Influence of Seed Physical Properties and Speed on the External Mechanical Damage Index and In-Row Spacing Uniformity in an Automatic Potato Planter

In this study, a new fully automatic potato planter was used in which the planting metering mechanism consists of cups and hold pins attached to the circumference of a vertical disc. In order to evaluate performance of the planting metering mechanism, in-row spacing uniformity and external mechanical damage index were determined (P<0.01). Also skips and doubles that caused deterioration of in-row spacing uniformity were investigated. In experiment, two potato varieties (Marfona and Agria) and two potato sizes (25-45 and 45-65 mm) were used. The ground speeds of potato planter were chosen as 3.5, 5.4 and 7.3 km h. The results obtained from the experiment indicated that the ground speeds and tuber physical properties significantly influenced tuber spacing accuracy and external mechanical damage index (P<0.01). While the ground speed increased, accuracy of tuber spacing decreased. The highest coefficient of variation (CV%) values were obtained from ground speed of 7.3 km h, Marfona variety, and tubers between 25-45 mm, with 26.80%, 25.58% and 26.27%, respectively. The highest damage index values were obtained from ground speed of 7.3 km h, Agria variety, and tubers between 45-65 mm, with 1.34, 1.18 and 1.23, respectively.


Introduction
Potato which is a versatile, carbohydrate-rich food and highly popular worldwide has an important place among plants cultivated in the world.Potato is consumed as a vegetable as well as processed.Potato is grown in more than 100 countries, under temperate, subtropical and tropical conditions (Potato 2008).The potato crop is usually grown not from seed but from "seed potatoes".To fulfil the food demand of an increasing population, it is necessary to increase yield of the potato which has the ability to produce 2 or 3 times more food material from per unit area when it is compared with cereals (Hussain & Gillani 1985).Planting, cultivating, and harvesting are three main components of potato production.Planting is the first component which has an affect on potato yield.The uniformity of plant density is one of the most important subjects of potato production, so it affects the cost of seed, plant development, yield and quality of potato (Bussan et al 2007;Güllüoğlu & Arioğlu 2009).There are two important factors that effect the plant density of potato planting with machinery.One of those is in-row plant spacing and the other one is inter-row width.Inter-row width largely depends on the tractor's wheel spacing, as well as the machinery used in potato production.Thus, inter-row width of a potato planter is fixed.However, in-row plant spacing plays very important and dynamic roles on potato yield and quality in terms of evaluating the performance of a potato planter (Seyedbagheri 2006).Uniform plant spacing has been suggested to increase potato yield and quality (Klassen 1974;Klassen 1975).Rupp & Thornton (1992) determined that a 10% increase or decrease in plant number from the optimum potato population reduced grower returns from 2% to 12%.In addition, they determined that planter skips and doubleclumped seed pieces typically reduced economic return compared with uniform in-row spacing.James et al (1973) studied to estimate the yield losses by removing randomly different percentages of plants from plots at emergence and/or later stages.They determined that the average losses in yield for 10, 20 and 30% misses were 0, 5.6 and 11.1%, respectively.Nevertheless, it is stated by some researchers that the potato is able to compensate for missing plants, judging by reports that the yield reduces about 0.3% for each 1% of plants missing (Hirst et al 1973;James et al 1973).
However, yield alone is not enough to determine the economic worth of potatoes.The size of the tuber also plays a crucial role (Rupp & Thornton 1992).The importance of spacing uniformity is influenced by the size grades that are used to define marketable yield.Therefore, changes in the size of tuber can directly affect grower returns (Schotzko et al 1983;Thornton et al 1983;Rex et al 1987;Rupp & Thornton 1992;Creamer et al 1999;Love & Thompson-Johns 1999).Love & Thompson-Johns (1999) reported that seed pieces of russet-type cultivars planted at 8 cm in-row spacing resulted in higher total tuber yields than seed pieces planted at wider intervals (15 to 91 cm).However, the total yield from the 8 cm spacing treatment returned fewer dollars per ha than those from wider spacing treatments due to a high percentage of undersized tubers.They reported that average tuber weight decreased as seed pieces were planted closer together, and vice versa.Similarly, Rupp & Thornton (1992) and Halderson et al (1992) found an increase in the yield of small undersized tubers from seed pieces that were clumped together versus those spaced farther apart.Sieczka (1986) determined that the in-row plant spacing of fields in New York was not uniform, and coefficients of variation (CV %) ranged from 43 to 70% with the average seed placement 4 cm wider than adjusted.Mechanical failure of planters was cited as the primary reason of plant missing.The mechanical failure was presumably due to a combination of factors, which included planting speed, poor planter condition and maintenance.
Capacity and accuracy of plant spacing are the main parameters of machine performance.High accuracy of plant spacing results in a high yield and a uniform sorting of the tubers at harvest (McPhee et al 1996;Pavek & Thornton 2003).The function of the potato planter is to place potatoes singly in furrows at predetermined regular intervals.The spacing is dependent on the efficiency of the feeder.Usually the spacing is done mechanically either by simple hand fed or by relatively more complicated, fully automatic potato planter.One of the most important parts of potato planters is planting metering mechanism.The accuracy of plant spacing is affected by constructional properties of the planting metering mechanism, the planter ground speed, the shape of potato and seed size.

