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a Univ. of Hawaii, 1955 East West Road, Agric. Sci. 205, Honolulu, Hawaii 96822, and Int. Rice Res. Inst. (IRRI), DAPO Box 7777, Metro Manila, Philippines
b IRRI, DAPO Box 7777, Metro Manila, Philippines
c ICRAF, United Nations Ave., P.O. Box 30677, Nairobi, Kenya
* Corresponding author (tgeorg1{at}attglobal.net)
Received for publication December 3, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: HI, harvest index • NERICA, New Rice for Africa
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Upland rice is commonly perceived to be a low-yielding crop grown for subsistence by resource-poor farmers in the highlands. In Asia, upland rice yields average only about 1 Mg ha-1 vs. 4.9 Mg ha-1 for irrigated lowland rice (IRRI, 1997). Because of its low yield and association with the infertile uplands, upland rice is generally considered to be unsuitable for management aimed at high yields. The low yield of upland rice, however, is largely a consequence of its production being limited to low harvest index (HI, ratio of grain to total biomass) varieties (George et al., 2001) and to infertile or drought-prone uplands.
Indications are that upland rice can also be high yielding if the genotype is improved for yield and the crop is not subject to nutrient and drought stresses. With improved upland rice, yields approaching 7 Mg ha-1 can be obtained, even in highly acidic upland soils, given adequate amounts of lime, N, and P (George et al., 2001). In the highly acidic soil of the 240-million-ha Cerrado region in Brazil, upland rice is a commercial crop in rotational systems (Guimarães and Yokoyama, 1998), producing about 5 Mg ha-1 with fertilizers and irrigation (Stone et al., 1997). In China's Huang-Huai-Hai River plains, with no soil acidity constraints, yields approaching 7 Mg ha-1 have been observed for improved upland rice lines on irrigated dryland (Huaqi Wang, personal communication, 2000).
While high rice yields may be possible in aerobic soil, there are also indications that they might not be sustained even with fertilization. A few reports from the 1970s indicate a rapid and sharp decline in yield when upland rice was monocropped (IRRI, 1976, 1977, 1978; Sanchez, 1983; Ventura and Watanabe, 1978; see also Gupta and O'Toole, 1986). Initial yields in these reported cases ranged from only about 1 to 4 Mg ha-1, but yields declined in succeeding rice crops in all cases regardless of fertilization. It is not known whether such a yield decline is inevitable in upland rice.
We have been examining the prospects of high-yielding rice in aerobic soil with the goal of developing a highly productive aerobic soil culture of rice. In experiments carried out in the Philippines, we assessed the limits to yield of rice genotypes in aerobic soil. In this paper, our objective is to examine whether high and stable rice yields could be obtained and maintained in aerobic soil under relatively nutrient-rich and drought-free growing conditions.
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MATERIALS AND METHODS |
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Treatments, Experimental Designs, and Field Management
Claveria Experiments
An essentially common objective for all four experiments (Table 3) was improving the productivity of upland rice–based cropping systems by increasing the yields of component crops of upland rice, maize, and legumes through nutrient management and sequence cropping. Nutrient treatments in the four experiments were select combinations of lime, N, and P and were assigned in a randomized complete block design with four replications. Upland rice variety UPLRi-5 was used in Exp. 1 to 3 and variety ‘IR30716’ (about 22 d earlier maturing than UPLRi-5) in Exp. 4. Maize following UPLRi-5 in Exp. 2 and 3 grew poorly and failed frequently because of late seeding after the harvest of UPLRi-5.
After tilling and harrowing twice, lime as CaCO3 was broadcast as designated and incorporated to 15-cm depth 2 wk before seeding. Furrows were spaced 25 cm apart in Exp. 1 to 3 and 30 cm apart in Exp. 4. Designated amounts of P in Exp. 1 to 3 and 11 kg P ha-1 in Exp. 4 as single superphosphate were applied into furrows before seeding. Seeds were drilled into furrows at 100 kg ha-1 in Exp. 1 and 4 and at 75 kg ha-1 in Exp. 2 and 3 and then covered with soil. Treatment amounts of N in Exp. 1 and 4 and 50 kg N ha-1 in Exp. 2 and 3 were applied as urea in two splits, half at 14 d after emergence and the other half at 1 wk before panicle initiation. Weeds were controlled by interrow cultivation and hand weeding and insects by furrow application of furadan (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate) and seed treatment against root aphids.
