African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 4, Num. 1, 1996, pp. 63-70

Upright starbur weed competition with soybean

O.A. CHIVINGE, A.B. MASHINGAIDZE and M. MUTSVAIRO

Crop Science Department, University of Zimbabwe, P.O. Box MP 167, Mt Pleasant, Harare, Zimbabwe.

(Received I June, 1995, accepted 30 October, 1995)

Code Number: CS96041
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ABSTRACT

The effect of upright starbur (Acanthospermum hispidum DC) time of emergence and duration on its competition with soybean (Glycine max (L.) was studied under field conditions during the 1991/92 and 1992/93 rainy season at Chegutu in Zimbabwe on a Rhodic Nitisols. Soybean grain yield under weedfare conditions was 3.6 t ha^-1, but full season competition from the weed reduced soybean grain yield by 1.84 t ha^-1 and extended the flowering period by 30 days. Upright starbur that emerged 2-3 weeks after soybean emergence delayed flowering by 14 days. Competition from soybean reduced upright starbur growth, stem diameter by 2.4 to 4.8 mm and seed yield by 910-960 kg ha^-1. Flowering and seed set in upright starbur was delayed by 14 days when the plants emerged 2 and 3 weeks after soybean emergence. Upright starbur plants which emerged with soybean and competed for the full season reduced growth and grain yield of the crop. Upright starbur plants that emerged 2 and 3 weeks later were less competitive and produced fewer seeds.

Key Words: Acanthospermum hispidum DC, Glycine max (L.) Merrill, flowering, grain yield, senescence, weed competition growth, seeds.

RESUME

L'effet du temps de germination de l'Acanthospermum hispidum DC et de sa duree de competition avec le soja (Glycine max (L.) etait etudie en champ au cours des saisons de pluies 1991/92 et 1992/93 a Chegutu au Zimbabwe sur un nitisol rhodique ayant un pH de 6,5. Le rendement de soja-graines en champ sarclee etait de 3,6 t/ha, mais sa culture non sarclee pendant route la saison reduisait la croissance du soja et son rendement en graines a 1,84 t/ha tout en prolongeant sa periode de floraison a 30 jours. L'Acanthospermum hispidum qui levait 2 a 3 semaines apres le soja, retardait sa floraison de 14 jour. La competition du soja reduisait la croissance de l'Acanthospermum, son diametre de tige par 2,4 a 4,8 mm et son rendement en grains de 910 a 960 kg/ha. La floraison et formation de semences de l'Acanthospermum etaient retardees de 14 jours lorsque les plants levaient 2 a 3 semaines apres le soja. L'Acanthospermum qui levait avec le soja et entrait en competition pour toute la saison, reduisait la croissance et le rendement en graines de la culture de la culture. Les plants de l'Acanthospermum qui levaient 2 a 3 semaines plus tard etaient moins competitifs et produisaient peu de semences.

Mots Cles: Acanthospermum hispidum DC, Glycine max (L.) Merrill, floraison, rendement en grains, croissance, semences, senescence, concurrence des mauvaises herbes

INTRODUCTION

Upright starbur, (Acanthospermum hispidum DC) first recorded in Zimbabwe in 1933 (Brian, 1937), is an ubiquitous weed (Chivinge, 1988). The weed infests most crops (Thomas, 1972; Chivinge, 1983). Heavy infestations occur in tobacco (Tobacco Research Board, 1987, unpubl.) and soybean (Mudzana, 1988). Its seed dormancy and nonsynchronous seedling emergence makes it difficult to control in soybean (Glycine max (L.) Merrill) and other crops (Mutsvairo, 1992). A glasshouse study (Mudzana, 1988) showed that upright starbur that emerged with soybean reduced growth and pod number per plant of the latter by 23.9%. Marginal reduction in crop growth occurred when the weed emerged 11 and 22 days after soybean. Chivinge and Schweppenhauser (1994) reported upright starbur to be one of the most difficult weed species to control in soybean.

