The Control of Established Weeds with Foliage-Applied Herbicides

Chapter 7

THE C O N T R O L OF E S T A B L I S H E D WEEDS WITH FOLIAGE-APPLIED HERBICIDES

Apart from the principal use of herbicides, for pre-emergence control of weeds in cane fields, there should be other effective measures at our disposal for post-emergence situations. For example, a lingering monsoonal rain during planting may make the field application of pre-emergence herbicides impossible for an autumn crop; and when the rain stops, the weeds have already appeared profusely. Sometimes, even the planting is postponed due to untimely bad weather, and has to be carried on among the established weeds. On non-cropped areas where cultivation is never done, the weedy vegetation is present all the year round, and weed control is especially needed. To deal with such situations we need not only to wipe out the established weeds but to prevent their regrowths for a desirable period. This can be achieved by combining a contact and a residual herbicide in one overall or directional post-emergence application. With a knapsack sprayer it is convenient to direct the nozzle and sprays to cover and rinse the weedy vegetation and soil surface beneath, in order to utilise fully both kinds of herbicide and to avoid the cane plants, for their protection.

1. CONTROL OF EMERGED B R O A D L E A F WEEDS A N D CYPERUS

SPECIES WITH 2,4-D

Among the herbicides provided for evaluation by chemical companies in Taiwan, the sodium salt of 2,4-D was first tested and recommended for use in the cane fields nearly 30 years ago (Chow et al., 1956). Applied either pre-emergence or post-emergence, and having both translocative and residual activities, this compound kills broadleaves, and is especially effective in controlling nut-grass, the commonest species in all cropped and non-cropped lands in subtropical regions. Before the introduction of chemical weed control, nutgrass was usually regarded as a nuisance, because even frequent hand-hoeing could do no more than superficially cut the top growths, and the weed soon resprouted from the seemingly inexhaustible underground nuts (tubers). According to Chang and Sze (1963), the nuts are able to germinate and send out shoots even after being buried at 50 cm depth. Their surprising vigor and ability to multiply is demonstrated by the fact that 368 tubers can be produced from one seed tuber in three months. The sodium salt of 2,4-D applied at the rate of 1.6 kg/ha of the active ingredient found capable of eliminating this weed after repeated postemergence sprays on regrowths. The tuber systems were completely destroyed by this translocative herbicide. The sodium salt of 2,4-D was accepted for general weed control by most of the sugarcane plantations after a local pesticide manufacturer, being part of Taiwan's sugar industry, had been able to synthesize it and distribute it at a fairly low price.

178 Typically its use is one pre-emergence application of 3 kg/ha, followed by two to three post-emergence applications of 2 kg/ha each, during the first five months of growth and before spaces are shaded by the close-in cane leaves. Gramineous weeds such as Elusine indica, Cynodon dactylon, Panicum repens, and broadleaves such as Physalis angulata and Mimosa invisa, being more tolerant, increased, apparently at the expense of those weeds which were controlled. The relatively short residual life of 2,4-D (two to three weeks) and the ease with which it decomposes in the soil accounts for some erratic results. To compensate for the shortcomings of the sodium salt of 2,4-D, one of the substituted ureas and triazines supplied for weed control a few years later was Karmex (diuron) and atrazine. Both compounds were found to control all broadleaves and most gramineous weeds for two to three months when they were used in pre-emergence application at rates of 3 to 5 kg/ha respectively (dosages that present the least danger of phytotoxicity to germinating cane seedlings). These herbicides have only a limited effect on Cyperus rotundus, and on perennials such as Panicum repens and Cynodon dactylon, at the pre-emergence application rates. The chemicals remain on the surface, and so are usually less effective in preventing deep-seated tubers of nutsedges and old rhizomes of perennials from sending out new shoots later. In practice both herbicides, at rates of 2 kg/ha for diuron and 3 kg/ha for atrazine, are used in combination with the sodium salt of 2,4-D at 2 kg/ha to compensate for the limitations of the individual herbicides, thus broadening the spectrum of weeds controlled. Another advantage of using the mixture is that it is less expensive at these application rates. The quantities of the high-priced diuron and atrazine can be reduced, while retaining the herbicidal effectiveness (due to the synergism of the compounds) of the mixture as a whole. Such pre-emergence application serves to suppress initial germination of all weeds in cane fields. However, one or two sprays of the sodium salt of 2,4-D (2 kg/ha each) one month later are still needed in order to kill any scattered regrowths of nutsedges that might not have been destroyed by the preemergence application. Figure 32 shows the results of weed control about two months after application of the pre-emergence herbicides followed by supplementary application of the 2,4-D sodium.

2. DANGER OF INDISCRIMINATE USE OF 2,4-D

Unaware of the systemic injury that might be caused by the auxin-type herbicide to cane plants, some plantations, more than a decade ago, preferred to use the sodium salt of 2,4-D in overall application as frequently as possible (in one instance up to ten applications were made over one crop). They hoped to attain an early 3—4 months' weed control for a crop by making such multiple applications to compensate for the short persistence of this herbicide. There was a sufficient supply of this locally-synthesized, cheap product at their disposal, and so the plantations often used it instead of the imported, and much more expensive diuron or atrazine, which, in soil treatment, could have achieved better results. Subsequently, there were large areas of affected young stalks, which produced retarded internodes, particularly at lower

179

Fig. 3 2 . Weed-free conditions of a cane field, about two months after application of the preemergence herbicides followed by supplementary post-emergence 2,4-D sodium to control all seed-germinated weeds and nutsedges.

portions, from which adventitious roots grew — the typical toxic symptoms caused by this hormone-like herbicide. The growers, however, suspected that this was caused by the prevalent underground pests that preyed on the root-stock of sugarcane. In order to convince the plantation-owners that this anomaly was the result of improper use of this auxin-type herbicide, an experiment with a different number of foliar applications of 2,4-D sodium, repeated on the cane variety F 156 growing in a nursery, was conducted in 1970—71, and the results are shown in the following table: N o . of foliar applications of 2,4-D Na at 1.6 kg ai/ha accumulated at intervals of 15 days

Unsprayed CK 1 2 3 4 5 6 7 8 9

Observed 6 m o n t h s after planting single-eyed cuttings o n July 2 6 , 1 9 7 0 (average of 4 replicates) N o . of stalks per plot

Wt. of stalks kg/plot

%

48 55 51 48 49 50 49 64 45 50

5.84 5.17 4.48 4.30 4.10 3.95 3.85 3.80 3.62 2.72

100.0 90.3 76.7 73.6 70.2 67.6 65.4 65.2 62.0 47.3

180 It can be seen that plant weights of the 6-month-old cane decreased proportionally to the increase in number of foliar applications of 2,4-D sodium starting from the sprouting of the cane plants. Just one application of this translocative compound reduced the plant weight by 10% compared to unsprayed plants. With less than ten applications, 52.7% loss was incurred. Figure 33 shows the range of reductions in plant weight per plot caused by accumulated applications of this compound. That the number of sprouting shoots per plant was not affected, but all the stalks showed retarded elongation, and that adventitious roots were produced from the shortened basal internodes, are typical phyto toxic symptoms caused by the systemic actions of 2,4-D. Furthermore, not only did this herbicide's foliar applications result in this serious injury to sugarcane, but its one pre-emergence treatment at the same dose, commonly thought harmless to sugarcane because of its weak and short residual activity in soil, also caused 50 days' suppression in the plant growth of another susceptible variety, F 153, as mentioned in chapter 5. The plantation-owners were then warned of the danger of indiscriminate use of 2,4-D, and advised that no more than two directional foliar applications for killing nutsedge among young cane should be used, following diuron or atrazine, for more effective pre-emergence treatments.

Fig. 3 3 . Proportionate reduction in plant weight of the 6-month-old cane of F 156 caused by foliar application of 1.6 kg ai/ha 2,4-D sodium accumulated from 1 to 9 times, at intervals of 15 days.