Siniloan Experiments
A rice monocrop experiment with two crops per year began in the 1998 dry season. The main objective was to test the limits of rice yield in irrigated and fertilized aerobic soil under successive cropping with varieties and plant population densities as treatments. For lack of a proven set of high-yielding upland rice varieties, a reference variety, Apo (IR55423-01), and a changing set of other varieties were used. The varieties were chosen based on yield information available on them before each season. Apo was used as the reference because of its good performance observed previously (B. Courtois, personal communication, 1997; George et al., 2001). Three or four varieties and three plant populations were treatments during the first four seasons. The varieties were Apo, Lubang Red (a traditional Philippine upland japonica), ‘IR72’ (an improved lowland indica), and Magat (IR64616H, a lowland hybrid rice, S.S. Virmani, IRRI, Philippines) for the 1998 dry season; Apo, Magat, Brazilian upland rice varieties ‘Primavera’ and ‘Maravilha’ (E.P. Guimarães, EMBRAPA Arroz e Feijão, Goiás, Brazil) for the 1998 wet season; Apo, ‘Mestizo’ (IR68284-H, lowland hybrid rice), and Primavera for the 1999 dry season; and Apo, Mestizo, and Magat for the 1999 wet season. Plant populations were 120, 240, and 480 plants m-2 during both seasons in 1998 and 60, 120, and 240 plants m-2 during both seasons in 1999. After a break from rice with a maize–cowpea [Vigna unguiculata (L.) Walp.] intercrop in the 2000 dry season, Apo and six other varieties [Magat, ‘Canastra’ (B. da Silveira Pinheiro, EMBRAPA Arroz e Feijão, Goiás, Brazil), UPLRi-5, and West Africa Rice Development Association's (WARDA) NERICA (New Rice for Africa) lines ‘WAB 450-11-1-P31-1-HB’, ‘WAB 450-24-3-4-P48-3-1’, and ‘WAB 450-I-B-P-38-HB’ (M. Jones, WARDA, Côte d'Ivoire)] were grown at a target population of 320 plants m-2 in the 2000 wet season. Because no seeding density effect on grain yield or total biomass was found, this treatment was not included in the fifth rice crop. The experiment was in a randomized complete block design with four replications each in the 1998 dry and wet seasons and three replications each in the 1999 dry season through 2000 wet season in plots of 34 m2.
An additional experiment was conducted in the 2000 wet season simultaneously with the monocrop experiment in new plots, hereinafter referred to as first-season rice plots, on adjacent land that had not been grown to rice since 1998. In this experiment, the same seven varieties and population densities as in the monocropped rice plots were grown in 16-m2 plots replicated four times in a randomized complete block design.
The field was tilled to 30-cm depth with a tractor. Bunded plots were then constructed and soil rototilled to 15 cm. Inputs except N were incorporated to 15-cm depth before seeding. Lime as CaCO3 was applied at 3 Mg ha-1 to the first crop in 1998 and again at 1.5 Mg ha-1 to the 2000 wet-season crop. The area used for the 2000 new experiment was limed at 1.5 Mg ha-1 a year earlier for soybean [Glycine max (L.) Merr.] and cowpea crops. Fertilizers except N applied to each crop were P, 100 kg ha-1 as single superphosphate; K, 100 as KCl; Mg, 50 as MgSO4; and Zn, 25 as ZnSO4. Nitrogen management was to maintain N sufficiency (SPAD reading of approximately 35; S. Peng, personal communication, 1998) by applying 10 to 20 kg N ha-1 as urea weekly until flowering. Nitrogen applied was 100 kg ha-1 in both the 1998 dry and wet seasons, 110 in both the 1999 dry and wet seasons, and 150 in the 2000 wet season. Target plant populations were achieved by ensuring that three plants grew every 2.5, 5, 10, or 20 cm in the rows in 1998 and 1999 and two plants every 2.5 cm in 2000. This was achieved by slightly overseeding and then thinning seedlings as necessary within 10 d of emergence.
All crops were irrigated as needed to maintain volumetric soil water content within approximately ±10% of field capacity (48% v/v) in the top 15 cm based on tensiometer readings. The plots were kept weed-free by hoeing and hand weeding. Furadan granules were always applied at seeding, and insect pests were controlled as needed. Except for bacterial sheath brown rot (Pseudomonas fuscovaginae Tanii, Miyajima & Akita; Cottyn et al., 2002), which affected the 1999 wet-season crop, there were no diseases. During this season, the crop grew normally until disease appeared during postflowering, resulting in almost no grain filling. Following the fourth crop, a maize–cowpea intercrop was grown in the 2000 dry season to break the continuous cycle of rice. The maize–cowpea biomass was slashed after 2.5 mo and burned within plots to prepare for the 2000 wet-season crop.
Sampling and Measurements
Claveria Experiments
Plant height from soil level to plant tip at maturity was determined on 10 random hills. The number of panicle-bearing tillers was counted from 2-m lengths of rows. In all experiments, rice plants from an area of 6 m2 were cut at the soil surface from each plot. Threshed grains were cleaned, sun-dried, and weighed, and moisture content was measured. Grain yield was expressed at 14% moisture content.