Soybean is sensitive to competition from broadleaf weeds that emerge at different stages of its growth. Soybean interaction with morning glory (Ipomoea pupurea J. Hered) delayed maturity of the former by 2 weeks (Oliver et al., 1976). Soybean stem height was reduced due to competition with common cocklebur (Xanthium strumarium L.) (Bosza and Oliver, 1990) but was increased in the presence of jimson weed (Datura stramonium L.) (Regnier and Stoller, 1989). The presence of X. strumarium in soybean for the first 2-3 weeks had no effect on the crop but flowering nodes were substantially reduced by the 4th week (Bozsa and Driver, 1990). Soybean pod number was reduced from 37% per plant due to D. stramonium infestation (Hagood et al., 1980) and by 62.6% with upright starbur (Acanthospermum hispidum DC) (Mudzana, 1988) but no reduction occurred with volunteer maize. Infestation from giant foxtail (Setaria faberi Herrm) (Knake and Slife, 1962) and D. stromonium (Hagood et al., 1981) did not affect seed weight but velvet leaf (Abutilon theophrasti Medic).(Hagood et al., 1980) and jerusalem artichoke (Helianthus tuberosus L.) (Wyse et al., 1986) reduced seed weight. Yield reduction in soybean due to full season competition from commom lambsquatter (Chenopodium album L.) was 20% (Crook and Rennet, 1990) and 22-57% with 4-16 plants m^-1 of X. strumarium but not with 1.5 to 2 plants of the same species (Hagood et al. 1980). Abutilon theophrasti, pricky sida (Sida spinosa L.) and stockrose (Hibiscus trionum L.) that emerged with soybean reduced grain yield by 1,010 kg ha', but only by 480 kg ha^-1 when they emerged later. Full season weed interference reduced soybean yield by over 50% (Hinson et al., 1982).

The effect of time of emergence and duration on competition between upright starbur and soybean under Zimbabwean field conditions has not been previously studied. Information on the effect of time of upright starbur emergence and duration of its competition with soybean is essential in designing effective control strategies for this weed. The objective of this study was, therefore, to determine the effect of time of upright starbur emergence and duration of competition on growth, development and seed yield of the two species.

MATERIALS AND METHODS

A field experiment was conducted during the 1991/92 and 1992/93 rainy seasons at Chegutu (18^o 11'S, 30^o 20'W). The site has red soils Rhodic Nitisols (FAO, 1988) with a pH of 6.5 (CaCl2) and a clay content of 40%.

Fields were fertilised with 200 kg ha^-1 of compound D (8% N, 14% P, 7% K). Soybean, Cultivar Roan, and upright starbur were treated with thiram at 1 g per kg seed against seedling damping-off fungal diseases. Soybean seed was also inoculated with Rhizobium japonicum in order to enhance nitrogen fixation.

Soybean and upright starbur were planted by hand to achieve a density of 300,000 and 150,000 plants ha^-1, respectively. Upright starbur was planted 3 weeks after soybean planting. Plot size was 7 m long by 3.6 m. Soybean seed was purchased from a commercial seed house in Zimbabwe while upright seed were collected from fields adjacent to the study site. Row spacing was 0.45 m. Upright starbur seeding rate was increased to 375 000 seeds ha^-1 in order to achieve an approximate population of 15 plants m^-1 in each row. This is the average population for a moderate infestation observed under field conditions. This weed does not germinate at once; only about one third of the seeds germinating at a time.

To get upright starbur emerging 2 and 3 weeks after emergence (wae) the weed was planted 1 and 2 weeks after soybean, respectively. Pure stands were planted at the same time as the rest of the plants competing full season. Treatments were replicated four times in a randomised complete block design.