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Generally, although the external mechanical damage index is used to determine ability of potato harvesting machines, this can be also used to determine the ability of potato planter.Then it can be said that external mechanical damage index is one of performance parameters of a potato planter.External mechanical damage is a visible injury without cutting the tuber.The external mechanical damage index is determined and classified with different formulas and statements in the UK, Holland and Germany (McRae 1985).In addition, several workers have published methods of assessing the damage sustained by potatoes.However, damage index described by Robertson (1970) is used as a useful measure to compare the scale of external damage for crops of different cultivars or from different sites.
The objective of the current study was to evaluate effect of different tuber sizes and shapes, and ground speeds on external mechanical damage index and the uniformity of seed spacing of the full automatic planter's planting metering mechanism.

Material and Methods
Full automatic potato planter with two rows was used in the experiments (Figure 1).The main frame is supported by two wheels.The hopper of the potato planter is placed above the main frame.There are two planting metering mechanisms under the hopper.The driving wheels do not only carry most of the weight of the potato planter, but also drive the planting metering mechanism.
The planting metering mechanism consists of 14 pieces of cups and hold pins (Figure 2) attached to the circumference of a vertical disc (Figure 3).The hold pins and the cups mounted on the vertical disc constantly rotate with the vertical disc.The hold pins are opened by the mechanism which is expressed as the pin-position cam.The pin-position cam is fixed.While the cup and the hold pin rotate, the roller on the hold pin hits the pin-position cam, and the hold pin is opened.Then, the cup passes through the tubers.Meanwhile, the planting mechanism selects a seed piece from the seed hopper.Then, hold pin moves onto the cup with the effect of a helical spring after the vertical disc rotates 170 0 degrees, and the potato seed is compressed by the hold pin and cup.After the cup rotates 190 0 degrees, the hold pin is opened by the pin-position cam, and potato seed drops into the furrow.This process repeats continuously.There is one planting metering mechanism for each row.(2) (3) Some physical properties of the two varieties of potato used in this study are given in Table 1.The angular speeds were determined by taking into consideration the ground speeds (3.5, 5.4 and 7.3 km h - 1 ) of the potato planter.The angular speeds of the vertical disc were calculated as 1.56, 2.39 and 3.22 rad s -1 , respectively for these ground speeds.The feeding rates for these angular speeds were 207, 319 and 431 potatoes min -1 , respectively.In order to obtain these angular speeds, a three-phase induction motor driving sprocket on the axle of the vertical disc with a roller chain was used.
An AC speed control unit was used to change the rpm of the three-phase induction motor.The speeds were measured by a contact type digital tachometer.A laser measurement system was used to measure the time interval between consecutive tubers.The laser measurement system has an accuracy of 0.01 s.The laser measurement system consists of a laser pointer, a light dependent resistor (LDR), a software written using Matlab, a sound card and a computer (Figure 4).The laser measurement system was placed under the planting metering mechanism (Figure 5).Potato passed through the pipe cut laser light so a change of voltage on the LDR occurred.The change of voltage was determined with the help of a PC sound card, and the time interval between two voltage peaks was measured with the software.Standard deviation of the time interval between consecutive tubers was used as a measure of plant spacing accuracy.The coefficients of variation (CV%) of plant spacing were considered the most important measure of performance of the potato planter.CV% values are changed by skips and doubles.Therefore, to understand why CV% values are high or low, skips and doubles should be determined because skips and doubles are important indicators that determine the performance of potato planters.A miss or skip is defined as a gap equal to or greater than twice the intended center-to-center plant distance.A double was recorded when the space between adjacent seed pieces (center-to-center) was 0.07 m or less (Misener 1982;Zoraki & Acar 2000).Skips, doubles and CV% were calculated by the software.The experiment was laid out as a factorial arrangement consisting of two levels of seed potatoes cultivars, two levels of potato sizes, and three angular speed levels of vertical disc with three repetitions.For each repetition, the vertical disc was rotated 10 times.Potatoes that passed through the laser measurement system were dropped into soft plastic bowl.Only one of the vertical discs was used in this arrangement.The seed potatoes were used only once in experiment.
To determine the external mechanical damage of tubers, 40 tubers were taken randomly from within plastic bowl at the end of every repetition.Then, to make external mechanical damage assessments, the tubers were divided into four categories; 1: undamaged, 2: scuffed, skin only broken but no flesh damage or slightly damage, 3: peeler, flesh damage to a depth not 18 (2012) 126-136 131 greater than 2 mm, 4: severe, flesh damage deeper than 2 mm (Robertson 1970;McGechan 1980;McGechan 1981;Maine & Caligari 1988;Misener et al 1988;Altuntas 2001;BPC 2011).The weight of the tubers in each category was recorded.Then, external mechanical damage index was calculated based on the percentage of tubers in each category multiplied by 0, 1, 3, and 7, respectively.According to Robertson (1970), McGechan (1981) and BPC (2011) the potato external mechanical damage index was calculated as follows: Where EMDI is the external mechanical damage index, A is the weight of undamaged tubers, B is the weight of scuffed tubers, C is the weight of peeler tubers, D is the weight of severe tubers, and E is the weight of total tubers