Siniloan Experiments
Shoot biomass was determined at 3- to 4-week intervals by cutting plants at soil level from eight 0.2-m row lengths and oven-drying at 65°C until constant weight. At harvest, plants were cut from a 15-m2 bulk-sampling area in all crops, except for Lubang Red, which was cut from a 9-m2 area in the 1998 dry season. Fresh weights were determined on the bulk and subsamples, which were then oven-dried. Biomass and grain yields were calculated from the plot bulk samples using percentage dry weight and HI determined on the subsamples. In the 1999 wet season, only the total biomass was measured at final harvest because a majority of grains were unfilled due to bacterial sheath brown rot. In the 2000 first-season rice plots, only one shoot sampling was made at 13 wk after seeding as continuing this crop to maturity was impossible because of lodging and crop damage from heavy rains, typhoons (Reming and Seniang, 25 Oct. to 5 Nov. 2000; Toyang, 27 Nov. to 3 Dec. 2000), and flash flooding. For the same reason, the grain yield measurements made in the monocropped rice plots in the 2000 wet season were also deemed unreliable and not presented.
Data Analyses
Data underwent analyses of variance and mean separation procedures as appropriate. Data from Claveria Exp. 1 were analyzed separately for each field because of heterogeneity of variances. The 3-yr data from Claveria Exp. 2 and 3 underwent a repeated-measures data analysis by using a combined analysis of variance across years. For Exp. 4, data only from treatments of 0 or 50 kg N ha-1 applied to both rice and maize were analyzed. In Siniloan, planting density did not influence yields; hence, mean values are presented. In the 1999 dry and wet seasons, the 10-wk biomass was estimated by linear interpolation using biomass at 65 and 78 d in the former and 61 and 86 d in the latter. Also in Siniloan, biomass data from the monocropped rice and the first-season rice plots in the 2000 wet season underwent a combined analysis of variance.
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RESULTS AND DISCUSSION |
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Given that (i) yield responses to fertilization were not consistent across fields and significant only in a few cases; (ii) crops were seeded simultaneously and managed similarly; and (iii) yield-reducing pest or disease outbreaks were absent, there appears to be another source of strong variability. Both Plaridel and Cabacungan 1 were locations receiving lighter rains early in the season, whereas Compact 1 and Ane-i received heavier rains. For example, the weekly rainfall in Cabacungan was only 45 to 63% of that in Compact during 10 out of 11 wk from 1 May to mid-July 1989 (Fig. 1) . Thus, it seems that the differential early-season rainfall would have differentially influenced the early growth of rice seeded simultaneously in the different fields. Yields as high as 7 Mg ha-1 could have been expressed only when adequate nutrient supplies coincided with relatively drought-free conditions throughout (George et al., 2001).
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When the UPLRi-5 performance across fields and years is taken together, four main observations can be made: (i) in four of a total of 10 crops, UPLRi-5 yielded more than 4 Mg ha-1 grain in one or more input treatments; (ii) only in the first season of cropping on fields with no immediate history of upland rice were high biomass in all crops and high grain yield in all except one crop obtained; (iii) in the three fields where rice was grown for 2 or 3 yr, yield loss was rapid and drastic in successive years despite relatively favorable soil water and nutrient conditions; and (iv) reduced grain yields in successive years were associated with reduced plant height or tiller production, indicating that yield loss resulted from depressed biomass production. An emerging conjecture is that UPLRi-5 has the potential for high yield, the expression of which is associated more often than not with the first season of cropping, and that yields decline rapidly afterward, even under favorable growing conditions.
First-Season Yield of Lowland and Upland Rice in Siniloan, an Irrigated Acid Upland
Grain yield, biomass, and HI of lowland and upland rice varieties grown in Siniloan, Laguna, in the 1998 dry season are presented in Table 8. High biomass in all varieties and high grain yield in some varieties were achieved. The lowland hybrid rice Magat yielded 7.8 Mg ha-1 vs. only 2.1 Mg ha-1 for the upland traditional rice variety Lubang Red. The improved upland variety Apo and the lowland variety IR72 yielded similarly, about 5 Mg ha-1. The 7.8 Mg ha-1 average yield of Magat in aerobic soil in the dry season is comparable to its reported yield of about 7.2 Mg ha-1 in paddy culture in the dry season (IRRI-IRIS database, http://www.iris.irri.org/; verified 20 May 2002).
