African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 10, Num. 2, 2002, pp. 157-162

African Crop Science Journal, Vol. 10. No. 2,  2002, pp. 157-162

Competitive influence of Eleusine indica  and other weeds on the performance of maize grown under controlled and open field conditions

A.M. Rambakudzibga, A. Makanganise¹ and E. Mangosho¹

Agronomy Research Institute, Department of Research and Specialist Services, P. O. Box CY 550 Causeway, Harare, Zimbabwe
¹Department of Research and Specialist Services, Weed Research Team, Henderson Research Station, P. Bag 2004 Mazowe, Zimbabwe

(Received 13 December, 2000; accepted 5 February, 2002)

Code Number: cs02015

ABSTRACT

Delayed weed removal is the primary cause of maize yield loss in smallholder agriculture. The slog for weed management could probably be reduced if the initial weed control removal is restricted to the in-row weeds, followed soon after by elimation of inter-row weeds before competition sets in. This strategy would most likely minimise the competitive effect of both intra-row and inter-row weed infestations.  However,  the width of the area that must be weeded along the crop row and the timing of the subsequent inter-row weeding are largely unknown. Experiments were carried out in 1993/94 rainy season under controlled conditions to assess the area of influence of Eleusine indica (L.) Gaertn  (Rapoko grass) on maize. The study was also carried out under field conditions during the 1994/95 and 1995/96 rainy seasons, and in addition to weed width to be removed, the timing of the subsequent inter-row weeding was also studied. While E. indica plants spaced 40 cm were as effective as those closer to the maize row in reducing grain yields under controlled conditions, grain yield reduction was significantly (P<0.05) less under field conditions. Furthermore, results of both seasons suggest that inter-row weeds must be removed by four weeks after crop emergence to avoid maize grain yield reduction.

Key Words: Maize yield, Rapoko grass, weeding time, Zea mays

Résumé

Le désherbage retardé est la cause première de la perte de rendement chez les petits agriculteurs. Le coût due à la gestion de mauvais herbes peut etre diminué si le control initial est reserve aux herbes dans les lignes, accompagné juste après de l'enlevement des herbes entre les lignes avant que la compétition ne s'installe. Cette stratégie permettra certainement de minimiser l'effet de compétition des mauvais herbes d'entre et dans les lignes. Cependant, l'étendue à désherber dans les lignes et le rythme des désherbages ultérieurs entre les lignes sont largement inconnus. Des experiences ont été conduits pendant les saisons pluvieuses de 1993/1994 pour determiner l'influence de l'Eleusine indica (L.) Gaertn (Rapoko) sur le maïs. Cette étude a été menée dans les conditions des champs pendant les saisons pluvieuses de 1994/1995 et 1995/1996. En plus de l'étendue à désherber le rythme de désherbage entre les lignes a été étudié. Indica E. à 40 cm d'espacement était aussi éfficace que ceux plus proches des lignes de mais en reduisant le rendement en grains en conditions controllées. Cette reduction était significativement moindre dans les conditions des champs. Les resultats de deux saisons suggèrent  que les herbes entre les lignes doivent etre enlevées quatre semaines après la germination pour éviter les pertes en rendement.    

Mots Clés: Rendement de maïs, Rapoko, rythme de sarclage, Zea mays

INTRODUCTION

Weeds are one of the most important limiting factors to crop productivity in smallholder agriculture in Zimbabwe (Chivinge, 1994). Indeed, time spent removing weeds represents a significant percentage of the time invested into crop production (Vernon and Parker, 1983; Shumba, 1988). Eleusine indica (L.) Gaertn (Rapoko grass), a weed with a very high seed production capacity (Schwerzel, 1970), has been identified as the most aggressive weed in Zimbabwe (Budd, 1976; Chivinge, 1983). The weed is very prominent  across experimental sites in three different agro-ecological regions of Zimbabwe (Mabasa and Rambakudzibga, 1993). It has been considered one of the most devastating weeds of maize (Zea mays) Zimbabwe and South Africa (Ivens, 1967).