Soybean growth and development was observed at intervals of 15 days after crop emergence (dae) using the growth stages of Herman (1985) as shown in Table 1. Growth stages of upright starbur were also observed at similar times. Height, stem diameter and leaf area were measured on 10 randomly selected plants from the two outer rows of each plot. Grain (seed) yield was measured from the four centre rows. Height was measured using a metre rule, stem diameter using a string around the stem, and leaf area using a leaf area metre model LI - 3100, LI - Cor. Inc, Lincoln, Ne 68505, USA. All other measurements, except grain yield were done at physiological maturity. Plants were divided into three equal parts to get the top, middle and bottom portions. Soybean grain yield and upright starbur seed yield per plant were taken at harvest which was done using sickles when the plants were dry. The plants were then dried in the sun for a week, put in jute bags and threshed, winnowed and weighed using an ordinary field scale. Soybean grain and upright starbur seed yield were adjusted to 13% moisture content. Data were analysed using a two-way ANOVA and the Least Significant Difference (LSD) used to compare treatment means. Data from the two seasons were combined for analysis after Bartletts' tests (Sokal and Rohlf, 1969) had shown homogeneity of variances.

RESULTS

The growth stages of soybean that emerged at the same time as upright starbur were retarded at 15 days after emergence (DAE) as shown in Table 2. Full season upright starbur interference of soybean extended its (soybean) vegetative period by 30 days and resulted in lodged plants with six branches (Table 2). Soon after soybean flowering, subsequent reproductive stages occurred so rapidly that some of them could not be detected between treatments particularly between 90 and 120 DAE (Table 2).

Upright starbur that emerged 2 and 3 weeks after soybean emergence had limited effect on soybean branching but extended flowering by 14 days (Table 2). Plants that emerged 2 and 3 WAE caused a soybean development delay of one reproductive stage compared to those in the pure stand.

Soybean plants that experienced full season interference from upright starbur were significantly (P Soybean grain yield was 1.76 to 3.60 t ha^-1 (Table 6) and competition from upright starbur for the whole season significantly (P Full season competition from soybean severely reduced vegetative growth in upright starbur (Table 7). Plants were thin and as a result, lodged and had fewer branches than those in the weedfree treatment. Weeds which emerged 2 and 3 WAE had spindly, lodged plants with 25% less flowers; flower and seed set in upright starbur were delayed by 15 days (Table 7).

TABLE 1. Soybean development stages followed in the study

--------------------------------------------------------------

R    Reproductive stage. 
R1   One open flower at any node in the main stem 
R2   Open flower on one of the two uppermost nodes of the main

     stem with a fully developed leaf. 
R3   Pod is 5 mm long at one of the four upper most nodes on   
     the main stem with a fully developed leaf. 
R4   Pod 2 cm long at one of the four upper most nodes on the  
     main stem with a fully developed leaf. 
R5   Seed is 3 mm long on pod at one of the four upper most    
     node on main stem with fully developed leaf. 
R6   Pod containing a green seed that fills the pod cavity at  
     one of the four uppermost nodes on the main stem with a
     fully developed leaf. 
R7   One normal pod on main stem that has reached its mature
     pod colour. 
R8   95 per cent of the pods have reached their mature pod
     colour.
--------------------------------------------------------------
Source: Herman (1985)

TABLE 2. The growth and developmental stages of soybean due 10 different durations of interference by upright starbur

Days                 Upright     Upright      Weedfree
after  Full season   starbur     starbur      whole 
emer-  interference  planted     planted      season  
gence                2 WAE* of   3 WAE of 
                     soybean     soybean  
--------------------------------------------------------------
15  Vegetative    Vegetative  Vegetative      Vegetative
    stage         stage       stage           stage, 12
                                              branches

30 Vegatative    Vegetative  Vegetative stage Flower buds   
   and six       stage 12    10 branches      on lower
   branches      branches                     branches  

45 Vegetative   Flower buds on    R1 stage    R2 stage 
   stage and    lower branches 
   semi-lodged 
   plants

60 Flowerbuds     R2 stage        R2 stage    R3 stage 
    on lower 
    branches

75  R1 stage      R3 stage        R3 stage    R3 stage
                                              
90  R3 stage      R6 stage        R6 stage    R6 stage
                                              
120 R7 stage      R8 stage        R8 stage    R7 stage
                                              
135 R8 stage      R8 stage        R8 stage    R8 stage
                                              
150 R8 stage      R8 stage        R8 stage    R8 stage
--------------------------------------------------------------

R1- R8 are soybean development stages shown in Table 1 - Weeks after emergence of soybean.