181 3. REINFORCED P A R A Q U A T FOR CONTROL OF ESTABLISHED WEEDS

When using herbicides to control the common, seed-germinated annual weeds in sugarcane, the most efficient technique is to broadcast a pre-emergence treatment under conditions which will allow for the normal functioning of the toxicants in the soil. In this way, the germinating weed seeds can be killed by the active elements of the herbicides at the minimum effective concentration with the least possible adverse effect on the cane plants. However, in the cane belt of southern Taiwan, where a distinct alternation of rainy and dry seasons is typical of its sub-tropical climate, the planting of cane and the following application of pre-emergence herbicides each fall are often interrupted by prevailing monsoon rains. The prolonged heavy rain which usually accompanies a typhoon may force the cancellation of scheduled pre-emergence spraying and, in many cases, delay the planting of the cane for a few weeks after the preparation of the seed bed. During the delay caused by the rains, the field is open for the unchecked infestation and establishment of the weeds. With both cane and weeds emerged, the blanket spray originally recommended for pre-emergence treatment, diuron -I- 2,4-D Na at 1.6 kg/ha each, performs poorly, particularly against some gramineous species. In some cases, an indiscriminate application of the mixture would cause severe injury to the cane seedlings of a few susceptible varieties. Even if a spraying of pre-emergence herbicides is completed at the proper time, a sudden monsoon rain may follow, as they so often do, and neutralize the effectiveness of the herbicides. In such cases, the weeds appear as usual. This illustrates how easily the residual activity of soil sterilants, particularly the atrazine and 2,4-D mixture, is influenced by the combination of intensive rain and high temperature during the fall planting period. New techniques of chemical weed control with other types of herbicides are, therefore, needed to kill the emerged weeds in cane, while giving the cane plants fairly good protection. Such a one, if commercially feasible, would give the cane growers a remedy for weed control when the pre-emergence treatment fails. In cases where the preemergence treatment is especially hazardous, such as hillside plantations where sheet erosion of land during the rainy season can occur, such a technique could be used instead for controlling weeds in autumn-plan ted sugarcane crops. A series of field experiments to this purpose was conducted (Peng, 1969); the results can be summarised as follows: ( 1 ) Herbicidal properties of paraquat Early in 1962, a few samples of Gramoxone, the commercial formulation of , 1 ,r-dimethyl-4,4 -dipyridylium dichloride (paraquat dichloride) were obtained for weed control tests at the Taiwan Sugar Research Institute. The compound was compared, on an active-ingredient basis, at 1.3 kg/ha, 2.6 kg/ha, and 1.3 kg/ha + 0.01% Agral 90 (a wetting agent), along with other compounds in a primary evaluation trial with an autumn-planted crop of varieties F 148 and N:Co 310. The data for weed control performance, and the effect on the cane plants of this chemical, are shown in Table 7.1.

182 TABLE 7.1 Weed control performance of paraquat (Gramoxone) and its effect on cane plants of the 1 9 6 3 - 6 4 autumn crop Percentage of weed cover

Paraquat applications (kg/ha)

Post-emergence

Pre-emergence

1.3 2.6 1.3 + 0.1% Agral 9 0 Hand-weeding

25 th day

60th day

Before treatment

15th day

40th day

14.0 14.0 10.0 35.0

100.0 100.0 75.0 100.0

24.0 50.0 48.0 20.0

4.0 1.0 1.0 90.0

40.0 28.0 17.0 100.0

Effect on cane Stalk count ( 1 2 m row)

Stalk length (cm)

1.3 2.6 1.3 + 0.1% Agral 9 0 Hand-weeding

One m o n t h after treatment

10 months after treatment

One m o n t h after treatment

10 months after treatment

(pre)

(post)

(pre)

(post)

(pre)

(post)

(pre)

(post)

25.9 23.6 28.5

7.2 12.4 16.0

243.0 239.6 249.9

224.9 227.1 235.0

53.0 68.0 130.5

35.0 46.5 47.0

172.5 185.0 182.0

128.0 147.5 148.5

26.0

31.4

237.0

260.8

112.0

63.0

205.0

152.0

Cane planted August 11th, pre-emergence application August 14th and post-emergence application September 9th.

The data illustrate that when paraquat was sprayed as a pre-emergence treatment, it was almost ineffective in killing weeds, at all three rates used. The tillering of sugarcane during the early growing stage, however, was somewhat adversely affected due to an incomplete cover of soil on the cane cuttings which exposed part of the live buds to the herbicide sprays. The cane plants, however, later recovered with normal tillering. The elongation of the cane stalks was not influenced in this case. When the herbicide was broadcast sprayed over the tops of both cane and weeds in a post-emergence application, all the broadleaved weeds and the greater part of the grasses were killed within a few days with the 2.6 kg/ha rate, the herbicide containing the 0.1% tank mix of Agral 90 being the most effective. About six weeks later all the weeds, particularly the nutsedges, resumed free germination, indicating an end of the herbicidal effect. In addition, the one-month-old cane seedlings which received sprays of paraquat became desiccated. However, they resumed growth, and new tillers increased gradually, so that little difference between the herbicide-treated and handweeded plants was found in observations 10 months later. These results confirmed the characteristics of paraquat put forward by Robson

183 and Procter (1963). Their findings indicated that: (1) paraquat acts by contact on the green photo synthetic tissues of plants only; (2) it acts consistently, with results independent of the presence of moisture in the root zone, a factor which is required for residual herbicides; (3) it acts rapidly after application; (4) it is completely inactivated on contact with the soil; and (5) it does not penetrate brown, non-photosynthetic tissue. (2) The use of paraquat for weed control in sugarcane Paraquat has been used as a single herbicide for weed control in sugarcane in most parts of the world. According to Darter (1967), to control heavy infestations of Cyperus esculentus in young cane in South Africa, Gosnell and Thompson recommend over-the-row spraying with paraquat at 0.25 to 0.75 lbs. per acre before the young cane reaches the six-leaf stage and about three weeks after the germination of the Cyperus. With this timing, the maximum translocation of paraquat within the weed takes place, giving good control. The cane suffers a severe reduction in the shoot population, but this is followed by an increase in tiller formation. Tests by Myatt in Queensland showed similar results: good weed control and recovery of the cane plants from extensive leaf and shoot desiccation after 0.25 lbs. per acre of paraquat was sgrayed over the rows of the cane crop. In order to minimize damage to the cane plants which usually occurs in over-the-row applications, or in blanket post-emergence applications, directed spraying with the paraquat is suggested, with the work by Orsenigo in Florida as an example. (3) Use of paraquat reinforced by residual compounds for directed applications

post-emergence

To achieve an effective control of weeds in cane after emergence, the herbicide used should not only be potent enough to knock down weedy topgrowths, but should also have, for a sufficient period, effective residual activity against any regrowth from storage roots and any regermination from seeds. Both actions of the herbicide should be selective, so that the crop yield of sugarcane plants is not adversely affected. Owing to the characteristics of paraquat mentioned above, it is ineffective to use this chemical in directed sprays to minimize the contact damage to the cane plants. Even at the high rate of 2.6 kg/ha, the weed control achieved by paraquat only lasts about six weeks. For the autumn-plan ted crop in Taiwan, the weed control needed is as long as five months, to cover from cane germination to the close-in of the cane leaves. Therefore, the addition of some soil sterilants to paraquafs knock-down property is necessary to give a residual effect against re-emergence of the weeds. Paraquat reinforced with dalapon and 2,4-D, at 0.64, 4.25 and 1.6 kg ai/ha, respectively, as directed post-emergence application (DPA) treatments was tested with two successive (1965—66 and 1966—67) crops in Taiwan. Such DPA treatments conducted at the end of the monsoon for both crops afforded the best weed control and the least effect on the cane. This was because after the mixture had completely killed the emerged weeds among the cane plants by contact actions, the

