by varied speed of the thresher cylinder. Analyses of variance made for respective years showed that the oats grain macrodamage was mostly affected by grain moisture at harvest, and the share on macrodamage in the total variability was higher than 70% (Table 1).

The highest percentage of macrodamage (25%) in the cultivars tested was obtained when the grain moisture at harvest ranged from 19.3 to 22.9% (1^st date), and grains with damaged endosperm constituted the most numerous group (7-12%). At this moisture content, embryo damage, which determines the sowing value, at the high cylinder speed, was up to 10%. The earlier reports suggest that the lack of lemma and palea in naked barley cultivars results in their increased susceptibility to direct injuries of embryos during mechanical threshing [10]. According to Kolasińska and Boros [10], the most frequent damage during threshing with a combine harvester involves breaking off grain fragments together with part of the embryo. In the present study, irrespective of the cylinder speed, in all the cultivars the smallest group were broken grains, which represented the fraction below 1.75 mm during cleaning.

The regression relationship between the moisture content and the frequency of grain macrodamage was estimated for oats cultivars harvested in successive years and at harvest dates (Fig. 1). In the samples of grains threshed at the lower rotation speed (1.6 m·s^-1), the regression equation showed that the most optimal conditions for threshing at that cylinder speed occurred for the grain moisture content of 15.3%, when the number of damaged grains was lowest and it did not exceed the value of 3%. Similarly, according to Strzelecki [17], the range of optimal grain moisture content for triticale threshing was 13-16% and, according to Slipek and Złobecki [19], for wheat – 17 to 26%. Threshing at cylinder speed 2.4 m·s^-1 showed a linear dependence of microdamage percentage on grain moisture at harvest (Fig. 2). The share of macrodamage depended in 58% on grain moisture content during threshing at high cylinder speed (r² = 0.58).

Microdamage constituted a more numerous group of grain injuries and ranged from about 10 to above 40% depending on the cylinder speed and grain moisture content. The variation components estimated for the share of microdamage indicate that their highest variability was caused by different harvest date (Table 1). Similarly as for macrodamage, the highest percentage of microdamage occurred on grain endosperm and on embryos threshed at high cylinder speed. Microdamage which occurred simultaneously on embryo and endosperm was low (below 5%). Mos and Zieliński [12] recorded similar results studying the microdamage of oat grains, additionally focusing on differences resulting from the presence or absence of lemma and palea. Husked cultivars demonstrated significantly, 2-3-fold, lower percentage of grains with microdamage. The authors noted that husked cultivars were highly resistant to mechanical damage during threshing, and the lemma and palea are helpful in limiting the effect of threshing device type on the grain damage area of traditional cultivars. In the present study, the maximal percentage of grain microdamage occurred at the highest moisture content and higher thresher cylinder speed. Damage to the embryo, reducing the sowing value most substantially was highest (about 20%) at high cylinder speed and decreasing with later harvest date (Fig. 3). According to Dziki [4], the higher the grain moisture, the greater the grain tolerance to the effect of external forces. Regression analyses in the present study made it possible to find a square equation in order to determine the microdamage trend depending on grain moisture content at harvest. Both hand and mechanical threshing gave similar results, thus they can be jointly interpreted. Despite a considerable spread of points for weighed sums of the microdamage percentages of hand-threshed grains and numerous observations reaching beyond confidence limits (Figs. 4 and 5), one may forecast that the lowest number of microdamage will occur at threshing grains of 14-18% moisture content. A disadvantage of such forecasting of microdamage share is the lack of data for grains with 15.5 to 19% moisture. For grains threshed at the high cylinder speed, a simple regression equation was found. At the same time, the high significant correlation coefficient value (r = 0.76) suggests a decrease in the number of oat grain microdamage together with the moisture reduction at successive harvest dates (Fig. 6). Furthermore, the high determination coefficient value indicates that nearly 60% of changes in the share of the weighed sum of microdamage resulted from grain moisture changes.