TABLE 3. Plant height of soybean and upright starbur emerging at different times

Treatment                      Height (cm)  Upright   
                                Soybean     starbur    
----------------------------------------------------
Full season interference          50.6      44.4    
Upright starbur emerging 2 WAE*   58.8      40.0    
Upright starbur emerging 3 WAE    57.5      38.2    
Soyabean pure stand               63.4        - 
Upright starbur pure stand          -       42.0    
------------------------------------------------------
LSD (0.05)                         7.3       NS   
CV (%)                             6.0      13.3   
------------------------------------------------------

*Weeks after emergence

TABLE 4. Stem diameter of soybean and upright starbur emerging at different times

Treatment                     Diameter (mm)  Upright 
                              Soybean        starbur
------------------------------------------------------
Full season interference          23.6    14.6 
Upright starbur emerging 2 WAE*   21.9    13.8 
Upright starbur emerging 3 WAE    22.1    12.9 
Soyabean pure stand               26.0      -
Upright starbur pure stand          -     17.7
------------------------------------------------------
LSD (0.05)                         NS      2.3 
CV (%)                             8.9     9.2
------------------------------------------------------

* Weeks after emergence

Although pure stand upright starbur reached 75% senescence at 120 DAE, it was only 50 and 25% in those which emerged 2 and 3 WAE, respectively (Table 7). The soybean crop did not affect the height of upright starbur (Table 3), but stem diameter in upright starbur was significantly (P Total leaf area at the top and bottom strata was significantly (P<0.05) increased in upright starbur plants under full season interference compared to all other treatments (Table 5,). The increases were 57.0 and 232.0% at the top and bottom, respectively. While soybean generally produced more leaf area in the top stratum, the weed had more leaves in the middle stratum in all treatments except for the full season competition.

TABLE 5. Leaf area of soybean and upright starbur at the 100 (T), middle (M) and bottom (B) strata

Treatment                            Leaf area (cm^-2)
                                 ---------------------------
                                    Soybean       Upright      
                                                  starbur 
                                 -------------  ------------ 
                                  B    T    M    B    M    T
------------------------------------------------------------- 
Full season interference         125   98   84   22   45   63 
Upright starbur emergence 2 wae* 118  105   88   16   48   15
Upright starbur emergence 3 wae  102  105   82   12   39   16 
Soyabean pure stand              122  108   78    -    -    -  
Upright starbur pure stand        -    -     -   14   34    19

--------------------------------------------------------------
LSD (0.05)                        NS   NS    NS  11   12    18
--------------------------------------------------------------

TABLE 6. Grain yield of soybean and seed yield of upright starbur when emerging at different times

Treatment                          Yield (t ha^-1)
                                   ---------------
                                 Soyabean  Upright
                                           starbur
Full season interference             1.76    0.09 
Upright starbur emergence 2 wae*     1.93    0.07 
Upright starbur emergence 3 wae      2.31    0.04 
Soyabean pure stand                  3.60      -
Upright starbur pure stand            -      1.00
--------------------------------------------------
LSD (0.05)                           1.71    0.61 
CV (%)                              11.32   15.00
--------------------------------------------------

"Weeks after emergence

Upright starbur yielded 0.4 to 1.0 t ha^-1 of seed. Soybean significantly (P DISCUSSION

Reduction in the number of branches and stem height in soybean due to competition from upright starbur resulted in fewer nodes and consequently a reduction in grain yield. Delayed flowering in soybean due to competition from upright starbur may be attributed to insufficient nutrients, water and sunlight required to support normal phenological growth. The dynamics of N, P and K are often in favour of these elements moving from older plant parts to young ones which act as sinks.

