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Timestamp: 2019-04-24 08:55:34+00:00

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Midmore D.J., Yang S., Kleinhenz V., Green S. and Tsay J.
In order to reduce virus infection and to improve chili pepper productivity five intercropping experiments with maize and chili were conducted in southern Taiwan. Among the factors studied were relative date of sowing maize and transplanting pepper, planting crops in adjacent or paired rows, maize of contrasting plant height, pruning or removal of maize following harvest, and a range of maize population densities.
Intercrop treatments which were effective in reducing spread of virus in pepper plants (e.g. tall vs. dwarf maize, greater vs. less maize population) also led to greater reduction of pepper yields. In general number rather than size of fruit was reduced by maize intercrops; maize yields were rarely, however, reduced by pepper intercrops.
At high maize populations, particularly for grain maize, sowing two rows of maize per bed alternating with a bed of pepper led to less pepper yield reduction than sowing one row each of maize and pepper on the same bed. Double-row strip-cropping was more amenable to relay-cropping of legumes with pepper following maize harvest than intercropping on the same bed.
Intercropping maize and pepper on the same bed was effective in raising intercrop productivity if the duration of the pepper crop was not markedly greater than that of the maize.
The importance of maximizing light interception, either through inter-cropping with maize or raising pepper population density, was highlighted.
Yield of chili peppers in the tropics are often low, usually a result of poor agronomy and rampant virus infection. Surveys in Indonesia indicate that farmers yields are markedly less than those achieved on experiment stations; pests and diseases, particularly virus and anthracnose are important con­straints, as are lack of purchased inputs (Vos and Sumarni, 1990). In fact, in some countries chili pepper yields per unit land area are declining over time (Subijanto and Isbagyo, 1988). Virus infection can lead to complete loss of the crop (Lockhart and Fischer, 1974) or of marketable yield (Marte and Wetter, 1986), but more often viruses are responsible for a reduction in yield potential.
Although rarely pinpointed as such, poor agronomic practices and virus infection limit the extent to which the pepper crop intercepts solar radiation and assimilates CO2 for the production of dry matter. Combined growing of pepper with other crops (i.e. intercropping) may result in a greater overall interception of solar radiation than by the pepper crop alone, which could lead to an improved cropping efficiency. Relay-planting pepper one month after garlic (Asandhi, et al. 1987), although reducing pepper yields by 13%, resulted in improved land equivalent ratios (LER - the amount of land planted in monocrops required to produce the same yield as that when crops are intercropped; Willey, 1979). Intercropping maize and pepper has also been reported to result in LERs greater than unity (Quasem et al. 1987). Besides improving total interception of radiation, taller species intercropped with peppers can potentially benefit the pepper crop through a wind break effect, causing a reduction in evapotrans­piration, and a disruption of aphid alightment and spread of viruses. Additionally, if the taller species is a C4 crop with special photosynthetic adaptation to the tropics, a more efficient use of high instantaneous receipts of solar radiation is evident in the understory intercrop (Midmore, 1990). The magnitude of such advantages will depend upon the relative planting times, densities and spatial distributions of pepper and the companion crops (Midmore, 1992). Detailed studies on these are presented herein.
Five field experiments were conducted in southern Taiwan during the period 1990-1992. All plots received 25 t/ha compost, and 200:120:120 kg/ha of N:P2O5:K2O as ammonium sulfate, superphosphate and potassium chlo­ride, respectively. Normal crop production practices for hot pepper and maize were followed, but insecticides were not applied.
Peppers were transplanted on 31 August 1990 (experiment I), 25 Sep­tember 1990 (experiment II), 13 March 1991 (experiment Ill), 15 August 1991 (experiment IV) and 27 April 1992 (experiment V).