184 weak residual activity of the mixture in the soil was able to control the subsequent regrowth in the following dry season, freeing the cane plants from weed competition and possible herbicidal effects (Peng and Sze, 1967; 1969b). Encouraged by these results, certain compounds, known as effective pre- and postemergence herbicides, were used in comparison studies with the reinforced paraquat in DPA treatments under different conditions of weed composition, cane cropping, and soil types and climates. For the tests, two autumn crops were planted separately in early August 1966 on a location (plantation) in Huwei (cane variety F 146, sandy soil) and Nanchow (cane variety N:Co 310, gravelly soil): a northern and a southern district of Taiwan's cane belt. Another spring crop of F 156 was planted, early February 1967, on a location in the middle district of Chaotow with clayey soil. All the pre-emergence treatments of herbicides followed within a week from planting the cane. The DPA treatments were conducted about one month later for the autumnplanted crops, when the weeds had established themselves (quickly during this warm season). Only three months later did the DPA treatments follow the spring-planted crops in Chaotow, due to the slow and scarce growth of weeds under the cold and dry climate. In addition to the paraquat combinations, the DPA treatments included: linuron + dalapon + 2,4-D; bromacil + 2,4-D; diuron + 2,4-D; and diuron + dalapon + 2,4-D, each combination at three rates for each compound except 2,4-D, which was applied at a constant rate of 1.6 kg/ha. To minimize contact of the sprays on the cane plants in the DPA treatments, 2,4-D was used alone in one location as a broadcast pre-emergence spray. This technique was used to delay the growth of the weeds so that the spray nozzles used in the DPA treatments could be kept as low as possible, optimally covering just the shorter weed population, without reaching the top leaves of the cane plants. In two other locations, however, an improved technique was devised. Diuron plus 2,4-D, each at 1.6 kg/ha, were first band-sprayed as pre-emergence treatments on the cane rows (in furrows). This allowed the DPA treatments later to be limited to the interrow weeds, and the top leaves of the cane rows were not touched by the sprays, as seen in Fig. 32. In all the experiments, the layouts of which were a randomized complete block design, standard broadcast pre-emergence treatments with diuron + 2,4-D, atrazine 4- 2,4-D, and hand-weeding in both common and clean degrees, were also entered as controls. The results obtained were in favor of the paraquat + dalapon + 2,4-Dcombinations in DPA for all crops planted in the fall season. Other herbicides in the DPA tests either performed poorly in weed control or had too phytotoxic an effect, so that reductions in the sugar yields resulted. In spring-planted crops, however, the ordinary pre-emergence treatments were more effective, because less decomposition of the herbicides in the soil occurred during the dry season. All the treatments were evaluated on the basis of weed control performance and safety to crop. Parts of the data comparing paraquat combinations and control treatments are summarized in Table 7.2. Table 7.2 shows that, for an autumn-planted crop in areas infested by annual weed species, as shown by Huwei, a formula of paraquat + dalapon 4- 2,4-D, at 0.64, 2.13 and 1.6 kg/ha, respectively, in DPA, may be used to achieve both a satisfactory

185

Fig. 3 4 . The method of treating established weeds among cane plants: post-emergence application with the mixture of paraquat + dalapon + 2,4-D sodium was directed on the shorter weeds while avoiding contact with top leaves of cane rows.

weed control and the highest sugar yield. A weed control of 77% was observed four months after application, and the sugar yield of cane stalks, after growing for about 18 months for this crop, was 8.6% higher than the 19.44 t/ha given by common hand weeding. In areas such as the plantation of Nanchow where the soil, composed mostly of gravels, is especially suitable for infestation by the perennial Cynodon dactylon Pers. (Bermuda grass), the paraquat combination at higher rates was found especially effective against the weed. In this case, 1.92 kg of paraquat, 8.50 kg of dalapon and 1.60 kg of 2,4-D per hectare achieved an 81% control, as observed two months after application. The results are illustrated in Figs. 35 and 36. The sugar yield of 16.30 t/ha thus obtained, also ranked as the highest among all the chemical treatments. In this instance, the ordinary broadcast pre-emergence treatments with diuron and 2,4-D were inferior for controlling the weed, and produced a lower sugar yield. Obviously, for the autumn-planted crop, the paraquat combination in DPA used at the end of the rainy season has the advantage of being less subject to the influence of the monsoon climate. By contrast, the pre-emergence herbicides which are sprayed about one month earlier than DPA during the culminating period of the monsoon are more vulnerable to the combined attack of the prevailing high rainfall and high temperature. As shown in the pre-emergence diuron plus 2,4-D tests, higher rates of diuron may be more resistant to soil inactivation resulting from the activity of the prevailing climate, but they may cause a reduction of the sugar yield by their toxicity. On the other hand, lower rates of diuron are more subject to soil inactivation, and the poorer weed control as a result will cause a reduction in the sugar yield. It should be pointed out that preceding a DPA treatment using the paraquat combination, a partial pre-emergence treatment with diuron plus 2,4-D, usually in a 30-cm band on

05) 01) 77 70 63 63 88 73 53 67 78 72 85

21.12 13.76 10.96 14.99 17.07 17.87 18.13 21.12 15.20 19.44 17.36 1.01 1.35

Directed post-emergence paraquat + dalapon Directed post-emergence paraquat + dalapon Directed post-emergence paraquat + dalapon Pre-emergence diuron + 2,4-D at 0.80 + 1.60 Pre-emergence diuron + 2,4-D at 2.40 + 1.60 Pre-emergence diuron + 2,4-D at 4 . 0 0 + 1.60

% 5 30 81 2 33 18 7 0 2 17 93 13.43 9.61 16.30 12.05 13.91 12.35 9.45 14.97 15.23 16.52 14.50 1.90 2.54

+ 2,4-D at 0.64 + 2.13 + 1.60 kg/ha. + 2,4-D at 1.28 + 4.25 + 1.60 kg/ha. + 2,4-D at 1.92 -I- 8.50 + 1.60 kg/ha. kg/ha. kg/ha. kg/ha.

108.2 70.8 56.4 77.1 87.8 91.9 93.3 108.6 87.6 100.0 89.3

%

Sugar yield (t/ha)

7 8 9 10 11

47 76 60 63 60 90 67 63 81 67 47

%

Weed control (60th day)

6.83 6.62 6.93 7.33 8.44 6.08 8.25 7.25 6.92 7.50 8.12 0.74 0.99

Sugar yield (t/ha)

91.1 88.3 92.5 97.8 112.5 81.1 110.0 96.7 92.2 100.0 108.3

Check plot %

Chaotow Clayey soils; annual weeds; springplanted F 156

Pre-emergence atrazine + 2,4-D at 0.50 + 1.60 kg/ha. Pre-emergence atrazine + 2,4-D at 1.50 + 1.60 kg/ha. Pre-emergence atrazine + 2,4-D at 2.50 + 1.60 kg/ha. C o m m o n hand-weeding, two times. Clean hand-weeding, six times.

81.3 58.2 98.7 72.9 84.2 74.7 57.2 90.6 92.2 100.0 87.8

Check plot %

Weed control (68th day)

Check plot %

Weed control ( 1 2 0 t h day)

Sugar yield (t/ha)

Nanchow Gravelly soils; Cynodon dactylon\ autumn-planted N:Co 3 1 0

Huwei Sandy soils; annual weeds; autumn-planted F 146

All measurements calculated on active-ingredient basis.

*1 2 3 4 5 6

Treatment No*

Effectiveness of paraquat combinations in DPA treatments in weed control and their effect on the sugar yield of cane as compared with standard treatments and hand weeding at three locations.

7.2 00 Os

187

Fig. 3 5 . The field o f an autumn-planted sugarcane was seriously infested by Bermuda grass (Cynodon dactylon) which was unable to be controlled by pre-emergence diuron plus 2,4-D, generally applied for control of the annual seed-germinated weeds.

