Eco-Physiological Studies on some Important Weeds of Wheat

Abstract

Reproductive Potential

1. Studies on the reproductive potential (i.e., fruit and seed yield) of seven weed species, form irrigated wheat fields of Lahore and four weed species from rain-fed (Barani) wheat fields of Maldeo District Jhelum were carried out.

2. Weeds of irrigated wheat fields are divided into three groups according to the order of their reproductive potential i.e., Chenopodium album and Rumex dentatus with every high; Melilotus parviflora and Phalaris minor medium; Medicago denticulate, Convolvulus arvensis and Lathyrus aphaca with moderate to low reproductive potentials.

3. Weeds with lowest fruit and seed yield i.e., L. aphaca and C. arvensis> M. denticulata> M. parviflora>P. minor>R. dentatus> C. album. Seeds of L. aphaca and C. arvensis were 20 to 25 times heavier than those of R. dentatus and C. album.

4. Amongst the weeds of rain-fed wheat fields Asphodelus tenuifolius has highest output of fruits and seeds followed by Malva parviflora and Trigonella Vicia sativa has lowest reproductive potential but heaviest polycerate seeds.

5. Frequency and Constancy

19 weed species were recorded in February 1982, from 20 irrigated wheat fields around New Campus. Lahore. Of these Medicago denticulate, Melilotus parviflora, Anagallis arvensis, Chenopodium album, Fumaria indica, Senebiara didyma and Cynodon dactylon showed a high degree of frequency of occurrence and constancy.

These weeds were not uniformly distributed in the various fields and exhibited dynamic changes. Senebiara didyma and Cynodon dactylon showed a gradual decline in its frequency of occurrence from all fields with the passage of time. Cyperus rotundus, on the ither hand became frequent and wide spread in many fields by April, 1982. Phalaria minor and Lathyrus aphaca though restricted only to be a few fields were very abundant in them.

6. Only 14 weed species were recorded from 20 barani (rain-fed) wheat fields of Maldeo, Distt, Jhelum, of which Trigonella polycerate, Asphodelus tenuifolius, Anagallis arvensis, Vicia sativa, Convolvulus arvensis and Fumaria indica were most prevalent and abundantin February 1982. By April, 1982 F. indica disappeared from all the field and C. rotundus instead had appeared in great abundance in most of the fields.

7. Although quite a good number of weeds are common in irrigated and rain-fed wheat fields; the major weed species with high degree of frequency and constancy are quite different in the two areas. Trigonella polycerata, Asphodelus tenuifolius followed by Cyperus rotundus and Convolvulus arvensis are most common in barani wheat fields while Medicago denticulate, Melilotus parviflora, Senebiera didyma, Chenopodium album and Phalaris minor commonly occur in irrigated fields.

Weed Density

8. Weed density per unit area in the various fields is also variable. Usually one or two weeds dominate isn a field as the wheat crop reaches maturity. M. parviflora, M. denticulata, A. arvensis, F. indica, P. minor and S. didyma show a very high level of relative density in the irrigated fields in which they occur.

9. A. tenuifolius and T. polycerate exhibited highest population density in all barani wheat fields. C. rotundus replaces F. indica in most of the barani wheat fields late in the season and also show good density.

10. Weeds show a marked reduction in their density per unit area with the passage of time both in the barani and irrigated wheat fields.

11. In contrast to most of the irrigated wheat fields, wheat density per unit area in rain-fed wheat fields was much lower; consequently total weed proportion in these fields was much higher i.e., upto 85%.

12. P. minor, L. aphaca and C. arvensis were the dominant weeds both with respect to density and biomass per unit area in their respective wheat fields in irrigated areas. L. aphaca caused maximum reduction i.e., 30% in the total biomass per unit area as compared with 17%and 14.4% reductions by P. minor and C. arvensis in their respective fields.

13. In rain-fed wheat fields T. polycerate and A. tenuifolius were dominant both with respect to density and biomass per unit area in their respective fields. On the average these weeds caused about 25% reduction in total biomass per unit area in their respective fields.

14. Soil from irrigated and rain-fed wheat fields was analysed both for the physical and chemical characteristics. Soils of rain-fed wheat fields are predominantly sandy (loamy sand) with relatively lower moisture content; water holding capacity and organic matter content as compared with the soils of irrigated wheat fields.