Details of treatments specific to experiments I-IV are presented in Table 1. In summary, experiments I and II were designed to quantify the influence of sowing maize before or after transplanting of sweet or hot pepper, with both pepper and maize growing on the same 1.0 m wide bed. Experiments III and IV investigated the influence of two pepper populations, the planting of intercrops in single or paired rows i.e. two rectangularities of planting, two maize types of contrasting height, and complete removal or pruning of Maize to the pepper canopy height following maize harvest in a factorial design. With a systematic design the influence populations (0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 plants/m2), two pepper populations (3.33 or 6.67 plants/m2), and two row directions (N/S or E/W) were studied in experiment V. This experiment was curtailed by serious flooding 70 days after transplanting (DAT) whereas the other experiment ran for > 200 days.
Sole hot pepper, two rows per bed.
In experiment I and II, in separate treatments both sweet pepper (var. Blue star) and hot pepper (var. Szechwan, selection PBC 076-9-10-3 (OP)) were intercropped with the grain corn variety Tainan 351. In experiments III and IV the hot pepper variety Hot Beauty F1 (Known You Seed Co.) was intercropped with the sweet corn variety TN236 and the grain corn variety TN11 (Table 5). The hot pepper variety Hot Beauty and grain corn variety TN11 were utilized in experiment V. Following harvest of maize in experiment Ill some plots (i.e. where maize plants were completely removed) were planted with the AGS 292 variety of vegetable soybean, and in experiment IV snapbean (Taiwan Seed # 1459) was sown in all intercropped plots following maize grain harvest.
Row directions in experiments I and II were N/S, in experiments Ill and IV they were E/W. Row direction comprised one of the treatments in experiment V. In experiments IV and V all treatments were transplanted or sown on the same date, in experiment III maize was sown 1 week after transplanting pepper, and in experiments I and II relative sowing and transplanting dates varied according to treatment. In all but experiment V plot size was 30 m2 (five beds, each 6-m long).
Data collected included the fresh yield of pepper fruits, dry maize grain yield, fresh cobs for sweet corn, light interception (with a LI-COR LI-1000 radiometer) and incidence of cucumber mosaic virus (CMV) on 20 plants per central portion of each plot. Infection with CMV was taken to represent an index of infection by all aphid-borne non-persistent viruses. One leaf from the upper, middle and lower portion of the canopy were taken from individual plants and on a per plant basis were tested for CMV by DAS-ELISA. In experiments III and V numbers of aphids were also counted on 20 maize and pepper plants per plot. Data were subjected to analyses of variance.
The results are presented in relation to the three main areas of interest within these experiments: a) virus infection, b) interception of solar radiation, and c) crop yields and indices of productivity.
Maize consistently reduced the incidence of CMV on intercropped peppers, whether sweet or hot peppers. Evaluated two months after pepper transplanting, simultaneously sown maize (experiment I) led to greatest reduc­tion in CMV, however at later samplings the later sown maize was more effective in reducing CMV incidence (Table 2). Thirty-seven days after trans-planting in experiment Ill, CMV incidence was almost nonexistent, but by 67 DAT up to 27% incidence was evident in the low density sole pepper crop and least incidence in the pepper planted on the same bed as grain corn (Fig. 1). By 88 DAT, incidence of CMV had risen to 55-58% in sole pepper plots.
Taller grain maize continued to suppress CMV incidence (10-14% incidence) compared to the shorter sweet corn (29-32%), but there were no differences between CMV incidence in plots with maize in the same or on alternate ridges to the pepper. Aphids in experiment III were more prevalent on maize than pepper plants 37 DAT, and more so in maize intercropped with pepper on the same than on alternate double row beds (Table 3).
Table 3. Influence of intercrop treatments on aphid populations on pepper and maize plants 37 DAT (mean number per plant). Experiment III.
By five weeks after transplanting in experiment IV no meaningful differ­ences between treatments were evident in CMV infection; all were less than 10% (Table 4). Three weeks later virus incidence was greatest in the low population sole pepper crop.
Table 4. Infection with CMV (%), of pepper grown as sole or intercrops. Experiment IV.
Eleven weeks after transplanting virus was more evident in sole pepper plots with least virus infection in alternate bed planting of sweet corn or same bed planting of grain maize. With increase in population of maize (experiment V) the reduction of virus incidence was more notable (Fig. 2), however neither row orientation nor the direction of the gradient of maize population had significant effects on virus incidence (data not presented). On average more aphids were present per plant at the high pepper plant population (Table 5), but there was no relationship between aphid population and incidence of CMV.