Fig. 36. The established Bermuda grass was almost completely eliminated by the mixture of paraquat + dalapon + 2,4-D in DPA treatment, while it prevented subsequent regrowth from the seed-germinated annuals for about 3 months, before 'close-in' of interrows by the cane leaves.

188 the cane rows, also means taking a chance that it may be washed out by the monsoon rains. However, the loss, if it occurs, is only one-fourth of the blanket spray with the same herbicides. For spring-planted crops, particularly on heavy soils, the DPA treatments tested showed poorer results than those from ordinary broadcast pre-emergence operations, as seen by the data from Chaotow. Due to the lower temperature and the scarcity of rainfall during the planting period, the weeds germinated and grew much more slowly than in the summer. The herbicides used in these applications, therefore, were sprayed on the soil surface rather than on the weed foliage. A much poorer weed control resulted because a large proportion of the herbicides was soon inactivated by the soil, and the regermination of weeds went virtually unchecked by any residual effect. The ordinary blanket pre-emergence application with diuron 4- 2,4-D or atrazine 4- 2,4-D mixtures should, therefore, be employed as usual for the spring-planted crops. (4) Tests of the synergistic activity of herbicides in combinations To test the synergistic activity when paraquat is used in a mixture with such chemicals as dalapon, diuron and 2,4-D, an experiment with torpedo grass (Panicum repens) as the test material was conducted in January, 1968. The stems and rhizomes of this 2 2 weed were collected and planted at 2 kg m in flat beds, 0.5 m in size. Individually, and in combinations of 2, 3 , and 4, the chemicals were sprayed on the weed during the 3-node growing stage, at a total dosage rate of 10 kg/ha. The layout of this experiment was a randomized complete block design, with each treatment replicated three times. Periodical counts of the surviving and regenerated plants per plot were recorded, as an assessment of the initial killing ability and residual effect of the herbicides on the weed. The data are in Table 7.3. The data presented in Table 7.3 demonstrate that, within the limits of experimental error, all the combinations of two and three components forming a total dosage rate of 10 kg/ha were significantly superior in their initial killing ability on the torpedo grass to diuron or 2,4-D used alone at the same rate, with the exception of the pairs among paraquat, 2,4-D and dalapon, and the combination of all four compounds. With the exception of the pairs, the synergistic activity of the three herbicides in mixture was very noticeable. Observations of the residual effects of the herbicides, made three months after spraying showed that diuron was more potent and persistent than other compounds. The paraquat 4- dalapon 4- 2,4-D, at the same dosage rate of active ingredients as the diuron combinations, were one of the weakest combinations for residual effect on the grass plants at this stage. This showed that when this mixture was used at only onethird of the rate used for killing emerged annual species in cane plants after the monsoon season, the short residual effect of the herbicides did not have much influence on the subsequent growth of the cane plants. However, once the topgrowths of the weeds were killed by such a mixture, any regermination from the seeds or storage roots of the annual species could be easily controlled by some residual effect, in cooperation with the prevailing dry conditions. Successful weed control for the autumn-planted sugarcane would then be secured with the paraquat mixture.

189 TABLE 7.3 Comparison of the effects of herbicides sprayed individually or in combinations on torpedo grass at its three-node growing stage Herbicides*

Rates (kg ai/ha)

Number of survived and regenerated weed plants per square meter 16th day

Ρ di 2 ρ + di ρ + 2 di + 2 ρ + di + da ρ + da da + 2 ρ + da + di + da ρ + di + ρ + di +

2

2 da da + 2

10 10 10 4 + 4 + 6 + 2 + 10 4 + 6 + 2 + 6 + 2 + 2 +

6 6 4 6 + 2 6 4 6 + 2 4 6 + 2 4 + 2 + 2

Unsprayed check plot LSD ( 0 . 0 5 ) (0.01)

Percent of check plot

96th day

Percent of check plot

653.2 1014.4 1241.2 506.8 714.8 438.8 530.8 1425.2 1168.0 708.0 565.2 349.2 390.8 1002.8

30.0 46.6 57.1 23.3 32.9 20.2 24.4 65.5 53.7 32.6 26.0 16.1 18.0 46.1

841.2 33.6 1509.2 36.0 1596.0 106.4 101.2 1094.4 1554.8 1026.8 1054.0 48.0 113.2 2392.0

36.7 1.5 65.8 1.6 65.9 4.6 4.4 47.7 67.8 44.7 65.5 2.1 4.9 104.2

2174.8 229.4 308.5

100.0 10.5 14.2

2294.8 280.1 377.8

100.0 12.2 16.5

*p = paraquat-dichloride, di = diuron, 2 = 2,4-D (dimethyl amine salt), da = dalapon.

Since it can completely kill the topgrowths, and has longest residual activity against regrowths of the annual weeds, and can be used in component doses (hence lower prices), the combination of diuron + paraquat 4- 2,4-D amine (see Table 7.3) has been used commercially for the control of vegetation on railroads and other non-cropped areas, following another series of extensive, comparison trials (Peng, 1968). This will be discussed in detail in a following chapter. (5) Early weed competition

before DPA treatment on young cane

Some sugarcane growers, perhaps, suspect that the month-long competition of weeds and cane plants before a DPA treatment is used to knock down the emerged weeds may cause some harm to the cane seedlings. That it does not is shown by evidence gathered in another experiment with autumn-planted crops (Peng and Sze, 1969a). Clean hand-weeded plots, which produced on average six stalks per stool and a 123.5 cm stalk length after 10 months of growth, were compared to a plot on which weeds were left intact for 3 weeks after planting, and then clean hand-weeded, to keep it weed-free throughout the season. The latter produced a few more stalks per stool, with an insignificant reduction in the stalk length. The tillering of cane plants was also

190 unaffected in those plots in which only the cane rows were clean hand-weeded, with the interspaces being left open for weed infestation for nearly three months after the planting. The length-wise growth only was slightly affected. The two partially weeded plots ultimately produced the highest cane yields of all the treatments, including nonweeding, clean hand-weeding and clean chemical weeding. This further justifies the safe use of the DPA treatment for weed control in an autumn-planted crop. The competition of the early-growing weeds does not influence the growth of the cane seedlings perhaps because the seedlings still depend on the planted cane cutting for nutrition. For large-scale use, a knapsack sprayer mounted with a flat spray 8004 TeeJet nozzle is found to be suitable for DPA treatments in Taiwan. The plane of the spray projection should be parallel to the cane rows, covering the inter-row weeds with a left-to-right motion of the spray lance, without intercepting the upper cane leaves, and moving forward at a constant speed. Drenching the weedy foliage and the soil surface beneath is necessary to achieve a satisfactory result. This new chemical weed control technique, employing a formula of paraquat + dalapon + 2,4-D, has been recommended for general use with the autumn-planted cane crops in Taiwan.