In smallholder environments, labour-shortages  are  commonly encountered during the onset of the rainy season. Inevitably the time of first weeding is spread from one  to six weeks after crop emergence and the weeding operation is done once, or twice but rarely three times (Mabasa et al., 1995). Such delayed weed removal exposes the crops to weed competition effects.  Effective weed management in such circumstances will require development of appropriate techniques and judicious weeding to cater for labour constraints.

It has been demonstrated that, at least during the early stages of crop-weed competition, weeds located closest to the crop row are most critical to crop growth (Carson, 1987). Maize grain yields  can be maintained if weeds growing within the crop row are removed initially with inter-row weeds removed later before exerting competition effects when labour shortage is not critical (Mabasa, pers. comm.). Reduction of the labour-requirements for weeding would facilitate the timely weeding of more planted area than would have been possible. Early control of in-row weeds could either be accomplished using hand-hoes or banded application of herbicides at reduced rates (Carson, 1987). Since the use of ox-drawn cultivators is widespread in some smallholder farming areas of Zimbabwe (Mabasa et al., 1995), the removal of inter-row weeds could be done with minimum recourse to human labour.  However, concerns on the width of the strip that should be weeded initially and whether there is any interaction between the proximity of the weeds to the crop row and the timing of initial weed removal are clear. The objective of the study, therefore, was  to quantify the influence of E. indica, in the first and second rainy season, and a mixed weed population in the third rainy season, on the performance (growth, development and yield) of maize. Furthermore, the interaction between the distance of the weeds from the maize row and the time of initial weed removal was also investigated.

MATERIALS AND METHODS

The competitive effects of Eleusine indica on maize grown under controlled conditions. The experiment was conducted on an open cement floor at Henderson Research Station (Zimbabwe) during the 1993/94 rainy season. Two-hundred-litre capacity steel drums (85 cm long and 58 cm wide) were cut length-wise into halves and filled with weed-seed free sandy subsoil. Approximately 30 g compound D fertiliser (8% N, 14% P₂O₅, 7% K₂O) equivalent to 600 kg ha^-1 was broadcast and incorporated into the top 5 cm of the soil profile. Maize cultivar R215 was planted along the centreline of the length of the drums. Eleusine indica seeds were sparingly dribbled along either side of the maize row at distances of 10, 20, 30 or 40 cm. A weed-free maize stand was included to act as a control treatment.  The experiment was arranged in a randomised complete block design with four replicates per treatment.  Both maize and E. indica were planted simulteneously. Maize and E. indica were thinned to two and five plants per stand, respectively one week after emergence. All other weeds, except the required stand of E. indica, were hand-pulled as soon as they emerged. Approximately 18 g ammonium nitrate (34.5% N) equivalent to 350 kg ha^-1 was split applied at 4 and 8 weeks after crop (maize) emergence (ACE). The plants were irrigated twice a day. Data taken included maize plant height at weekly intervals, maize and E. indica shoots dry weight (DW), maize cob and grain weight at harvest.

The competitive effects of  Eleusine indica on maize grown under open field conditions. The competitive effects of weeds on maize were examined in a factorial experiment arranged in split plots with three replicates. The distance of the weeds row from the maize row and time of weed removal were the main plot factor and the sub-plot factor, respectively. The field experiment was conducted in 1994/95 and 1995/96 rainy season on a sand-veld experimental site at Henderson Research Station. In both seasons, maize cultivar R215 was planted at an inter-row and in-row spacing of 90.0 cm and 22.5 cm,  respectively. The plots were 6.0 m long and 5.4 m wide. A basal compound D fertiliser was applied at 450 kg ha^-1. Ammonium nitrate was applied 4 and 8 weeks ACE at 350 kg ha^-1, but unlike for the previous experiment, the plants were not irrigated.

