Section 1:Influence of harvesting time around grain maturity on rice cracking and head rice yield in the Mekong River Delta of Vietnam " pdf

5 SECTION 1 Influence of harvesting time around grain maturity on rice cracking and head rice yield in the Mekong River Delta of Vietnam... Influence of harvesting time around grain ma

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SECTION 1

Influence of harvesting time around grain maturity on rice cracking and head

rice yield in the Mekong River Delta of Vietnam

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SECTION 1 Influence of harvesting time around grain maturity on rice

cracking and head rice yield in the Mekong River Delta of Vietnam

ABSTRACT

Timely harvesting plays an important role in controlling rice cracking Reduced whole rice grain yield due to cracking causes the value loss and reduces the farmers’ income Field experiments were carried out to study the effect of harvesting time around crop maturity on rice cracking and head rice yield for seven common rice varieties (OM1490, OM2718, OM2517, OM4498, AG24, IR50404 and Jasmine) in three different locations during two cropping years (2006-2008) in the Mekong River Delta, Vietnam The results showed that the rice cracking was strongly influenced

by both the variety and time of harvesting around maturity There was a general trend of increase

in percentage of cracked rice with late harvesting in relation to estimated grain maturity date The head rice yield also followed the same trend in response to delayed harvesting A delay of

4-6 days reduced the head rice yield by 11.3 % an average and up to 50 % Similar trends were observed in both wet and dry seasons The large varietal difference in percentage of cracked grain (0.9 to 60.5%) on 6 days after maturity date indicated that the level of rice cracking caused

by late harvesting time can be minimized by the selection of suitable varieties

INTRODUCTION

Head rice yield, which is defined as the weight percentage of rough rice that remains as head rice (the kernels that are at least ¾ of the original kernel length) after milling, is considered as the main quality indicator because the broken rice has often half the commercial value of whole grain rice It has been shown that timeliness of harvesting can influence milling yield

significantly Harvesting rice at crop maturity can give a maximum head rice yield (Kester et al

1963, Bal and Oiha 1975) Any delay in harvesting time causes reduction of head rice yield (Bal

and Oiha 1975, Ntanos et al 1996, Berrio et al 1989) and extended delay in harvesting can lead

to significant losses in head rice yield Berrio et al (1989) showed that among 16 investigated

rice varieties studied the whole-milled grain was reduced by 18% when harvesting was delayed

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by 2 weeks However, it was also found that there was no impact of harvesting time on sensory

perception of rice (Champagne et al 2005, Chae and Jun 2002)

The incidence of rice fissuring in the field has a potentially significant impact on head rice yield Cracking can develop in the field as a result of changes in grain moisture after the rice matures due to hot sunny days followed by humid nights Harvesting time affects proportion of cracked rice and hence head rice yield Large quantities of immature rice kernels can be detected in early harvested rice Immature kernels are usually thinner and defective, and are easily cracked during subsequent milling (Swamy and Bhattacharya 1980) In contrast, late harvested grain is often associated with a grain product that is too dry and more prone to fissuring Investigations by Chau and Kunze (1982) showed that cracking can develop in low-moisture content kernels (13%

or 14% wet basis) before harvesting as a consequence of the swings in relative humidity in the atmosphere Furthermore, improper post-harvest practices, such as a delay in threshing when rice stacks are left in the field, can also provide the potential of moisture adsorption due to an uneven moisture content and uneven maturity within the bulk rice (Kunze and Prasad 1978)

Reduced whole rice grain yield due to cracking is one of the major issues that directly reduce income and availability of staple food to the farmers in the Mekong River Delta of Vietnam Mekong River Delta is the largest rice production region in Vietnam The cracking or partial fissuring of rice kernels may occur right in the paddy field due to incorrect harvesting time and improper harvesting practices, and occur also due to adverse post-harvest drying conditions and inappropriate milling operations The weather pattern (temperature and humidity) in Mekong River Delta is unique The rice is grown and harvested in both wet and dry seasons Weather conditions at around harvesting period are different between the two seasons and this can impact the rice fissuring and cracking during milling However, there is no experimental data available

on the impact of harvesting time on rice cracking and head rice recovery on the rice varieties grown at different seasons in the Mekong River Delta This research work is an attempt to systematically collect the rice cracking and head rice yield data based on field experimentations

in four consecutive harvesting seasons between 2006 and 2008 The main factor considered in this study during the collection of data was harvesting time- before and after grain maturity The