4. POST-EMERGENCE A N D PRE-PLANTING TREATMENTS WITH FOLIAGE-APPLIED HERBICIDES FOR CONTROL OF WEEDS ON SALINE SOILS

Along the coastal plain of south eastern Taiwan, nearly 4,000 hectares of cane land originating from slate alluvial or sand-stone shale alluvial soils are affected by salinity. Contaminated mostly by sodium salts and chlorides from intruding seawater, the ground water beneath such land becomes more or less saline, or saline-sodic. Poor drainage leaves the salty ground water table only 80—130 cm below the surface during the dry season, and nearly at the surface layer during the wet months. The cultivation zone is, therefore, prone to resalination by direct wetting of the saline ground water in the monsoon, or by soil evaporation in the dry months that draws up salts and deposits them on the surface layer. Such salinity-affected land is usually unsuitable for growing cane before certain reclamation measures have been introduced to bring down the soil's electric conductivity from above 6 mmhos/cm to below 3 mmhos/cm, and the soil acidity to pH 7.8—8.3. The installation of open and tile drainage to lower the ground water table, the desalinization of the upper soil by frequent flushing with fresh water pumped from deep wells or down-stream river, and soil amendments, have been employed for improvements (Yen, 1971). With the continual washing down of residual salts from the upper soil during cane growing, special submerged planting and r i c e cane interplanting are practised on such land. Generally, cropping on a saline field begins with land preparation and impounding of water, early June-July after the land has been idle for a few months after harvesting the last cane crop in the spring. The field is divided into blocks by small leaves and fresh water is irrigated, and impounded within the blocks. Depending on the texture of the soil, several weeks are required before cropping for the impounded water

191 to percolate, along with the dissolved salts from the upper soil, down to the underground stream. Reponding of fresh water by irrigation is indispensable even after cropping, lest the field surface becomes dry, soil clods form, and resalination by soil evaporation occurs. Therefore, after planting the cane in September, ponding of water in the field is maintained for 5—6 months until the banking-up of cane rows. Then flood irrigations every 20—30 days are, still, required until the cane is 10—12 months old. A rice—cane intercropping is necessary for the newly-reclaimed saline land. Four rows of paddy rice 25 cm apart, either direct-seeded or transplanted, in July, are alternated with one empty row to be planted with cane later in October when the rice has grown for about 3 months. This rice—cane intercropping has the advantage of removing more salts from the soil, due to the continual ponding of water for the summer rice, and the easier percolation of salts which is aided by the downward growth of the rice roots. The rice is harvested in December, leaving the cane to grow alone to harvest. The two-eyed cane cuttings are planted in the empty rows at an angle of about 45°, with the lower bud buried in the submerged soil, and the upper bud exposed to the air. Such 'slant planting' of cuttings permits the buried bud to develop roots in the soil, while the exposed one can respire and sprout in the air (both set-roots and shoot-roots are not produced from the upper portion of a seed piece as long as it is exposed to air). After about 5 - 6 weeks the field is temporarily drained and the slantwise planted seed pieces are one by one stepped to lay flat in the soil by a worker, letting the exposed sprouts develop normal roots. After a field has undergone several years of rice-cane intercropping, and the salinity has become mild, sole cropping of sugarcane is then followed, as described before. In most cases, ratooning is not done, because of the generally lower crop yield due to shallow root development in the salinity-affected soil. For many years the cane growers have been frustrated by the failure of such conventional soil herbicides as diuron and atrazine to control weeds on salinity-affected fields that are predominantly composed of salt-tolerant perennials. Different approaches to dealing with the new problems must be tried, and several field and pot experiments already completed in the years 1970—75 have been fruitful (Peng et al., 1975a). Their results are as follows: (1) Field trials for practical methods of control (a) For sole cane cropping on saline soils The first field experiment devoted to screening suitable herbicides to control weeds with the submerged culture of sole cane on saline soil was conducted at the coastal Ao-ku plantation with a 1973—74 autumn-planted crop. The field had been under cultivation for growing sugarcane ever since it was reclaimed from a salty and weedy swamp a few years ago. In the beginning the field was prepared, and plots were embanked to impound irrigation water, which was pumped from a deep well for about two months. The field was then temporarily drained and the 2-eyed cuttings of cane variety F 160 were planted, using the slant planting method, on Sept. 15th 1972. By

192 then the weeds had profusely covered the entire field and consisted of three main perennials: Bermuda grass, nutsedges and torpedo grass, together with a few annual plants like barnyard grass (Echinochloa crusgalli) and goose grass (Elusine indica). As buds on the exposed portion of the half-planted cuttings would have been injured by a herbicide sprayed soon after planting, the application of herbicides was delayed a week until the exposed upper buds had developed into spikes and attained their greatest tolerance, protected by their leaf-sheaths (Peng and Yeh, 1971). For testing the herbicides' effectiveness for weed control under pre-emergence conditions, the weeds on the plots were hoed off in advance, expsoing the soil surface and the protruding cuttings with their sprouted buds (spikes). Then the soil compounds (all doses on an active ingredient basis) Outfox at 2 and 3 kg/ha, and diuron at 1.6 kg/ha of 2,4-D (sodium salt), were sprayed as the pre-emergence treatments. The contact and translocative compounds MSMA (3.6 kg/ha), paraquat (0.8 kg/ha) and asulam (2.5 kg/ha), each reinforced by mixing with a residual compound such as diuron, 2,4-D or Actril-D at ordinary or double doses, were used for overall post-emergence treatments until the cane shoots unfurled 2—3 leaves. The unsprayed and unweeded plots were entered as control. All treatments (12 in total) and the control were laid out according to a randomized complete block plan, with replication of four. The plot size was 8 rows x 1.25 m row spacing x 8 m row length. The measurements of the harvest of weeds and the growth and yield of sugarcane used to assess all treatments' effectiveness of weed control and effect on the cane crop are shown in Table 7.4. From Table 7.4 it is seen that even the combination of diuron + 2,4-D Na, each at 1.6 kg/ha (Treatment 3), which was normally safe and effective for the pre-emergence control of weeds for sugarcane on ordinary soil, caused some leaf-yellowing of the cane seedlings and achieved only 47.8% control of regrowths of the mostly perennial weeds for about one month before inter-cultivation and embankment of the cane rows. It also gave only 41.7% reduction in the underground rhizomes of torpedo grass, as observed at harvest of the cane crop about 18 months after application. The cane yield of 77.1 t/ha with this treatment, being influenced by both phytotoxicity and weed competition, which had caused evident reductions in tillering, was the lowest, being 33.6% less than that for the check plots. The frequent irrigations and ponding of water, and the soil salinity, could account for the reduced effectiveness of the herbicides, and the susceptibility of the cane plant. After the dose of diuron in this combination was doubled, to give Treatment 4, control of the weed regrowths was improved by only 4.0%, but the rhizomes of torpedo grass were reduced by 20.6%. This increased the cane yield to 128.6 t/ha, although with the same phytotoxicity on young cane. When 0.8 kg/ha paraquat was added to this combination, to give Treatments 9 and 10 in overall post-emergence applications, control of the weed regrowths was much improved, to 76.6-85.7%, but control of underground rhizomes of torpedo grass was ineffective because the later treatments gave them more time to multiply (hence higher tolerance). Other treatments of MSMA combinations did not achieve any better results. Asulam, at 2.5 kg/ha combined with 2,4-D sodium (1.6 kg/ha) in foliar sprays (Treatment 12), not only killed up to 70% of the top-growths of these perennial

2,4-D Na 2.5 + 1.6 kg/ha (12). Unsprayed and unweeded (13).

c of increasing degree of leaf-yellowing from + to + + to + + + . cance at 1% level of probability. ignificant.

6.84* * 2.86 3.83

6.22 6.90 5.26 4.86 5.88 4.03 2.93 8.61 2.36 1.44 3.53 3.05 10.08

t/ha

28.4 38.0

61.7 68.5 52.2 48.2 58.3 40.0 29.1 85.4 23.4 14.3 35.0 30.3 100.0

%

Harvest of topgrowths before breaking field ridges b (Oct. 3 1 , 1972)

1.42 NS

1.23 0.74 0.85 0.55 1.75 1.95 1.62 0.66 1.74 1.88 0.55 0.35 1.46

t/ha 84.1 50.0 58.3 37.7 122.3 133.8 110.8 45.5 118.9 128.6 37.7 24.0 100.0

%

Harvest of torpedo rhizomes (Apr. 9, 1974)

-

-

2.25 NS

1.61 1.74 1.49 1.30 1.41 1.60 1.69 1.75 1.44

1.72 1.74 1.69 1.84

m

Av. ht of plant (Dec. 15, 1972)

-

2.86 NS

75.8 123.3 67.6 95.1 108.6 121.0 58.2 32.4 63.3 52.5 103.0 90.8 65.5

N o . of tillers per row (Dec. 15, 1972)