15. Differences in the chemical characteristics were also market. Calcium and Potassium contents in rain-fed fields were higher than their irrigated counterparts. On the other hand, average values of Exchangeable and Soluble sodium as well as Sodium Adsorption Ratios were higher in soils of irrigated as compared to the rain-fed wheat fields. CEC in irrigated fields was of medium type and almost double that of their rain-fed counterparts (low type). Both types of soils are salt free (non-saline) with EC in circum-neutral (pH;7.5) rain-fed wheat fields being slightly higher than rather basic (pH:7.79) irrigated wheat fields. The soils of both types of fields are non-sodic Seed Germination.

16. Seeds of M. denticuluta, M. parviflora, Carthamus oxycantha, C. oxycantha, C. arvensis, L. aphaca and P. minor from irrigated and A. tenuifolius, T. polycerata and V. sativa from rain-fed wheat fields were exposed to different temperature regimes (5-30ᵒC) both in light and dark. Different weeds were favoured by different temperatures as regards their optimum germination.

17. Low temperatures (5 to 10ᵒC) greatly favoured germination in M. denticulata and M. parviflora both in light and in dark. C. arvensis responded positively toa wider range of temperatures in light as well as in dark. Germination increased progressively from 10ᵒ to 25ᵒC followed by slight decline at 30ᵒC in light. In dark there was a sharp decline at 25ᵒC. In P. minor, germination percentage increased with the percentage increase in temperature upto 20ᵒC followed by sharp decline at 25ᵒC. At all temperatures light favoured germination. Optimum germination (94%) occurred at 25ᵒC in light. In C. oxycantha, 15ᵒ and 20ᵒ caused remarkably high percentage germination both in light and dark with highest values at 20ᵒC in dark. Higher temperatures had a depressing affect with negligible percentage germination both at 25ᵒand 30ᵒC temperatures. L. aphaca showed a sharp rise in germination (95%) at 20ᵒC both in light and dark followed by slight to moderate decline at higher temperatures. Higher temperatures of 20 to 30ᵒC also stimulated early germination in contrast to the low temperatures.

18. Of the barani wheat fields, A. tenuifolius showed a maximum of 35% germination at 20ᵒC both in light and in dark. In V. sativa there was a sharp in germination from 15ᵒ to 20ᵒC which continued steadily upto 25ᵒC with around 90% germination both in light and dark. There was a slight decline at 30ᵒC. T. polycerata showed negligible (less than 10%) germination at all temperatures.

Weed-Wheat Competition.

19. On the basis of the results of various field studies on the frequency, constancy, density and biomass per unit area of three weeds, five weeds of irrigated wheat fields i.e., M. denticulata, M. parviflora, P. minor, C. arvensis and L. aphaca were selected for a weed-wheat competition experiment.

20. The experiment was carried out in a wire-netting enclosure under natural conditions at New University Campus, Lahore. Different densities of these weeds were grown on competition with a constant wheat density per unit area. There were six weed density treatments varied from 0.5:1 to 6:1 except M. denticulata and L. aphaca.

21. Tow harvests were taken to measure the impact pf weeds on wheat growth and yield. Harvest I was taken at the prime of vegetative growth of both wheat and weeds i.e., 14 weeks from sowing. Harvest II was taken at maturity i.e., 23 weeks from sowing.

22. M. denticulata, P. minor and mixed weeds caused significant reductions in wheat biomass at 1:1 weed-wheat density ratio. This reduction in wheat biomass per unit area increased gradually with increasing weed densities. At harvest I, different weeds can be arranged in the following order according to relative loss of wheat biomass caused by them. M. denticulata> P. minor=mixed weed= M. parviflora> L. aphaca> C. arvensis.

23. There was a marked reduction in the actual weed-wheat density ratio in the various treatments at the time of harvest II because of thinning of weeds especially in the high density treatments. L. aphaca and M. parviflora were most while P. minor least sensitive to density pressure.

24. As regards inter-specific competition of weeds in the mixed weed plots, P. minor was dominant both in number and biomass per unit area at maturity followed by M. parviflora, L. aphaca, M. denticulata and C. arvensis respectively.

25. Mixed weeds and M. parviflora caused significant reduction in grain yield even at the lowest (0.5:1) weed-wheat density ratio. P. minor and M. denticulata caused significant reduction at slightly higher density levels. L. aphaca caused significant reduction only at higher density levels while C. arvensis was effective only at the highest (6:1) weed-wheat density ratio. The loss in grain yield similar to that caused by mixed weeds. M. parviflora at higher density levels was relatively less and C. arvensis least effective.