Table 5. Influence of pepper planting population on light interception, total fruit number and yield, aphid number per plant and virus infection (mean of two row directions and 7 maize populations). Experiment V.
Neither sole hot pepper nor sole sweet pepper crops in experiment exceeded 60% light interception (LI) whereas maximum LI in sole maize or intercropped plots for both pepper types varied between 80 and 90% (Fig. 3). Initially LI was greater in intercropped maize plots sown at the same time as with peppers, and less in the maize intercrop treatment planted 1 week later than the rest, but by 100 DAT no differences between sole maize only or intercrops were evident. Following maize harvest 125 DAT, LI was similar in sole and inter-cropped pepper plots, with a tendency for it to be greater in the sole plots, and in the hot pepper plots.
A similar trend for LI was evident in experiment II, with peak LI 75 DAT, and following maize harvest maximum LI was evident in the sole hot pepper plots. Light interception by short sweet and tall grain corn in experiment III was incomplete, reaching 65% by 60 DAT (Fig 4a). Both single and double row (PO and PT respectively) pepper reached 90% LI, with the latter reaching its maximum 9 days earlier (Fig 4a). Pepper and sweet corn planted on the same bed had superior LI to intercropped paired rows of each crop on separate beds, an effect maintained after maize harvest (Fig 4b). A similar improvement of LI in associated planting of pepper and grain corn on the same bed over intercropped paired rows of each crop on separate beds was evident. This effect was also maintained after maize harvest (Fig. 4c). Following harvest of sole maize, LI of the subsequent vegetable soybean crop reached > 60% by 120 DAT (Fig. 4a). Intercropping of soybean in pepper plots was only done where maize was removed in paired row plantings. This resulted in increases in L1, especially following grain maize (Fig. 4c). There was no influence of maize type (sweet or grain) on the subsequent pepper LI once maize was cut or removed. Light interception by maize in experiment IV was also poor (Fig. 5). Light interception by high pepper population exceeded that of the low popula­tion treatment by 10 to 20%. Intercropping pepper with single vs. double rows of grain maize (i.e. on the same bed vs. alternating beds) led to greatest and least LI respectively.
Total LI was consistently greater in intercrops with greater maize popu­lations (experiment V), however, at the greater maize population considerably less radiation was intercepted by the pepper crop (Fig. 6). Even the lowest population of maize intercepted > 40% of incoming radiation by nine weeks after sowing.
Yields of pepper were invariably less in intercropped than sole cropped treatments.
Cumulative yields of pepper over harvest dates in experiment I are presented in Fig. 7. Major contributions to total pepper yield were evident over the period 90-140 DAT and 240-280 DAT, the period 140-240 DAT representing the coolest (winter) period when fruit set was minimal. Intercropping with maize planted at its sole crop population severely reduced pepper yields, but less so for sweet pepper when sowing of maize was delayed. Differences between maize yields in sole or intercropped plots were not significant (data not presented). Maize yields did, however, seem to be greater with sweet corn and less with hot pepper in comparison to sole maize yields.
Cumulative pepper yields in experiment II showed similar trends to those in experiment I, but yields before the cool winter w ere notably less for hot pepper and greater for sweet pepper (Fig. 7). Relative yield reduction of both hot and sweet pepper in intercrop treatments was greater in experiment II than experiment L Following the cool spell renewal of fruit production was superior in hot than sweet pepper. Maize yields did not differ significantly between treatments, although there was a tendency for the maize sown before the pepper to have superior yields to that sown at the time pepper was transplanted (data not presented).