+ ++ ++ +++ +++ + +

-

+ +

-

_

Toxicity on cane 3 months oldc

-

-

1.97 NS

133.6 114.5 77.1 128.6 134.4 120.3 108.6 99.3 149.4 118.4 104.7 126.3 116.1

t/ha

Cane yield (Apr. 10, 1974)

112.2 98.7 66.4 110.8 115.8 105.8 93.5 78.6 128.7 102.1 90.0 108.8 100.0

%

C o m p o s e d of 80% Bermuda grass, 20% torpedo grass, nutsedges and a few annual grasses.

b

a A U doses on active ingredient basis, pre-emergence: Outfox 2.0 kg/ha (1), and 3.0 kg/ha (2); Diuron + 2,4-D Na 1.6 + 1.6 kg/ha (3), and 3.2 + 1.6 kg/ha (4). Overall post-emergence: Outfox 2.0 kg/ha (5), and 3.0 kg/ha (6); MSMA + diuron + 2,4-D Na 3.6 + 0.7 + 1.6 kg/ha ( 7 ) , MSMA + 2,4-D Na 3.6 + 1.6 kg/ha (8); paraquat + diuron + 2,4-D Na 0.8 + 1.6 + 1.6 kg/ha (9), and 0.8 + 3.2 + 1.6 kg/ha ( 1 0 ) ; asulam + Actril-D 2.5 + 0.4 kg/ha (11);

F test LSD (0.05) (0.01)

1 2 3 4 5 6 7 8 9 10 11 12 13 (CK)

Treat. No.a

Comparison in effectiveness of weed control and effects on sugarcane for herbicide treatments tested with 1 9 7 3 - 7 4 autumn crop planted on saline soil of Ao-ku plantation, average of four replicates

7.4

194 weeds, but also reduced the rhizomes of torpedo grass by 76.0%, at cane harvest and so was the best of all the treatments. This treatment, which was screened from evaluation tests for controlling annual grasses, described in a preceding chapter, was also highly selective to the cane plants which had lowered tolerance due to the influence of soil salinity. (It had caused no reduction in tillering and had given one of the highest stalk yields). (b) For rice-cane intercropping on saline soils The second field trial was held with the 1974-75 crop at another salinity-affected plantation, I-wu, where the rice—cane intercropping had been practised and where the weed population consisted of the same perennial and annual grasses as in the Ao-ku plantation. As planting sugarcane on the empty rows is preceded by planting summer rice about two months earlier, the application of herbicides for weed control for the cane crop should take into account the susceptibility of the summer rice. Therefore, a pre-planting application of compounds having a more translocative than residual activity is practicable. The sodium TCA, dalapon, asulam and metribuzin are suitable for this and were thus employed for the tests. Each of these compounds, at two dosages, except sodium TCA as a single herbicide, in combination with 1.6 kg/ha of 2,4-D amine, was in a three split foliar application on the same plots on May 25th, June 20th and July 5th 1973, during fallow of the field. Unsprayed and unweeded plots were used as the check. The test involved an RCB layout with four treatment replications. Following the fallow treatments (for comparison) the field was ploughed and the plots prepared again according to the original layout. Rice and cane (variety F 160) were planted on Aug. 10th and Sept. 27th, respectively, to test the herbicide effect. Complying with the cultivation systems, one half of each plot, which was 10 rows of 20 m row length, had either sole cane cropping or rice—cane intercropping, so that there were the two different cropping systems on each plot. From the measurements of harvested weed regrowths, yields of rice plants and sugarcane, the best treatment, that achieved the highest 91.4% weed control at the end of fallow and caused less toxicity to the ensuing rice, was metribuzin + 2,4-D, each at 1.6 kg/ha. The second was asulam 4- 2,4-D at 3.0 4- 1.6 kg/ha, which was a little less effective but more selective to the rice plants. These combinations, having once been proved suitable for fallow treatment of weeds on saline soils (with ensuing sole cane cropping and rice—cane intercropping), were again tested for economical use in one, total, application rather than being split to achieve the same effectiveness of weed control. Similar tests with the 1975—76 autumn crops, were made on two more salinity-affected plantations, San-ku-tzu and Shi-hu. There, there was a split and a total application on plots of the two herbicide combinations, compared with the unsprayed and unweeded plots as the check. The three split applications began in early June and ended in mid August 1974, at intervals of 2 - 3 weeks, during fallow of the fields. The one total application was conducted on the same day as the third split application. The sole cane cropping and the rice—cane intercropping followed in mid September. The results again confirmed that, for the sole cane cropping, the total application with metribuzin 4- 2,4-D at 4.8 4- 1.6 kg/ha, about one month before planting cane, could eliminate most of the perennial and

195 annual grass weeds and leave the cane plants growing without any toxic effect. However, for the rice-cane intercropping (the summer rice was far more susceptible) the three split applications with asulam + 2,4-D each at 1.6 kg/ha caused the least toxic effect on the rice which, due to its vigorous growth, had helped to control regrowths of weeds for the later, interplanted sugarcane. This combination was less potent. This is shown in Fig. 37. (2) Responses of sugarcane and weeds to herbicides in salinity regime Despite desalination measures which attempt to remove salts from cultivation zones, sugarcane cropping in salinity-affected fields is still hazardous, due to the monsoonal weather that often occasions the typhoon raids. Strong winds accompanying heavy rains always cause flooding of the coastal plantations by intruding seawater during the planting season, and cane plants can be chronically injured by contamination from the salts. If the summer rice lacks competitive vigor by being injured by the flooding of seawater, and so is less competitive against regrowths, then the laterplanted sugarcane that relies upon this competition could suffer as usual, both from the weed competition and from salt injury. This is true even if the weeds have been eliminated by foliar herbicides in fallow.

Fig. 37. In a salinity-affected field that needed continuous irrigation and ponding of fresh water to wash off salts from the surface soil before and after cropping, 3 split foliar applications during fallow (pre-planting applications) with asulam + 2,4-D amine at 3.0 + 1.6 kg ai/ha had eliminated most of the perennial and annual grass weeds and had left the planting of rice and, two months later, sugarcane, without residual effect, particularly on the susceptible rice plants.

196 (a) Responses of sugarcane in salinity substrate The fact that sugarcane is prone to salt-induced injury has been determined by some workers. Shen and Tung (1961, 1962) of this Institute studied salt tolerance of sugarcane varieties twenty years ago. They added various concentrations of NaCl to a base nutrient solution (Hoagland's no. 2 solution which, together with the salt solution, was renewed every 7 - 1 0 days) as the medium for growing several cane varieties, and investigated the response in plant growth, and analysed the cane for the elements Na, Κ and CI: whether they were absorbed from the salty substrate. With a preliminary test, they found that the growth of primary shoots of two cane varieties N:Co 310 and l F 146 were clearly affected even in the initial 2330 ppm (5.92 mi2" /cm) concentration of the salt. The resulting reductions in dry weight of the 100-day-old plants were about 50% for N:Co 310 and 70% for F 146, as compared to control plants of the respective varieties, thus showing the greater tolerance of the former to the injurious effects of the salt. With another elaborate experiment, using the same water culture, to test the effect of salt at four levels: 0, 2000, 4000 and 6000 ppm (2.0, _1 5.4, 8.3 and 12.0 m£2 /cm, respectively), they found the same susceptibility of this _1 crop plant to the salt: at the initial 2000 ppm (5.4 m i 2 / c m ) salt concentration, the dry weight of the 50-day-old plants was reduced by about 25% for N:Co 310 and F 137, and by 42% for F 148. When the salt concentration was 6000 ppm, the dry plant weight was reduced by 53% for the former two varieties, and by 86% for the latter. Again, N:Co 310 showed much higher tolerance to salt than the other two varieties, since the reductions in the growth rate of the primary shoots, measured by dry plant weight as well as by number of set-roots produced for the 50-day-old plants, were far less for N:Co 310 than for the other two varieties, as observed with the lower to higher salt concentrations. By planting (in soil) 2-eyed cuttings of the four varieties N:Co 310, F 137, F 146 and F 148, and spraying them with 0, 2000, 4000 and 6000 ppm salt (on the basis of the dry weight of the soil), in another experiment for testing the effect of salt on the germination from buds and root primordia, they found that bud germination was much more suppressed by salt, even at 2000 ppm, with no more than 30% germination after 30 days for all varieties. With an increase in salt concentration, suppression rose sharply and there was no bud germination at all at 6000 ppm of the salt. A higher rate of bud germination was observed with N:Co 310. Germination of the root primordia was less influenced by the salt. Almost all the set-roots sprouted from the cuttings treated with 2000 ppm salt were as abundant and normal as those from the untreated plants, for all varieties, and there was only about 50% reduction in the number of setroots produced when the salt solution sprayed was increased to 6000 ppm. In treatments with higher concentrations of salt, F 146 produced more set-roots than the other three varieties. However, when treating the cuttings with salt in a continually renewed water culture, germination of the root primordia was suppressed to greater extent, and there was, for all varieties, about a 50% reduction in the number of setroots produced at the initial 2000 ppm salt concentration. Suppression increased with increase in salt concentration, and there was about an 80% reduction in the germination rate of set-roots for N:Co 310, and 96% for F 146 when the salt was at 6000 ppm, again verifying the higher tolerance of this South African variety N:Co310.