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objective of this experiment was to evaluate the effects of harvesting time of several rice varieties on the level of rice cracking and head rice yield in different seasons This study will assist to determine the optimal harvesting time for various rice varieties grown in the Mekong River Delta

MATERIALS AND METHODS

Rice samples

Experiments were carried out at three locations, namely Seed Centre (An Giang Province), Tan Phat A Cooperative (Kien Giang Province) and Tan Thoi 1 Cooperative (Can Tho City) in four consecutive harvesting seasons during two years (2006-2008) Seven rice varieties commonly cultivated in these cooperatives and seed centre were selected for field experiments as shown in Table 1 The grain maturity date of these rice varieties provided by the local extension centers were in the range of 86-98 days (Table 1) The maturity date is defined here as the harvesting date expressed in days after sowing (DAS) planned by the farmer as recommended by the extension centre based on the predicted physiological maturity of the grain

Table 1 Rice varieties and their maturity dates used for this study

Variety Crop season Recommended MD † Experimental MD ††

OM1490

OM2718

OM2517

OM4498

†

Recommended maturity date (days after sowing) given by local extension centre

††

Maturity date (days after sowing) chosen for this study

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Experimental design

Each experiment consisted of seven treatments corresponding to the harvesting times prior and

after expected maturity date for each of seven varieties These varieties were grown in different

paddies at the three locations There were seven harvesting times, two days apart commencing

six days prior to maturity date (MD) and ending at six days after MD Experiments were

designed in RCBD (Random Complete Block Design) method with five blocks (Table 2)

Table 2 Treatments (harvesting days) in relation to the maturity date (MD) 0, +2, +4, +6 and -2, -4, -6 are

harvesting days after and before estimated maturity, respectively A, B, C, D, E are the replication blocks

Block Treatment

(Days vs MD)

A B C D E

Experimental procedure

Some rice fields of selected varieties were chosen and used for the experiment Wet season

experiments were sown in March-April and harvested in June-July (in 2006 some varieties were

grown in late wet season and harvested in September), while sowing and harvesting for dry

season experiments were November-December and February-March, respectively Figure 1

depicts plot layout for harvesting time experiment of each rice variety Grains were harvested

the treatments from 6 days before to 6 days after maturity date (MD) with five replications for

each treatment (Figure 1) Cutting and threshing operations were done manually using a sickle

for cutting the rice stalk Rice was harvested in the morning to avoid intense sun light, aiming to

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reduce natural cracking due to sudden change of moisture distribution inside the kernel when it goes between wet night and dry day After cutting, rice was transferred into a shaded area for manual threshing and cleaning in which most bulk straw, chaff, immature grain, and very light and fine impurities were separated from the grains The straw and chaff were manually separated and the grain was dropped though a cross-wind to remove the lighter impurities

Samples were then transferred to the dryer after undertaking moisture determination Samples

Department, Nong Lam University Ho Chi Minh City, Vietnam until the moisture content reached 14 % wet basis Then samples were once again cleaned to remove residual immature grains, measured for moisture content using grain moisture tester (Kett Co Ltd., Japan), packed

in nylon bags, and transferred into lab for rice cracking and head rice yield analyses

1A 3B 4C 5D 7E 2A 1B 3C 6D 4E 3A 5B 1C 7D 6E 4A 2B 6C 3D 5E 5A 6B 7C 1D 2E 6A 7B 2C 4D 3E

Figure 1 Plot layout of harvesting experiments for each rice variety

Each plot is 2 m long and 1 m wide and the border around harvesting area is 1.5 m

2 m

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Measurements

Cracking of paddy rice

This is the direct indicator for effects of harvesting time on cracking Three 150 g paddy samples were taken from each plot to ensure the repetition of each plot Grains were dehulled by hand to avoid cracking developing during this procedure Fifty dehulled grains were randomly checked

to count cracking grains under microscope, and cracking fraction calculated

Head rice yield

Exact 180g of paddy was dehusked and then 100g of brown rice was whitened for 60 seconds using a laboratory milling system Whole kernels were separated by grader from broken kernels,

to determine head rice yield which is defined as the ratio of the mass of unbroken kernel to the total mass of paddy rice The head rice is composed of grains which maintain at least 75% of their length after milling