Results and Discussion
ANOVA results belonging to the variation of coefficient of plant spacing, skips, doubles and external mechanical damage index of tuber are presented in Table 2.According to results of ANOVA, uniformity of plant spacing was significantly affected by all three experimental factors: the variety, the size and the angular speed, and size x angular speed (S x A) twoway interactions.The skips were significantly affected by the variety and the size, and size x angular speed (S x A) two-way interactions.All three experimental factors and variety x size (V x S) two-way interactions on the doubles had a significant influence.External mechanical damage index of tuber was significantly affected by all three experimental factors and variety x size x angular speed (V x S x A) three-way interactions.The relative effects of potato varieties, potato sizes and angular speeds on planter performance were summarized in Table 3.  [b]  7.50 0.000 [a]  35.43 0.000 [a]  1.01 0.000 [a]  Size (S) 1 81.40 0.000 [a]  28.70 0.000 [a]  119.96 0.000 [a]  1.73 0.000 [a]  Angular speed (A) 2 38.36 0.000 [a]  2.14 0.018 [b]  7.75 0.000 [a]  1.14 0.000 [a]    [b]  5.65 0.000 [a]  1.12 0.069 0.00 0.287 V*S*A 2 0.60 0.840 0.10 0.800 0.35 0.399 0.02 0.012 [b]  Residual 22 3.39 0.44 0.37 0.00 Total 35 [a] Indicates significance at 1% level of probability [b] Indicates significance at 5% level of probability The performance of the potato planter was significantly affected by the potato shape, potato size and angular speeds of the vertical disc.The effect of potato varieties on CV% was statistically significant (P<0.05).The highest mean value of CV% was found to be 25.58% for Marfona.The lowest mean value of CV% was found as 23.96% for Agria.There was 6.8% difference between the CV% values of Agria and Marfona.It was observed that there was a contrary correlation between shape factor and CV% as the shape factor increased; the accuracy of tuber spacing became better.It may be said that oblong tubers held by hold pins were better than round tubers, and it is possible that oblong tubers deposited in cups better than round tubers.Similarly, Buitenverf et al (2006) stated that the standard deviation of the balls with a shape factor of 100 was higher than the oblong tubers.
Potato size was found to be an important parameter affecting the uniformity of tuber spacing.The highest mean value of CV% was found to be 26.27% for small potatoes seed pieces.The lowest mean value of CV% was found to be 23.26% for big potatoes seed pieces.As the tuber size increased, the accuracy of tuber spacing increased.Possibly, it was more difficult to capture small potatoes seed pieces than to capture large potatoes seed pieces by hold pins and cups.Similarly, Misener (1982) determined in a field experiment conducted to estimate the effect of seed piece size and type on different potato planters that as the tuber size increased, the accuracy of tuber spacing increased.  a] Means in a single column without the same letter are different at the 5% level using the LSD test.
According to analysis, the accuracy of tuber spacing, CV%, was significantly affected by angular speeds of the vertical disc.The highest mean value of CV% was obtained as 26.80% for 3.22 rad s -1 .The lowest mean value of CV% was obtained as 23.42% for 1.56 rad s -1 .However, the difference between 1.56 and 2.39 rad s -1 was statistically insignificant.As the angular speed increased, the accuracy of tuber spacing became worse.These results agreed with the results reported by Altuntas (2005).In addition, some researchers stated that as the angular speed increased, the accuracy of tuber spacing became better (Buitenwerf et al 2006;Sieczka et al 1986).For the accuracy of a different potato planter in same research, Sieczka et al (1986) stated that as the angular speed increased, the accuracy of tuber spacing became worse.Essentially, what is desired from a potato planter is protection the accuracy of tuber spacing at all ground speeds.In experiment, it was observed that there was a 14.5% difference between the highest CV% value and the lowest CV% value of angular speed of the vertical disc.Planting accuracy ranged generally from 20% to 80% in many farm researches (Entz & LaCroix 1983;Sieczka et al 1986;Misener 1982).Generally, it may be said that tuber spacing accuracy of the planter was suitable at all angular speeds of the vertical disc.There was an interaction of S x A on CV% (Figure 6).It was observed that it was hard to catch both sizes of tubers in the cups because of the increasing angular speed of the vertical disc.
The effect of potato variety on skips and doubles was statistically significant (P<0.01).It was observed that there was an inverse correlation between skips and doubles.As the percentage of skips increased, the percentage of doubles decreased.