197 As we may recall, this variety's roots structurally and physiologically tolerate the herbicidal effect more than do other F varieties, and this greater tolerance to osmotic pressure induced by a high concentration of salt in the soil or other substrates seems to shed more light on its adaptability to inadequate cultivation conditions. In salty nutrient solutions, the few set-roots produced were, in the beginning, whitish in colour. Then discolouration gradually set in, from yellow to dark brown, and they underwent suberization (conversion into cork tissues). Therefore, the retardation in the growth of sugarcane in a salinity regime is due primarily to physiological malfunction of the plant: apart from suppression, producing less than the normal number of set-roots from the root primordia, suberization of those few produced (both effects increase as osmotic pressure of the salty substrate increases) blocks the uptake of water, cf. Eaton (1941). Without adequate water absorbed in the plants, cell division in the meristematic tissues is stopped, and hence the growth of the whole plant is retarded. Due to the ease with which the coastal cane plantations can be contaminated by salts to cause severe root injury of the cane plants, the harvest of even an autumn planted crop with a normal yield is hazardous, and its ratooning is not generally practised. By analysing for the elements of sodium chloride, potassium and chlorine in the plants in the water culture, the authors found that the levels of Na and CI in plants of all the tested varieties increased as the concentration of salt in the nutrient solution was increased. The level of Κ otherwise was, however, found to decrease, the leaves showing worsening symptoms of K-deficiency. The fact that the uptake of Na by the plants was antagonistic to that of Κ was in agreement with Findings from Hawaiian sugarcane research, cf. Humbert and Martin (1955). (b) Combined effects of soil salinity, weed competition and herbicides on sugarcane The interactions of cane plants, weeds, herbicides and salts cause complex relations in cropping sugarcane on salinity-affected soil with chemical weed control. It is worthwhile looking into the differences between the responses of the cane plant to each of the two chemicals, and to both in combination, under the competition of a weed. The results of a factorial experiment conducted during the dry season from mid November 1975 to late February 1976 (Peng and Twu, 1979a) therefore follow. Earthenware pots filled with sandy loam soil were each planted on Nov. 21st 1975 with a one-eyed cutting of the variety F 177, and 200 g of rhizome sprigs of torpedo grass, one of the major perennial weeds in coastal, salinity-effected Fields. After 20 days, when cane and weed had both sprouted their shoots, they were irrigated weekly with NaCl solution at concentrations of 0, 5, 10, and 20 mmhos/cm. Then, nearly two months later, metribuzin and asulam (their usefulness for controlling gramineous weeds in such cane fields has just been described), were used at 0, 2, 4, 6, and 8 kg ai/ha in a foliar application. The pots of plants received, therefore, treatments of the two chemicals in 4 x 5 factorial combinations, with each treatment being replicated four times. Being compared with pots planted with either sugarcane or torpedo grass alone (which were treated neither by herbicides nor by the salt, and equally irrigated with freshwater), the treated plants were assessed for factorial effects on the two chemicals at harvest on Mar. 1st 1976, after having grown for 102 days. The results are in Table 7.5.

base of percentage, not included in factorial test of significance. cant at 0.01 level of probability. significant.

0 0

0 2 4 6 8

0 2 4 6 8

0 2 4 6 8

0 2 4 6 8

0*

0

5

10

20

Factorial F H S HXS

Rate of herbicide (kg ai/ha)

Salt cone. ( m £ 2 _ 1/ c m)

-

13.0 6.0 0 0 0

17.6 8.5 9.5 0 0

18.8 12.8 10.3 0 0

47.23** 6.18** 1.17 NS

56.5 26.1 0 0 0

76.5 36.9 41.3 0 0

81.7 55.7 44.8 0 0

96.9 71.7 54.3 0 0

-

22.3 16.5 12.5 0 0

100.0

37.5 24.3 53.8 49.8 50.5

68.5 40.3 56.8 54.8 60.0

77.0 41.0 60.0 55.3 60.2

-

90.3 70.3 68.5 56.0 61.8

92.8

9.48** 16.75** 2.80**

40.4 26.2 57.9 53.7 54.4

73.8 23.4 61.2 59.1 64.4

82.9 44.2 64.6 49.6 54.9

-

97.3 75.8 73.8 60.3 66.3

100.0

(%)

(g)

(g)

23.0

(%)

wt. of rhizomes

wt. of stems

Metribuzin treated

100.0

11.0

0 0 0 0 0

0 0 0 0 0

22.7 9.1 0 0 0

25.5 22.7 20.9 18.2 0

78.09** 241.41** 23.50**

2.5 1.0 0 0 0

2.8 2.5 2.3 2.0 0

50.5 27.3 25.5 22.7 20.9

_

_ 5.5 3.0 2.8 2.5 2.3

(%)

(g)

wt. of cane

11.0 9.3 13.8 12.8 11.5

12.5 11.0 14.0 13.0 12.5

14.3 12.8 14.2 13.8 13.8

-

15.0 14.0 16.0 16.5 13.8

23.0

(g)

1.45 NS 2.66 NS 0.10 NS

47.8 40.4 60.0 55.7 50.5

54.3 47.8 60.9 56.5 54.3

62.2 55.7 61.7 60.0 60.0

-

65.2 60.9 83.9 71.7 60.0

100.0

(%)

wt. of stems

Asulam treated

54.3 51.8 49.5 51.5 5 5.0

55.8 51.8 54.5 52.5 63.0

56.5 54.5 58.8 53.5 64.3

-

76.8 58.5 58.3 56.5 66.3

92.8

(g)

2.61** 4.29** 0.60 NS

58.8 55.8 55.3 55.5 59.3

60.1 55.8 58.7 56.6 67.9

60.9 58.7 63.4 57.7 69.3

-

82.7 63.0 62.8 60.9 71.4

100.0

(%)

wt. of rhizomes

0 0 0 0 0

2.3 0.8 0.8 0.5 0

2.5 1.5 1.3 1.0 1.0

4.5 3.3 2.8 2.3 1.8

11.0

_

(g)

44.29** 218.14** 6.14**

0 0 0 0 0

20.9 7.3 7.3 4.5 0

22.7 13.6 11.8 9.1 9.1

40.9 30.0 25.5 20.9 16.5

100.0

_

(%)

wt. of cane

Combined effects of soil salinity (S) and herbicides (//) on fresh plant weights of sugarcane (variety F 177) and torpedo grass growing for 102 days, average of 4 replicates