26. Marked differences in grain reduction caused by equivalent densities of different weeds can be attributed to the differences in their own survival and biomass per unit area at different density levels.

27. On the basis of the experimental evidence of the present investigation in relation to weed density under field conditions, it is suggested that the grain loss in wheat fields would be in the range of 22 to 40% depending on the actual weed-wheat density ratio at the time of crop maturity.

Nutrient Losses

28. Nitrogen, Phosphorus and Potassium losses caused by these weeds competing with wheat at different density levels as described earlier were also investigated.

29. N. P. K. uptake by weeds per unit area increased with increase in their density at the expense of corresponding decline in N. P. K. content of wheat. Most significant losses were caused by P. minor, M. parviflora, M. denticulata, L. aphaca and mixed weeds at all and especially at higher density levels.

30. There was a reduction in total nitrogen exploitation per unit area collectively by wheat and non-leguminous weeds (i.e., P. minor and C. arvensis) in all the treatment s whereas of the leguminous weeds i.e., M. denticulata and M. parviflora at all and L. aphaca at higher density levels caused a marked increase in the total nitrogen uptake per unit area. Total phosphorus and potassium exploitation per unit area collectively by wheat and all the weeds in different density treatments also showed a marked reduction proportionate to weed density.

31. Nitrogen content (dry wt. nitrogen percentage) of all the weeds at all density levels were much higher than that of the wheat of the respective treatments especially in M. denticulata, L. aphaca, C. arvensis and mixed weed treatments followed by M. parviflora and P. minor.

Phosphorus content (dry weight percentage) of M. denticulata in all density treatments were markedly higher than their wheat counterparts. Wheat phosphorus content showed successive reductions with increasing weed densities. On the whole, similar but not so marked response was exhibited by other weeds except C. arvensis. Here phosphorus content of wheat at all density levels were higher than that of its weed counterpart.

Dry weight potassium percentage in wheat was much higher than that of either M. parviflora, C. arvensis and P. minor at all density levels and M. denticulata only at higher density levels. However in mixed weeds and L. aphaca, potassium contents were higher than their counterparts at all density levels.

32. N. P. K. content of individual plant parts i.e., shoot and root of various weeds and wheat competing at different density levels were also determined. Root and shoot nitrogen content in all weeds (except root nitrogen in C. arvensis and P. minor) were higher than their counterparts at all density levels.

Root and shoot phosphorus content of P. minor, M. denticulata, L. aphaca (shoot only) and mixed weeds at all density levels and in M. parviflora only at higher density levels were higher than their wheat counterparts. The pattern was reverse in C. arvensis with root phosphorus content much lower than the wheat counterpart at all density levels.

Dry weight potassium % of shoot in P. minor, M. parviflora, C. arvensis at all and M. denticulata at higher density levels were lower than their wheat counterparts. In mixed weeds and L. aphaca shoot potassium contents were higher that their wheat counterparts. Response of root was varied. In P. minor and M. denticulata root potassium contents declined with successive density increases; in M. parviflora and L. aphaca it increased with increasing density levels and at higher density levels they were higher than their wheat counterparts. On the other hand in C. arvensis and mixed weed values of root potassium contents were higher than their wheat counterparts at all density levels.

33. Similar trends of N. P. K. losses of wheat were maintained at crop maturity. At 6:1 density ratio maximum N. P. K. losses were 47,52 and 56% respectively. P. minor, M. parviflora, mixed weeds and L. aphaca were relatively more effective than C. arvensis. At lower density levels M. denticulate caused maximum losses and at its highest ratio of 3:1, wheat P and K losses were more or less similar to those P. minor, M. parviflora or mixed weeds at 6:1 density ratio.

As regards individual plant organs, nitrogen and potassium contents of wheat grains were not affected by the increasing weed densities. However there was some reduction in their phosphorus contents at 1:1 density ratio of all the weeds especially P. minor, M. denticulata, M. parviflora and C. arvensis. This reduction gradually increased with increasing weed densities.

34. Increasing weed densities also increased soil N. P. K. losses as compared with the control. Since leguminous weeds (M. denticulata, M. parviflora and L. aphaca) especially at higher density ratios are capable of fixing adequate amounts of atmospheric nitrogen through their root nodules; they caused relatively less soil-N losses as compared with non-leguminous weeds.