In experiment Ill, the higher density pepper planting (treatment PT) resulted in 45% greater cumulative yield than the lower density treatment (P0, Fig. 8a), which itself was 100% more productive than the intercropped pepper yields. Pepper yields when intercropped with sweet corn were essentially similar across treatments (Fig. 8a); however, double paired row planting of grain maize with pepper led to greater pepper yield than if planted together on the same bed (Fig. 8b). Although cumulative yields of peppers remained low after removal of maize, there was no difference between maize type on subsequent pepper yields when both crops had been planted on separate alternating double row beds (data not presented). In contrast, when planted on the same bed as pepper, there was a residual effect of maize types: subsequent pepper yield after maize harvest was reduced more in plots previously inter-cropped with grain maize than in those with sweet corn (Fig. 8c).
Yields of sweet corn were consistently greater when planted on the same bed rather than alternate beds with pepper (Table 6). The exploitation of fertilizer applied to the pepper but taken up by maize on the same bed cannot be discounted when interpreting this effect. Lodging of some alternate double row plots of sweet corn was responsible for their low yields. Grain yield of field maize in alternate double row plots with pepper was somewhat less than that in intercrops with pepper on the same bed (Table 6), which in turn was less than sole maize grain yield.
In experiments III and IV, in some treatments space was availed between paired rows of pepper following maize harvest. This was subsequently sown with vegetable soybean (experiment Ill) or snap bean (experiment IV). Vegetable soybean yields were greatest in sole treatments (5.6 and 6.5 t/ha fresh weight following grain or sweet corn respectively). In intercropped treatments yields were greater in plots previously intercropped with grain maize (2.8 t/ha) than previously intercropped with sweet corn (2.0 t/ha), i.e. greater yield where pepper growth was more suppressed by the previous intercrop.
Table 6. Maize yield (t ha-1) as influenced by intercropping with hot pepper in Experiment Ill and IV.
Trends in cumulative pepper fruit yields in experiment IV were similar to those of experiment Ill: sole crop high density yields exceeded those of sole crop low density by 22% which in turn exceeded maximum intercrop pepper yields by 44% (Table 7). The greater residual debilitating effect of taller grain maize than shorter sweet corn on pepper yields was evident throughout the experiment (Fig. 9), as was the beneficial effect of removal of maize plants following their harvest on pepper yields (Table 7). The inverse relationship between maize and pepper yields observed across maize types or the positive effects of removal of maize after its harvest on pepper yields, were followed by inverse relationships between pepper and snap bean yields. For example, where more space was available for the snap beans, such as in strip intercropping, snapbean yields were greater (Fig. 10). Following maize harvest there was no evidence, however, that pepper yields compensated for apparent release from competition with the maize (Fig. 9).
Table 7. Pepper fruit yields as influenced by maize type and intercrop treatments. Experiment IV.
Heavy rain and flooding prematurely curtailed pepper harvests in ex­periment V. Doubling of the pepper population (two vs. one row of plants per 1 m wide bed) resulted in a 20% average increase in fruit yield (Table 5), however there was no effect of pepper population upon the intercropped maize yields (Table 8). Pepper yields were reduced from 7.5 t/ha in sole plots to 4.2 t/ha at a maize population of 3 plants/m2 (Fig 11). In the pepper intercrop maize yields rose linearly over the range 0 to 3.0 plants/m2 (from 0 to 2.5 t/ha). Yields at the maximum maize intercrop population (2.6 t/ha) were somewhat less than sole maize yield of 3.4 t /ha. There was some evidence that row direction, particularly N/S, favored maize and pepper yields (Table 9).
Table 8. Effects of pepper planting population on maize yield and components of yield (means of two row conditions and 6 maize populations). Experiment V.
Table 9. Effect of row direction on total pepper and maize grain yields (means of 7 intercrop treatments and 2 pepper populations). Experiment V.