Statistical analysis

Data were analysed by statistical software Statgraphics® 3.0 (StatPoint, Inc.) using ANOVA (Analysis of Variance) procedure

RESULTS AND DISCUSSION

Level of rice cracking

Percentages of cracked grain before husking obtained from seven varieties in four consecutive crop seasons, i.e., wet season 2006, dry and wet season 2007, and dry season 2008, are shown in Table 3 For each rice variety, level of rice cracking before husking were significantly different among harvesting dates (P<0.05) Early harvesting (before maturity date) showed lesser proportion of cracked grains There was a general trend of increase in the percentage of cracked rice corresponding to delayed harvesting day in relation to the time of grain maturity (day 0) For example, the highest level of rice cracking for all rice varieties was recorded at sixth day after maturity date, which was the last harvesting date in this experiment (Table 3) This indicates how important it is to harvest the paddy at correct time in its maturity period The result of this

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research work on Vietnamese rice varieties is in agreement with previous studies showing negative impact of delayed harvesting to rice quality with respect to level of rice cracking

(Ntanos et al 1996, Berrio et al 1989) Any over-drying in the field (or in the panicle) can result

in increased number of cracked grains

Table 3 Degree of rice cracking of seven rice varieties before and after maturity dates during two crop years

Cracked grain (%) before and after maturity day Variety Crop

Wet ‘06 0.80a 3.20a 9.60bc 4.80ab 10.80bc 15.20c 23.60d

OM1490

Dry ‘07 1.87a 0.53a 2.27a 2.80a 5.60a 14.40b 22.40c

Wet ‘07 2.00a 2.13a 2.27a 1.07a 1.33a 2.13a 2.40a

Wet ‘06 0.40a 0.40a 1.20a 2.80a 10.80b 4.00a 5.20ab

0M2718

Dry ‘07 2.40a 0.67a 6.27b 2.00a 3.20a 7.20b 8.53b

Dry ‘07 1.47a 2.00a 3.60a 5.73a 16.00b 33.60c 60.53d

Wet ‘07 3.47a 10.27b 15.73bc 18.67c 12.13b 12.67b 20.27c

OM2517

Dry ‘08 0.67a 1.73a 3.33a 8.13b 9.33b 14.13c 25.73d

Dry ‘07 3.73a 1.07a 1.47a 1.47a 1.07a 2.93a 9.33b

OM4498

Wet ‘07 2.53a 3.73ab 3.87ab 4.67ab 8.93b 10.40c 8.13ab

Wet ‘06 † 1.33a 0.13a 1.60a 0.53a 1.33a 5.47b 5.47b

AG24

Dry ‘08 6.50a 18.17bc 16.44bc 17.67ab 21.47bc 32.40c 53.07d

Wet ‘07 1.47b 1.60b 1.07b 0.67a 0.93ab 0.4a 1.33b

IR50404

Dry ‘08 0.80a 1.47a 2.80a 1.07a 1.73a 1.60a 12.27b

Jasmine Wet ‘06 † 4.00a 3.90a 5.18ab 5.14ab 6.00ab 8.66c 7.60bc

All data represent mean values of five replications The same superscripts in the same row indicates that the values are not significantly different (P>0.05)

†

harvested in ‘late wet season’ which was in September 2006

Increased rice cracking due to delayed harvesting also depended on the variety There was a large amount of cracked grains after maturity date for OM2517 (16.00-60.53%) and AG24 (21.47-53.07%) in dry season 2007 and dry season 2008, respectively In contrast, percentages of cracked grain of IR50404, OM2718, and OM4498 varieties had lower values in both wet and dry seasons (in the range of 0.4-12.27%, 3.20-10.80% and 1.07-10.40%, respectively) after maturity date This implied that there is a varietal difference on rice cracking and hence the selection of variety is important in decreasing cracked grain percentage

