SECTION Influence of harvesting time around grain maturity on rice cracking and head rice yield in the Mekong River Delta of Vietnam SECTION 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 46 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 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 by 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 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 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 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 Rice varieties and their maturity dates used for this study Variety Crop season OM1490 Recommended MD† Wet Dry OM2718 OM4498 90-95 85-90 90 86 Wet Dry 92 92 Wet Dry 92 92 Wet Dry OM2517 87-92 Experimental MD†† 90-95 90 91 Jasmine 95-105 98 AG 24 Wet 90-95 90 IR50404 † Wet Wet 90-95 92 Recommended maturity date (days after sowing) given by local extension centre †† Maturity date (days after sowing) chosen for this study 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 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 A B C D E Treatment (Days vs MD) (-6) -6A -6B -6C -6D -6E (-4) -4A -4B -4C -4D -4E (-2) -2A -2B -2C -2D -2E (0) 0A 0B 0C 0D 0E (+2) +2A +2B +2C +2D +2E (+4) +4A +4B +4C +4D +4E (+6) +6A +6B +6C +6D +6E 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 depicts plot layout for harvesting time experiment of each rice variety Grains were harvested from 35 sub-plots of m x m (total harvesting area is 70 m2) at harvesting dates according to the treatments from days before to 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 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 were gently dried at 35 oC using a laboratory tray drier developed by Chemical Engineering 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 1.5 m in nylon bags, and transferred into lab for rice cracking and head rice yield analyses 2m 4C 5D 7E 2A 1B 3C 6D 4E 5B 1C 7D 6E 4A 2B 6C 3D 5E 5A 6B 7C 1D 2E 6A 7B 2C 4D 3E 4B 3C 2D 1E 7A 1m 3B 3A 1m 1A Figure Plot layout of harvesting experiments for each rice variety Each plot is m long and m wide and the border around harvesting area is 1.5 m 10 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 For each rice variety, level of rice cracking before husking were significantly different among harvesting dates (P0.05) † 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 12 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 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 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 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 is similar to the maturity time shown in Table 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 13 rather small cracking problem but delay of 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 Change in head rice yield of seven rice varieties at different harvesting time (days after expected maturity date) Variety Crop Head rice yield (%) before and after maturity day season -6 -4 -2 +2 +4 +6 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 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 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 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 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 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 OM1490 0M2718 OM2517 OM4498 AG24 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 14 Table Seasonal trend of effect of harvesting time before and after maturity (4-6 days prior and 4-6 days later than the expected day of maturity) on the proportion of cracked grains (prior to milling) and head rice recovery Head rice yield is expressed as relative to the yield on maturity day Crop Rice season variety Proportion of cracked grain % Before maturity After maturity Relative head rice yield % Before maturity After maturity Optimum harvesting date 0.8-9.6 1.1-23.6 101-109 72-88 94 0.4-1.2 4.0-10.8 103-117 84-93 92 OM2517 3.5-15.7 12.1-20.3 90-114 105-117 94 OM4498 2.5-3.9 8.1-10.4 91-93 96-108 94 AG24 0.3-1.5 1.1-4.1 93-97 83-108 94 IR50404 1.1-1.5 0.4-1.3 103-105 99-106 90 Jasmine 4.0-4.5 6.0-7.7 75-99 87-99 98 OM1490 0.5-2.3 5.6-22.4 93-99 83-95 92 OM2718 0.7-6.3 3.2-8.5 98-101 92-98 92 OM2517 0.7-3.6 9.3-60.5 77-106 51-97 86 OM4498 1.1-3.7 1.1-9.3 75-93 90-98 91 AG24 6.5-16.4 21.5-53.1 133-145 86-102 88 IR50404 Dry OM1490 OM2718 Wet 0.8-2.8 1.7-12.3 105-107 86-95 88 CONCLUSIONS A few days early harvesting (before maturity) is better than late harvesting by to days because late harvesting will make the grain more sensitive to cracking Therefore, any delay or longer harvesting time can cause more losses, as is often the case of harvesting by hand The degree of harvesting time effect is also dependent on the variety REFERENCES Bal, S., & Oiha, T P., 1975 Determination of biological maturity and effect of harvesting and drying conditions on milling quality of paddy Journal Agricultural Engineering Resource, 20, 353-361 Berrio, L E., & Cuevas-Perez, F E., 1989 Cultivar differences in milling yields under delayed harvesting of rice Crop Science, 24, 1510-1512 15 Calderwood, D L., Bollich, C N., & Scott, J E., 1980 Field drying of rough rice: Effect on grain yield, milling quality energy saved Agronomy Journal, 72, 644-653 Chae, J C., & Jun, D K., 2002 Effect of harvesting date on yield and quality of rice Korean J Crop Sci., 47(3), 254-258 Champagne, E T., Bett-Garbet, K L., Thompson, J., Mutters, R., Grimm, C C., & McClung, A M., 2005 Effects of Drain and Harvest Dates on Rice Sensory and Physicochemical Properties Cereal Chemistry, 82(4), 369-274 Chau, N N., & Kunze, O R., 1982 Moisture content variation among harvested rice grains Transactions of the ASAE, 25(4), 1037-1040 Kester, E B., Lukens, H C., Ferrel, R E M., A., & FIinfrock, D C., 1963 Influences of maturity on properties of western rice Cereal Chemistry, 40, 323-326 Kunze, O R., & Prasad, S., 1978 Grain fissuring potentials in harvesting and drying of rice Transactions of the ASAE, 21(2), 361-366 Ntanos, D., Philippou, N., & Hadjisavva-Zinoviadi, S., 1996 Effect of rice harvest on milling yield and grain breakage CIHEAM-Options Mediterraneennes, 15(1), 23-28 Swamy, Y M I., & Bhattacharya, K R., 1980 Breakage of rice during milling- Effect of kernel defects and grain dimension Journal of Food Process Engineering, 3, 29-42 16 .. .SECTION 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... 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... 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