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Performance of Successively Cropped Rice in Siniloan
Table 9 presents the biomass and grain yield of the reference variety Apo grown in five seasons, three other varieties grown in two seasons, and UPLRi-5 grown in the fifth season. The 10-wk biomass declined in all four varieties that were grown in the second and subsequent seasons. The 10-wk biomass of Apo in the fourth crop in the 1999 wet season was only one-fourth of its first-season biomass of 6.9 Mg ha-1 in the 1998 dry season. This decline in 10-wk biomass of Apo with successive cropping occurred through both dry and wet seasons. Grain yield likewise declined from 5.3 Mg ha-1 in the first season to 3.4 Mg ha-1 in the third season. In the fourth season, grain yield was not harvested, but the total harvest biomass of Apo was just 3.4 Mg ha-1 (data not shown), indicating that grain yield would have been substantially lower than in the third crop even if grain filling was normal. A similar loss in crop performance was observed for Magat grown in the first and fourth seasons, Primavera grown in the second and third seasons, and Mestizo grown in the third and fourth seasons. Similar to Apo, the low harvest biomass of Magat and Mestizo (3.1 Mg ha-1 on average, data not shown) in the fourth season could have only resulted in low grain yield. It is unclear why the grain yield of Primavera in the first crop was not higher than in the second crop despite its 54% higher 10-wk biomass in the first crop. In any case, the overall trend revealed is one of a rapid loss in crop performance with successive cropping regardless of variety, an observation similar to that of UPLRi-5 grown in favorable rainfed conditions in Patrocenio and Cabacungan 2 and IR30716 in Compact (Tables 6 and 7). Further, UPLRi-5 also performed poorly when grown in the fifth season in the monocropped rice plots in Siniloan (Tables 9 and 10) although it was fully fertilized and irrigated unlike in Claveria.
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Rapid Yield Loss in Aerobic Soil, a Phenomenon Unique to Rice
Results from both favorable rainfed Claveria and irrigated Siniloan fields provide evidence for the existence of a rapid yield loss in rice when it is successively cropped in aerobic soil. The observations supporting this phenomenon include:
1. In two separate, favorable rainfed fields in Claveria, the first-season high grain yield or biomass of UPLRi-5 declined drastically in the second to third seasons. This yield loss was linked to reductions in tiller production and plant height.
2. In an upland rice–maize rotation in a favorable rainfed Claveria field, a drastic yield loss was observed in rice but not in maize, both fertilized the same.
3. The biomass and grain yield of four rice varieties were always lower in their second season in the irrigated and fully fertilized Siniloan fields. The only exception was the grain yield of Primavera, whose yield was already low in its first crop.
4. Despite abundant supplies of nutrients and water and the control of weeds and soil pests, the continuous cropping of the superior upland rice variety Apo resulted in a continuing and rapid loss in its performance.
5. Despite highly favorable growing conditions, the high-yielding UPLRi-5 grew poorly when grown for the first time in the fifth season in the Siniloan monocropped plots.
6. The 13-wk biomass of all seven high-yielding upland rice varieties, including UPLRi-5, Apo, and NERICA varieties, was twice as much when grown in the 2000 first-season rice plots compared with the fifth crop in the monocropped rice plots.
Although not fully documented, support for our finding of the rapid yield loss comes from the few reports from the 1970s that demonstrated drastic reductions in yield with monocropping of upland rice (IRRI, 1976, 1977, 1978, 1983; Ventura and Watanabe, 1978). The IRRI reports showed the first-season grain yields of about 1 to 3 Mg ha-1 on IRRI Alfisol sharply declining to about 
0.5 Mg ha-1 in three or more crops on irrigated dryland. Sanchez (1983) illustrated rapid yield reduction in monocropped upland rice on Peruvian Ultisols from about 3 to 4 Mg ha-1 to about 1 Mg ha-1 with fertilization and from about 1.5 to below 0.5 Mg ha-1 without fertilization. Likewise, Evenson et al. (1995) found that the first-season upland rice yield of about 2 Mg ha-1 decreased to a negligible amount in just three seasons of cropping, and additional application of lime, N, P, K, and Mg in the fourth season did not increase yield on an Oxisol in Sumatra, Indonesia. They also found that a yield decline in cowpea was reversed when fertilized. In all of the above reports, first-season yields were much lower than the high yields in our study and could not have exhausted soil nutrients rapidly, yet yields declined quickly. Further, it was also found that the nutrient status of the monocropped soil did not differ much from soil not cropped to upland rice (IRRI, 1978). It seems probable that the rapid yield loss is unique to rice grown in aerobic soil and is unlike the gradual yield decline traditionally observed in other upland cereal crops (Dalal et al., 1991) and flooded rice (Cassman et al., 1995).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Management options must also be developed to overcome the problem of the rapid yield loss. To this end, the cause(s) for the rapid yield loss must first be identified by assessment of factors such as soil nutrients and water, diseases and pests, and weather.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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