The phytotoxins present in upright starbur (Chivinge and Mudzana, 1989) probably also interfered with physiological processes in soybean. Despite full season competition from upright starbur, soybean still yielded more than 1.0 t ha^-1.

Soybean height advantage over the weed probably enabled it to capture more photoactive radiation in addition to other consumable resources for flower development and seed set. Upright starbur that emerged 2 and 3 WAE was not competitive enough to affect normal growth, development and yield of the crop adversely. Similar results have been reported by others (Hagood el at., 1981; Chivinge and Mudzamt, 1989; Bosza and Oliver, 1990).

Reduction in branching, flowering and seed set in upright starbur due to competition from the soybean crop accounted for the decline in seed yield of upright starbur. Upright starbur was unable to get sufficient consumable resources for normal development. Plants that emerged 2 and 3 WAE were heavily shaded resulting in a reduction in the potential to produce more seeds. Soybean used most of the resources at the expense of the weed. Radosevich and Holt 1994) have reported similar situations. Reduction in upright starbur or other weed species seed production due to competition from soybean has been documented (Quackernbush and Anderson, 1981; Chivinge, 1990). Shelly et al. (1982) indicated reduction in upright starbur yield due to shading from crops.

Where combine harvesters are used, the increased soybean lodging associated with competition from upright starbur plants will mean increased harvesting losses as the combine harvester cannot pick lodged plants. Delayed senescence of upright starbur weed when the soybean crop is ready for harvesting would result in fleshy leaves staining the grain and consequently lowering the grade and price. However. for hand-harvested soybean, delayed senescence in upright starbur could be of an advantage. The crop will be harvested before burs mature 10 inflict harm to people.

In conclusion, upright starbur competing with soybean full season reduced growth, development and grain yield. Upright starbur which emerged 2 and 3 weeks after soybean emergence, though effectively shaded by the crop still reduced soybean grain yield. Hence, soybean managed to be free of upright starbur up to at least 3 WAE. Emergence of the weed at 2 and 3 weeks after soybean emergence causes its shading by soybean resulting in reduced growth and seed production. Reduced seed production in upright starbur is important to reduce future weed pressure in subsequent seasons.

TABLE 7, The growth and development of upright starbur due to different durations of inteference by soybean

Day   Full season  Weed free   Weed free    Weedfree whole
after interference  2 WAE*      3 WAE       season
plant-
ing
--------------------------------------------------------------
15  Thin plants    Plants just  Plants not    Vegetative
    in vegetative  emerged      yet emerged   state
    stage 
30  Semi-logded    Reduced      Vegetative    Prolific
    plants with    branching,   stage and     branching,
    branches       lodging.Veg- branching     green veg-
    reduced        etative      stage and     etable
    Vegetative     stage buds   flower
    stage          on the lower
                   branches
45  Flowers        Flower buds  Flower buds   75% flowering
    formed on      on lower     on lower
    lower          nodes        nodes and
    branches                    branches
60    75%            50%           50%         75% flowering 
    flowering    flowering    flowering 
75    100%           75%           75%        100% flowering 
    flowering    flowering    flowering 
90  Seed set on      75%          100%       Seed set on lower

    lower        flowering    flowering      branches 
    branches
105 Lower,          100%      Seed set       Lower leaves 
    leaves       flowering    burs getting   yellow 
    yellow, fruits            dark and firm 
    drying 
120    75%        Seed set        25%         75% Senescence 
    Senescence                Senescence
135 Senescence       50%      Senescence      Senescence 
                 Senescence
150 Senescence   Senescence
--------------------------------------------------------------

* Weeks after emergence

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