Virus (CMV) incidence on pepper was reduced by intercropping with maize, especially when maize was sown at sole crop populations. That taller (grain) maize was more effective than shorter (sweet) corn suggests that the maize acted as a barrier toward the spread of the aphid vector of CMV. Arranging maize intercrops as single rather than double rows (with single or double rows of peppers, respectively) adds weight to the possible barrier effect, as does the greater effectiveness in reducing CMV incidence at the greater maize populations (Fig. 2). It is still not clear whether the barrier effect prevents entry of aphids into plots, or their plant-to-plant movement once inside plots. Nevertheless, the attractive influence of yellow-colored maize tassels cannot be disregarded. Fluctuations in aphid populations over time are common, yet by two months after transplanting > 30% sole crop pepper plants were infected with CMV. Reduction in CMV incidence in experiments I and II was more notable by maize crops which had tasseled closer to the date of sampling for CMV (Table 2: i.e. in experiment I at 59 DAT by the same day sowing, vs. 116 DAT and 91 DAT (experiment II) by the later sown treatments). This merits further study. Counts of aphids on pepper plants may not be highly associated with virus infection (Table 5) although occasionally they may be related to protection by intercropped maize (Table 3).
In spite of protection afforded by maize to pepper plants for the control of CMV, yields of pepper fruits were always reduced in intercrops. There was, however, no evidence for significant reduction of maize yields when inter-cropped with pepper. Compared with data from low populations of pepper (e.g. in experiments I and II), intercropping with maize raised total light interception (Fig. 3), particularly over the first 70 DAT. Total light interception at the high pepper population was not exceeded by intercrops at the lower pepper population in experiment 111, but it was equaled when maize and pepper were grown on the same bed (Fig. 4). In experiment IV only intercropping pepper with single rows of maize raised LI above that of the sole high population pepper crop.
Separating the light interception by intercrops into that intercepted by the pepper or maize (Fig. 6) illustrates the marked reduction in light energy available to the pepper plants from 40 DAT, particularly when grown with maize populations> 1.5 plants/m2. Total pepper fruit number (m-2) was inversely related to maize population (plants/m2) in experiment V (y = 227-37.3 (±3.6) x, r2 = 0.956, n = 7), but no compensation in individual fruit weight was evident across treatments, since pepper fruit yield and number per unit area were closely correlated (r2 = 0.979, n=7). Evidently fruit set of peppers both during and after the intercrop period was reduced compared to sole crops. This response may in part have been due to competition for light and in part for space (and general canopy expansion) with the associated maize crop. Mild shading (about 25% reduction of clear light conditions) has been reported to increase pepper yields, but heavier shade induced flower, bud, and lastly fruit abscission (Wien et al., 1989). Release from competition with maize after maize harvest did not result in compensatory growth by the peppers in any experiment, which suggests that yield potential as determined by canopy girth was fixed within the first 3-4 months after transplanting.
Pepper yields were reduced more so when grain maize was planted on the same as opposed to alternate beds (i.e. pepper intercepted less light when grown on the same bed as maize). Likewise, when maize was grown on the same bed as pepper, yield reduction of pepper was substantially less with the shorter (sweet) corn than with the taller (grain) maize.
Experiment I favored hot pepper yields (12 t/ha) compared to those of experiments II (10 t/ha) before the onset of cool conditions. The reverse was true for sweet pepper, which achieved only 7.5 t/ha in experiment I vs. 13 t/ha in experiment II. Earlier fruiting in the cooler conditions of experiment II (i.e. > 3 t/ha at 50 DAT vs. > 3.0 t/ha at 75 DAT in experiment I) favored sweet pepper yield in that experiment. The first yield peak in experiment Ill, transplanted in the spring, reached 16 t/ha in the low-density planting and 22 t/ha at the high density, both considerably greater than the autumn (experiments I and II) yields.
That the difference between autumn and spring yields might have been a variety effect is substantiated by total pepper yields of > 20 t/ha in the autumn planting of Hot Beauty in experiment IV. Cumulative yields of the variety tailed off 130 DAT (mid-December), most likely a low temperature effect. Pepper yields of experiment V were constrained by strong winds, rain and flooding, and only marginally exceeded 7.0 t/ha. Greater pepper and maize yields for the NI S row direction in experiment V (Table 9), transplanted in late spring, correspond with the prediction by Mutsaers (1980) that crop yields at 25°N will be greater in E/W row direction in spring, but N/S during the summer. A further experiment is presently in the field to confirm this result.