Figure 6-Plot of S X A interaction on CV% Şekil 6-S X A interaksiyonunun % CV grafiği
Probably, that was why the physical properties of Marfona were different than those of Agria.The highest mean percentage of skips was found as 3.81% for Agria.The lowest mean percentage of skips was found to be 2.90% for Marfona.The highest mean percentage of doubles was found to be 4.41% for Marfona.The lowest mean percentage of doubles was found to be 2.42% for Agria.The effect of small and large tubers on the percentage of skips and doubles was significant.The highest mean percentage of skips and doubles were found to be 4.25% and 5.24% for large potatoes seed pieces and small potato seed pieces, respectively.The lowest mean percentage of skips and doubles were found to be 2.46% and 1.59% for small potato seed pieces and large potato seed pieces, respectively.Missener (1982) & Altuntas et al (2004) reported that as the size of tubers increased, the number of skips produced by the tuber unit planter increased.In particularly, the percentage of doubles obtained from small tubers was higher than those obtained from large tubers because two or more small tubers frequently logged in one seed cup.These results were compared with those found by Missener (1982) who measured skips and doubles as having a range from 7.7% to 7.8% and from 14.1% to 23.7, respectively for <40 mm of tubers, and as having a range from 4.1 to 9.2 and from 6.6 to 16.5, respectively for > 40 mm of tubers.Skips and doubles were significantly affected by angular speeds of the vertical disc.The highest mean percentage of skips was obtained as 3.81% for 3.22 rad s -1 .The lowest mean percentage of skips was obtained as 2.80% for 2.39 rad s -1 .The highest mean percentage of doubles was obtained as 4.23% for 3.22 rad s -1 .The lowest mean percentage of doubles was obtained as 2.62% for 1.56 rad s -1 .As the angular speed of the vertical disc increased and percentage of skips and doubles increased.S x A interaction on skips and V x S interaction on doubles were significant.Plots of these interactions are showed in Figure 7a&b.It was observed that it was hard for the large tubers to be caught by the cups because of the increasing angular speed of the vertical disc.That was the reason for interaction (S x A) (Figure 7 a).The difference between the values of doubles obtained from Marfona and Agria for >45 mm of tubers was lower than those obtained from Marfona and Agria for <45 mm of tubers (Figure 7b).This effect may be explained as being due to having high sphericity of Marfona.The results of analysis indicated that the effects of the angular speeds of the vertical disc, potato varieties and potato sizes on external mechanical damage index were significant (P<0.01).The external mechanical damage index increased as the angular speed of the vertical disc increased.The highest mean value of the external mechanical damage index was obtained as 1.34 for 3.22 rad s -1 (Table 3.).The lowest mean value of the external mechanical damage index was obtained as 0.72 for 1.56 rad s -1 .The highest mean value of external mechanical damage index was obtained as 1.18 for Agria.The lowest mean value of the external mechanical damage index was obtained as 0.85 for Marfona.It was observed that Agria was more sensitive than Marfona.There was a significantly different between potato sizes on the external mechanical

Figure 3 -
Figure 3-Planting metering mechanism: 1, hold pinposition cam; 2,hold pin; 3,cup Şekil 3-Tohum dağıtma düzeni: 1, tutucu pinleri açma düzeneği; 2, tutucu çubuk; 3, kepçe Agria and Marfona which are cultivar (cv.) of seed potatoes were used in the experiments.The seed potatoes were stored in the warehouse at low temperatures for five months after harvest.Potato seeds between 25 and 45 mm created one group.Potato seeds between 45 and 65 mm created the other group.This grouping was done by considering the regulation on certification and marketing of potato seeds published by Variety Registration and Seed Certification Centre of Turkey.Some physical properties such as shape factor, sphericity and mass of tuber were determined.Fifty tubers were randomly selected and their dimensions were measured by vernier calipers with an accuracy of 0.05 mm.Shape factor (Sf) was determined as (Buitenwerf et al 2006):

Figure 7
Figure 7-a, Plots of S X A interaction on skips; b, V X S, interaction on doubles Şekil 7-a, S X A interaksiyonunun boşluk grafiği; b, V X S interaksiyonunun ikizleme grafiği