In the 1994/95 rainy season, E. indica seeds were dribbled along the maize row, and 20, and 40 cm on either side of the maize row. The E. indica seedlings were thinned to ten plants per metre row one week after emergence. All weeds other than the required stand of E. indica were hand-pulled as soon as they emerged. In the 1995/96 rainy season, however, a 5-cm wide strip of natural weed population was allowed to grow along the maize row, and 20, and 40 cm on either side of the maize row. During both seasons, the weeds were removed using hand-hoes at 2, 4 and 8 weeks ACE and the plots maintained clean thereafter.

Maize plant height was measured at weekly intervals. Maize cob, leaf, stem and grain weight were also determined at harvest. In the 1995/96 rainy season, the leaf area of a sample of two maize plants was measured at 108 days ACE. In both rainy seasons, weeds were oven-dried at 70 °C for 48 hr and dry matter weights (DW)  recorded.

RESULTS

Under controlled conditions, E. indica competition did not significantly (P<0.05) influence the phenological development of maize vis-à-vis time to anthesis, silking and physiological maturity (data not shown). Prior to 21 days ACE, the influence of E. indica competition on maize height was not significant (P<0.05) (Table 1). However, E. indica significantly (P<0.001) reduced maize plant height beginning at 42 days ACE. Distance from the maize row did not influence effect of E. indica on maize grain yields. Eleusine indica plants that were 40 cm from the maize row were as effective in reducing grain yields as those growing 10 cm from the maize row. The treatment where E. indica competed from within the maize row contained only 50% of the weed population that was present in other treatments. The weed population therefore is probably a more significant factor with regard to impact on yields than the distance at which weeds are from the maize crop. However, results indicate that E. indica can interfere with growth, development and yields of maize even from a distance of 40 cm away from the crop.

During the 1994/95 rainy season, the effects of E. indica distance from the maize row on maize development and grain yields was not significantly (P<0.05) affected by the time of E. indica removal, hence, only the main effects data are presented (Steel et al., 1997). Table 2 shows the effects of E. indica removal time on maize height, maize leaf area, cob counts and grain weight. Maize height was significantly (P<0.001) reduced when E. indica was left to compete with the crop for the whole duration of crop growth. However, delaying E. indica removal up to 8 weeks ACE did not significantly (P<0.05) affect maize height. Similarly, delayed weed removal did not significantly affect maize leaf area determined at 108 days ACE maize cob counts. On the contrary, maize cob counts were significantly (P<0.01) reduced when E. indica was not removed for the entire duration of crop growth.  Maize grain yields were significantly (P<0.001) reduced when the removal of E. indica was delayed up to 8 weeks ACE. However, E. indica removal earlier than 4 weeks ACE did not result in significant maize grain yield increases.

In the 1994/95 growing season, E. indica located within the crop row did not significantly reduce maize grain yields (Table 3). Eleusine indica population in this treatment were only 50% of those in the other treatments. Maize grain yields were, however, significantly (P<0.05) reduced by 20% when E. indica was spaced 20 cm compared to 40 cm from the maize row. On the contrary, varying the spacing of E. indica from the maize row did not significantly (P<0.05) influence leaf area determined at 108 days ACE as well as maize height and cob counts at harvest.

As was the case in the 1994/95 rainy season, the interaction effect between the spacing of the weeds from the crop row and the time of weeds removal were not significant (P<0.05). Accordingly, only the main effects were considered (Steel et al., 1997).  Data on the influence of distance between the maize plants and weeds on maize cob counts and maize grain yields are presented in Table 3. Although the treatment in which weeds competed with the maize plants from within the crop row constituted only 50% of the weed pressure of other treatments, weed competitive effects of this treatment significantly reduced  maize grain yields. Weeds located furthest from the crop-row (i.e., 40 cm) had less competitive effects to the maize plants as reflected in the grain yield gains.