Use of popular indices which quantify the productivity benefit of growing two crops together (e.g. the land equivalent ratio - LER (Willey, 1979)) are inappropriate if the intercrops vary in their crop duration (Hiebsch and McCollum, 1987).
To determine whether productivity on the intercropped plots exceeded that of planting the crops on separate pieces of land, the Area Time Equivalency Ratio (ATER - Hiebsch and McCollum, 1987) was used. This index takes into account the opportunity use of land in the sole plots of the shorter season crop once that crop has been harvested. Values > 1.0 indicate a productivity benefit for the intercrop. For experiment I, assuming that the intercrop was left until 280 DAT (Fig. 3), planting in sole crops would have been more advantageous (from the production viewpoint) than intercropping for both sweet and hot pepper (Table 10). If intercrop harvests had terminated prior to the cool period (starting 140 DAT, Fig. 7), intercropping of hot pepper would have been more productive than sole cropping, whereas for sweet pepper there was no advantage either way. Since in later experiment s higher pepper population led to greater yields, the greater ATER of intercrop plots may simply be due to greater total population in those plots. In experiment II for no treatment did the ATER markedly exceed unity, indicating that sole crop planting was equal or superior to intercropping.
Table 10. Area time equivalency ratios (ATER) for intercrop treatments in Experiment I.
ATER = ((Y(IA)/TI)/)Y(SA)/TSA)) + ((Y(IB)/TI)/Y(SB)/TSB)) when Y=Yield, T=Time, IA=intercrop B, SA=sole A, SB=sole B, I=time from initial planting to final harvest of intercropped plots.
When calculated on the basis of a 211-day intercrop period no ATER in experiment Ill (for maize and pepper) exceeded unity (Table 11). Pepper yields did not increase after 135 DAT, due to the detrimental effect of water-logging following a typhoon. Calculated on a 135-day intercrop period ATER's exceeded unity (Table 11) when compared to the low pepper population treatment (PO) but not compared to the higher population (PT). Pepper yield suffered more from competition with maize in intercropped treatments than did maize yield from competition with pepper.
ATERs calculated for experiment IV included snap bean yields. Intercrops with sweet corn produced a greater proportion of sole crop yields per day of season than intercropping with grain maize, and there was evidence that complete removal of maize plants favored overall productivity (Table 11).
In both experiment III and IV sweet corn intercropped with pepper on the same bed, while not reducing CMV as much as did grain corn, competed less for light, yet resulted in fair soybean or snapbean yields.
Advantages in productivity of all intercrop treatments were evident across maize populations in experiment V (Fig. 12). Values were greater at higher maize populations, due to relative yield stability of pepper under increasing shade, particularly for fruit size.
Greater productivity of intercrops than sole crops was not always evident in our experiments. The high amount of inputs (fertilizer, irrigation, weeding) might have obviated some of the traditional benefits associated with intercrops (e.g. reduced need to weed, better sharing of limited resources), and led to greater levels of competition, particularly for light, with consequent reductions in pepper yields.
- Intercropping pepper and maize at their sole crop populations, while beneficial for virus control, results in a reduction of pepper yield.
- At maize sole crop population, intercropping which involves shorter maize varieties, a delay in sowing relative to transplanting of pepper, or planting maize on alternate paired row beds, results in less reduction of pepper yield than taller maize, same day or same bed planting.
- Maize intercrop treatments which lead to reduced pepper growth (e.g. taller maize) but not necessarily yield (e.g. paired row plantings of maize) result in compensatory growth by subsequent intercrops alongside the pepper plants with poor canopy.
- Paired row planting with maize lends itself to relay cropping pepper and legumes better than single row planting, especially if maize plants are removed after harvest.
- Single row planting with maize is to be favored if duration of the pepper crop is not vastly greater than that of the maize crop, but maize population should not approach sole maize population. Reductions in pepper yield through intercropping result mainly from reduction in pepper fruit number; fruit size is more resilient to shade.
We wish to thank A. Massawe for collecting data on aphid populations in experiment Ill, Mr. Heestermans and Mr. Kramer for data collection and analysis in experiment V, and Ms. Shui for general field assistance.
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