Apart from the traditional method of mechanical damage evaluation, the computer image analysis was applied. The significant effect of all the sources of variation tested and their interactions on the share and area of oat grain microdamage was found (Table 2). Grain moisture content at the first harvest date resulted in the highest number of grain microdamage. During manual threshing (K) the average number of grains with microdamage accounted for 25%. Mechanical threshing at low (N) and high (W) cylinder speed increased microdamage by 8 and 15% on average, as compared to the control. Harvest at lower moisture content, 13.6 to 14.8%, decreased the number of injuries, and no influence of the threshing method on the share of microdamage was observed when the grain moisture content decreased to about 11%.

The largest area of damage was found at threshing when grain contained 22% of water (Fig. 7). High cylinder speed increased the damage area, but the differences for high and low cylinder speed were reduced together with a decrease in grain moisture at successive harvest dates. The oat cultivar-specific effect was observed at all harvest dates. The largest damage area occurred at the first and second date of harvest in Akt.

In the study of mechanical damage, some authors focus on the relation between the size and shape of grains. In naked cultivars, Peltonen-Sainio et al. [15] indicated the significant importance of grain length and the degree of embryo sticking-out beyond the grain outline and their susceptibility to mechanical damage. Oats grains that are longer and irregular in shape are more exposed to mechanical damage. In the same study, the grains of Lisbeth naked oats, highly resistant to damage, were longest, smallest in diameter and lowest in weight. In the present study, the average grain length ranged from 0.77 to 0.92 mm, the width from 0.21 to 0.28 mm, and perimeter from 1.75 to 2.18 mm. The highest values of those features in all the harvest years were reported in Akt. A significant effect of seasonal variability was found, and the year 2004 was most favourable for harvesting plump grains. In spite of the significant influence of the cylinder speed on the increase in the number of damaged grains, the relationship between the seed size and their susceptibility to mechanical damage cannot be finally explained. Both longest and widest Akt grains were most considerably damaged only at the moisture content ranging from 19.3 to 15.4%, irrespective of the cylinder speed. The other parameters measured using the computer image analysis, such as the number of damaged grains and the percentage of damage area did not indicate a particular cultivar, and a characteristic size of its grains, for which the damage would assume the highest values. Similarly, Thornton [20], making a comparison of oat cultivars in search of resistance to mechanical damage in naked forms, did not observe that grains with embryos sticking-out less, even for the same cultivar, were more resistant to mechanical damage.

The microdamage was assessed with the traditional (macroscopic) method and compared with the results generated in the computer image analysis (Table 3), which identified highly significant relationship between the sum of microdamage classes and the number of damaged grains (r = 0.47), the total damage area (r = 0.52) and the percentage of damage area (r = 0.55). The number of damaged grains was highly significantly correlated with all the morphological features, and the highest correlation was observed with the total damage area (r = 0.84).

CONCLUSIONS

1. Variability of grain moisture content at harvest modified the susceptibility of naked oat cultivars to mechanical damage caused by different speed of threshing cylinder.

2. Harvest at grain moisture content from 22.9 to 19.3% and applying the high cylinder speed of 2.4 m·s^-1 resulted in a significantly higher number of macrodamage, of which grains with damaged endosperm constituted the most numerous group (7-12%) and damage to the embryo was up to 10%.

3. A significant effect of the cylinder speed on the mechanical damage to naked cultivars was found; the increase in threshing cylinder speed from 1.6 to 2.4 m·s^-1 resulted in a 15% increase in the share of grains with microdamage and a 40% increase in the area of microdamage.

4. To limit the naked oats grain microdamage during threshing, the grain moisture content ranging from 14 to 16% seems most favourable.

5. The high correlation between the results of total grain microdamage determined with macroscope and the computer image analysis indicates the applicability of the latter to mechanical seed damage evaluation. Additionally, the possibility of evaluation of damage area and basic properties of grain, such as length, width, perimeter and shape, makes this technique even more useful for seed research.

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Accepted for print: 27.10.2007

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