Unlike the previous rainy season, maize cob count was not significantly (P<0.05) affected by the duration of weed competition though there was a consistent reduction in the number of cobs with  delayed weed removal (Table 4). However, maize grain yields were significantly (P<0.01) reduced with delayed weed removal. Removal of weeds as early as four weeks ACE minimised maize grain yield reduction. Regression analysis revealed that of the 35% variability in maize grain yields attributable to total weed DW in the 1995/96 rainy season, E. indica accounted for 32% of this variation. Indeed E. indica contributed 65% to total weed DW. The other weed species were Richardia scabra (Moq.) Gomez (Mexican clover), Tagetes minuta L. (Mexican marigold), Amaranthus hybridus L. (Pig weed) and Dactyloctenium aegyptium (Linn.) P. Beauv. (Crow's foot).

DISCUSSION

Through competition for nutrients, weeds can reduce the growth and yields of maize by influencing the availability of soil water (Thomas and Allison, 1975; Marais, 1985; Twomlow et al., 1997). This results, among other effects, in the temporary immobilisation of nutrients in the plough layer (Marais, 1985). Though tall growing weeds like Rottboellia cochinchinensis (Loux.) Clayton (Shamva grass) can compete favourably for light against maize (Thomas and Allison, 1975), this would not have been the case in these studies since the E. indica and R. scabra, which were the major weeds at the trial site, are relatively low growing. In this study therefore, most of the competitive interaction effects between maize and weeds could be explained in terms of competition for soil moisture and soil nutrients.

In studies under controlled conditions, maize height was not significantly influenced by E. indica competition at 21 days ACE. However, at 42 days ACE, the effect of E. indica competition on maize height was evident. According to Akobundu (1987), there is an initial period in the growth of both weeds and crops when negative interaction is absent. This period was quite evident in the confined drum environment. Similarly, under field conditions, maize height, maize leaf area and maize cob counts were not significantly affected by weed competition when weeds were removed as late as 8 weeks ACE. However, weeds should  be removed within four weeks ACE to avoid grain yield reduction.  Grain yield, therefore appears to be more sensitive to weed competition effects than other maize developmental attributes.

Besides the composition of the weed spectrum, the two rainy seasons were very contrasting with regard to rainfall amounts received. Total precipitation in the 1994/95 and 1995/96 rainy seasons was 439 and 891 mm, respectively. This probably explains the very low grain yields recorded in the former season. Mabasa and Rambakudzibga (1993) reported that, when several weedings are planned, initial weeding should be conducted 2 weeks ACE and the frequency of subsequent weedings should be higher under wet conditions. However, single weeding was more efficient when conducted 4 weeks ACE. The majority of smallholder farmers in Zimbabwe weed their maize crop once or twice in a season (Shumba, 1986).

In the drum experiment, E. indica plants spaced 40 cm from the maize row effectively reduced maize grain weight and was comparable to the treatment with E. indica spaced only 10 cm from the maize row. The same was also true in the 1994/95 rainy season under field conditions. However, in the 1995/96 rainy season under  natural weed infestation, and also under conditions of favourable rainfall conditions, weeds competing from a distance of 20 cm adversely affected maize grain yield than those competing from 40 cm. Whereas E. indica was thinned to ten plants per metre row in the first season, more weeds were left to compete with the crop in the second growing season. There was no interaction between the time of weed removal and the width of the in-row weed-free area in terms of maize crop development and grain yields.

The findings of this study reveal that, though there is scope for the use of in-row weed removal to reduce labour shortages for weeding, the inter-row weeds should be removed preferably by 4 weeks ACE to avoid grain yield reductions. A weed-free area of 20 cm on either side of the maize row is too narrow to avoid early weed competition.

ACKNOWLEDGEMENTS

We are grateful to Prof. O. A. Chivinge for his useful comments in the preparation of this manuscript.

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