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The cracking behavior of the rice in the field is expected to depend on the season due to the different patterns of temperature fluctuation during day and night, solar radiation intensity, sunshine hours and frequency of rain During the rainy season, the rice grain can develop cracks during the late maturity stage due to rewetting At the same time, during dry season it is likely that the grains over-dry if not harvested by its maturity However, data on Table 3 obtained from four consecutive crop seasons (wet 2006, dry and wet 2007, and dry 2008) showed that crop seasons did not have much impact on level of rice cracking as similar trend was observed in both wet and dry seasons

Head rice yield

The head rice yield as a function of harvesting time for seven rice varieties is presented in Table

4 The head rice yield was generally less at late harvesting time A delay of 4-6 days reduced the head rice recovery by up to 50% of the head rice yield at the expected maturity The head rice yield followed the opposite trend to rice grain cracking, indicating that the presence of cracks in the original paddy reduced the head rice recovery

The overall results as influenced by harvesting time are presented in Table 5 It should be noted that the head rice yield is affected by a laboratory milling system, as it is a function of milling efficiency Therefore, the head rice yield data is presented in relative term in Table 5, with the recovery on the harvesting at maturity (0 day) being assigned a value of 100% In addition, due

to the limited number of experiments undertaken, the values are presented as a range for each variety

In general, the optimum harvesting time presented in Table 5 is similar to the maturity time shown in Table 1 for all varieties used in this investigation Suggested optimum harvesting times

in wet season for OM1490 (94 days) and OM2517 (94 days) are 2-4 days longer than recommended maturity day by local extension centre It can be concluded that (1) even if the rice varieties were harvested about the right time, varieties differ considerably in the cracking and hence intervention opportunity of growing low cracking varieties such as OM2718 for farmers and developing such varieties for rice breeders, (2) harvesting at optimum harvest time had

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rather small cracking problem but delay of 6 days can cause major problem, and hence intervention opportunity here to ensure harvesting at the right time, and (3) varieties differ in their response to time of harvesting hence time of harvesting is more critical for some varieties than others, for example OM2517 was the most sensitive variety, and hence there is an opportunity for intervention to ensure quick harvesting of particular varieties

Table 4 Change in head rice yield of seven rice varieties at different harvesting time (days after expected maturity date)

Head rice yield (%) before and after maturity day Variety Crop

Wet ‘06 51.06cd 52.30d 50.73cd 48.08c 42.23b 36.51a 34.53a

Dry ‘07 63.13bc 66.21c 66.93c 67.90c 64.57bc 60.25ab 56.35a

OM1490

Wet ‘07 50.03a 45.10a 52.15a 45.56a 49.81a 49.26a 49.01a

Wet ‘06 45.41c 51.47d 43.54bc 43.91bc 38.76ab 36.83a 40.72abc

0M2718

Dry ‘07 67.93b 67.01b 66.40b 67.48b 66.22b 63.81a 62.41a

Dry ‘07 64.58d 41.09b 45.19b 56.68c 53.18c 43.74b 28.63a

Wet ‘07 48.01c 44.16bc 37.88a 42.19ab 44.47bc 49.24c 44.34bc

OM2517

Dry ‘08 65.68c 65.36c 64.67c 59.84c 60.55b 55.29a 52.90a

Dry ‘07 43.80a 54.35bc 54.02bc 58.33d 56.95cd 53.78bc 52.55b

OM4498

Wet ’07 36.64a 37.77a 35.83a 39.35ab 37.87ab 42.42b 35.35a

Wet ‘06 † 40.35b 42.35bc 40.76b 43.50bcd 46.99d 35.90a 35.35a

AG24

Dry ‘08 61.66c 55.42bc 52.38b 42.62a 43.55a 36.48a 37.94a

Wet ‘07 58.08c 56.94b 57.79c 53.27a 56.54bc 55.67abc 54.55ab

IR50404 Dry ‘08 64.28de 61.75cd 64.57e 60.28c 57.40b 56.99b 51.68a

Jasmine Wet ‘06 † 41.59a 54.65c 51.82bc 55.36c 54.59bc 48.15b 49.46bc All data represent mean values of five replications The same superscripts in the same row indicates that the values are not significantly different (P>0.05)

†

harvested in ‘late wet season’ which was in September 2006