1. RESPONSES OF NERICA RICE TO WEED INTERFERENCE IN SAVANNAH UPLANDS
I. K. Dzomeku1, W. Dogbe2,
E. T. Agawu1 and I. T. Duku1 1University for Development Studies, Tamale. 2Savannah Agricultural Research Institute, Tamale.
Africa Rice Congress, Dar Es Salaam Tanzania.
31st July – 4th August

2. ORDER OF PRESENTATION INTRODUCTION OBJECTIVE MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSION

3. 1. INTRODUCTION
With the development of NERICA rice (New African rice) varieties by WARDA and the programmed dissemination of at least NERICA 1 during the 2006 cropping season to Ghanaian farmers,
There was the need for the National Agricultural Research system to develop suitable agronomic packages for farmers to enhance the performance of the crop and farmer adoption rate.

4. 1. INTRODUCTION 2
Weeds are major constraint to increased rice production and farmers spend many hours hand weeding (Akobundu 1987); in face of lack of labour (Tollens, 2006).
Weeds interfere with rice growth and development by (1) reducing the available light, nutrient, water, CO2 and space, (2) secrete toxic exudates into the soil that depress growth and development of rice. (3) Harbour various pests (Moody, 1994, FAO, 1996).
The longer the weed-rice association remains, greater the reducing effects on rice productivity (Akobundu 1987, 1991; Moody, 1994).
Understanding “how long” weeds-rice could associate without damaging effect on rice is key to formulation of sustainable integrated weed management alternatives.

5. 2. OBJECTIVE
Determine the critical period of weed interference (i.e “how long” can weeds-rice coexist for onset of economic loss in rice productivity?).
It is necessary to help determine timing and targeting of weed control interventions to most crucial periods to avoid economic yield loss,
Helps to determine single weed control measures likely suitable for integration, and
Helps efficient use and management of farm labor.

6. 3. MATERIALS AND METHODS
Location of Trials: Tamale at the Savannah Agricultural Research Institute (SARI) in the Northern Guinea Savannah ecological zone of Ghana.
Rainfall: Unimodal pattern-mean annual rainfall of mm. Fairly distributed from April-November.
Temperature -mean monthly minimum of 23.4˚C and maximum of 34.5˚C.
RH: Minimum RH of 46% and maximum of 76.8% (SARI Annual Report, 1997).

7. 3.1. Experimental design and materials used
Ten treatments were compared in a randomised complete block design in four replications.
In one set of treatments, plots were kept weed-free up to 3, 6, 9 and 12 WAP and subsequently left weed-infested until harvest.
In the other set of treatments weeds were allowed to interfere with the crop for periods up to 3, 6, 9 and 12 WAP and subsequently kept weed-free until harvest.
Two checks were included as full-season weed-infestation or season-long weed-free regimes

8. 3.2. Agronomic practices 1
Weeding was done every 3 weeks, starting from the third week after planting with a small hand hoe.
Plot size was 5 m × 5 m,
New Rice for Africa (NERICAs 1 and 2 were evaluated in experiments 1 and 2 respectively).

9. New Rice for Africa at maturity
NERICA 2
NERICA 1

10. 3.3. Agronomic practices 2
Fertilizer rate of kg/ha NPK was applied as split,
basal fertilizer at a rate of kg/ha at 2 WAP.
N with urea fertilizer drilled between drills second application was broadcasted 5-6 WAP.

11. 3.4. Data collected (SES) Soil gravimetric moisture content (%)
Plant height
Leaf area index (Watson, 1952)
Tiller count/m²
Weed species and species dominance
Straw weight
Grain yield

12. 3.5. Weed species and species dominance
1 m² quadrat sampling unit for weed dominance data and identification.
The quadrat was thrown on the two diagonal transects of the field for frequency and density data summed dominance ratio (SDR) of weeds, determined by the relationship:
½ (F/∑F + D/∑D), where F = frequency of occurrence of a weed within the field, D = density of its occurrence on the scale of 0-4, where 0 = zero occurrence of a species per 1m² and 4 ≥ 20 stands of weed (Dangol, 1991).

13. 4. RESULTS 4.1. Gravimetric soil moisture content
Site 2
Site 1

14. 4.2. Plant height of NERICAs at 12 WAP

15. 4.3. Plant height of NERICAs at 15 WAP

16. 4.4. LAI of the NERICAs at 50DAP

17. 4.5. Tiller count/m² of Nerica 1 at 50DAP

18. 4.6. Straw yield of NERICA 1-site 1

19. 4.7. Grain yield of NERICA 1 - site 1

20. 4.8. Grain yield of NERICA 2 - site 2

21. NERICA 1

22. Nerica 2

23. Table 2. Relationship between grain yield and growth parameters of NERICA 1 and 2

24. Table 2. Relationship between growth parameters and grain yields of Nerica 1 & 2.

25. 5. DISCUSSION 1
Changes in gravimetric soil moisture content could be attributable to the fluctuation in rainfall during the season.
The fluctuation did not however, show any visual effect on the vegetative phase but potentially yields could been higher under more stable moisture.
The varieties however have an in-built ability to withstand short periods of drought (WARDA, 1999).

26. 5. DISCUSSION 2
At 12 and 15 WAP, the mean plant height for plots weeded for 6 weeks or more were similar to the weed-free check suggesting the required optimum weeding regime to maximise plant height was c. 6 WAP.
Indeed the 2 varieties attained 100 cm plant height within 6 WAP under 6 weeks of continuous weeding.

27. 5. DISCUSSION 3
Similar to other results, crops kept weed-free until harvest gave maximum LAI value similar to plants given continuous weeding up to 6-12 WAP showing absence of weed interference within 6 WAP, could optimise attainment of LAImax.
Weed infestation of the Nericas exceeding 6 WAP timings resulted in poor LAI, suggesting that the crop is a poor competitor with weeds beyond this period of development.
Usually LAI of rice is closely related to grain yield because at flowering it greatly affects the amount of photosynthates available to the panicle (Yoshida and Parao, 1976).

28. 5. DISCUSSION 4
Crops kept weed-free at least up to 6 WAP had better tillering plants kept initially weed-infested and until harvest probably due to the genetic tillering ability of ativa parent.

29. 5. DISCUSSION 5
Grain and straw yields was optimised by crops kept weed-free for at least up to 6 WAP suggesting the frequent removal of weeds eliminated weed interference resulting in enhanced growth performance such as in tiller numbers, with consequential higher grain and straw yields.
Notably an initial weed infestation for only up to 3 WAP did not affect most parameters including the grain yield compared with weed-free check. In effect, early weed infestation for maximum period of 3 WAP before weed removal might not cause any yield reduction.

30. ACKNOWLEDGEMENT
We are grateful to WARDA for financial support for the project and sponsorship of the Congress

31. 6. CONCLUSION
Weed infestation 6WAP or more significantly depressed grain yield and other growth parameters, especially plant height (at 12 and 15WAP), LAI and tiller count/m².
Initial weed infestation up to 3 WAP did not have adverse effect on the varieties.
Weed-free environment up to 6WAP recorded notably similar grain yield, LAI and tiller count/m² with the weed-free check.
The critical period of weed interference likely lies between 3 and 6 WAP(as reported for rice Akobundu, 1987).
As such two hand weedings at 3 and 6WAP could protect the crops against weed interference with consequential optimum productivity of Nerica 131 and 2.

32. Thank you