7.5

199 From Table 7.5, it is noted that when there was only irrigation of freshwater _1 (electric conductivity about 2 m i 2 / c m ) to the pots of plants, without herbicide treatments, the mutual competition caused sugarcane to lose 50—59% plant weight, approximately in agreement with the field observations mentioned previously. Torpedo grass suffered insignificant losses of 3.1—34.8% in its aerial stems, and 2 . 7 17.3% in underground rhizomes, by competition from sugarcane. This is new information. Under weekly irrigation with salt solution, even at the starting concentration of 5 m f i ^ / c m , sugarcane suffered 74.5—77.3% reduction in plant weight, both from salt-caused injury and weed competition. When the salt concentration was doubled _1 to 10 m£2 /cm in irrigation, the losses in plant weight were increased to 7 7 . 3 79.1%, and there was mortality of plants at 20 m H ' V c m of salt solution. Taking into account the combined effects of herbicides and weed competition, sugarcane suffered plant losses of 72.7—79.1% with treatment by metribuzin, and of 70.0—83.5% for asulam, as both compounds' doses were increased from 2 - 8 kg ai/ha. The herbicides' effect at 2 kg ai/ha application rate should, therefore, be about 10% reduction in plant weight, if the maximum effect of weed competition (causing 60% reduction) is subtracted. This may be due to the cane plant's becoming more susceptible to the herbicides under the destructive competition of torpedo grass, i.e. a result of the interaction of both factors. Under field conditions, there is usually not even leaf symptoms of the cane plants when one of the two herbicides is used at 2 kg ai/ha in foliar application. However, only a little more than 10% reduction in plant weight of sugarcane resulted, and there was no plant mortality when the herbicides' application rate increased 4-fold, indicating their still high selectivity to this crop. Torpedo grass, with its densely planted seed rhizomes, showed high tolerance to both herbicides in one foliar application, even at the highest 8 kg ai/ha and under continual irrigation by the salt solution at all levels of concentration, losing only 30.1—76.6% weight of rhizomes. Although higher reductions were caused by higher levels of both herbicides and salt, they lacked a relation as clear as that for sugarcane. However, aerial parts of the grass exhibited markedly, progressive reductions under increasing levels of both salt and metribuzin, significant interactions between both chemicals, and more contact action in metribuzin than in asulam.

5. TOTAL CONTROL OF WEEDS WITH HERBICIDES ON NON-CROPPED A R E A S

For chemical control of weeds on non-cropped areas, where there is no crop susceptibility to worry about, we can use the most powerful herbicides to kill the usually luxuriant vegetation as completely as possible, while preventing its regrowth as long as possible. This goal dictates prescription of some non-selective herbicide mixtures that should possess all the contact, translocative, and residual properties. Although the length of time of weed control on non-cropped areas may be varied by dosages of the residual compounds, such areas as ditchbanks, that rely on some perennial species' ramifying rhizomes and stolons for consolidating the soil against erosion, may want a herbicide mixture without too much translocative effect. However, as the climate, soil type, and weedy vegetation constitute the major factors influencing

200 the performance of herbicides, suitable formulae and timing of application is all the more important, and should be discovered by clear trials under prevailing environmental conditions. This begins with developing a formula for the total control of weeds on the railways and industrial sites, for which year-round freedom from weed infestation is desired (Peng, 1968). (I) Total control of weeds on railways and industrial sites For many years, the Taiwan sugar industry has been relying on a network of narrow-gauged railways for transportation. More than 3,000 km of railroads extend into the remote countryside to connect the widely distributed sugarcane plantations, which cover nearly 100,000 ha, with the 25 sugar mills in the southern part of the island. Some 68% of these tracks are used solely to convey harvested cane during the milling season, which usually lasts from late November to April when the cane crops reach maturity. The remaining 32% of the railroads, however, play an important role as year-round arteries for transportation of passengers and goods in the rural districts. Traditionally, all the railway lines had one overall hand-weeding each year, before the start-up of milling operations, to eliminate vegetation that might interfere with the transportation of the raw materials. The lines with heavier traffic in the rural districts often received one additional hand-weeding when the regrowth was about to cover the tracks again. Due to a limited budget and a shortage of labor, it has become more and more difficult to keep up with even this minimum of weeding in recent years. The quickly resumed growth of vegetation, particularly in some humid areas, frequently makes the tracks frictionless when trains are riding on them. The situation is occasionally worsened when ignorant farmers graze their cattle on the roadsides — a source of accidents for the trains. Weeding the roadbeds is quite different from weeding in the cane fields. Usually, picks, instead of hoes, are used to dig out the deep-rooted dense vegetation that never occurs in cane fields because of the cultivation of the crop (Fig. 8). Nearly eight times the labor force used in the cane fields is needed for the exhaustive hand-weeding of the roadbeds. Nearly 20% of the yearly budget for the maintenance of the railroads, and almost 30% of the repairmen's working hours are tied up by this job. Of course, the chemical method of weeding was a possible answer to the problem of freeing repairmen for more valuable mechanical work that was badly needed. Several experiments were thus made in 1966—67 to search for practicable solutions. (a) Initiation of experiments Since the alternating monsoon and drought seasons are distinct in this sub-tropical region, the use of chemical herbicides, to be successful, had to be in the beginning or at the end of the rainy months. During the wet months from early May to mid-September, there is the risk of washing-out the newly sprayed chemicals by the torrential rains. In drought conditions the dry soils, particularly in the well-drained roadbeds, prevent any effective results from the herbicides applied.

201

Fig. 38. Picks, instead of hoes, are the traditional tool used for laborious hand-weeding on railroads of the Taiwan sugar industry.

The experiments for total control of railroad weeds by chemical methods were thus conducted, twice, on October 5, 1965, and May 17, 1966. Three locations were chosen to represent a southern, a middle and a coastal district of this producing region, with varied, average annual temperature and total annual rainfall: Pingtung, 24.2°C and 2398.2 mm; Hsinying, 23.4°C and 1656.2 mm; and Huwei, 22.2°C and 1497.0 mm. Two different sections of the railroad at each site were tested at each application time, so the experiment was actually replicated six times. Nine different, commercially known, herbicide products (except Kleer-Trax F, which is an experimental product without a chemical name yet issued, by Amchem Products, Inc.), donated by leading companies, were employed in the experiment. All the testing compounds were used at graded product dosage rates, either singly or in mixtures. In all, there were 34 herbicide treatments, to each of which a constant 5 kg/ha of 2,4-D sodium salt was added, as shown in Table 7.6. The object was to secure satisfactory control of weed regrowths, for as long as possibly one year, so most of the treatments were used as foliar sprays. In some treatments, however, the residual compounds diuron and fenac, in mixtures with 2,4-D, were sprayed onto the bare roadbeds after the weeds had been removed by hand-weeding. This was to test the effectiveness of these soil sterilants when used at heavier rates in pre-emergence applications on non-cropped lands. Each treatment was replicated four times, and each was assigned, at random, to a plot 10 m long and 2 m wide, along the railway line. 1000 1/ha of water was used

202 TABLE 7.6 Comparison of the weed-control effectiveness o f various herbicide treatments o n railroads (average results of six replicated experiments) N o . Herbicide treatments (each mixed with 5 kg/ha 2,4-D Na)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

paraquat + diuron paraquat + diuron paraquat + diuron paraquat + diuron paraquat + diuron paraquat + diuron Hyvar X Hyvar X Hyvar X Hyvar X Hyvar X + dalapon Hyvar X + dalapon diuron + dalapon diuron + dalapon Hyvar X + W.K.

16

Hyvar X + W.K.

17 18 19 20 21 22 23 24

Hyvar X-WS Hyvar X-WS Hyvar X-WS Hyvar X-WS Kleer-Trax F + fenac Kleer-Trax F + fenac Kleer-Trax F + fenac Hand-weeding followed by diuron Hand-weeding followed by diuron Hand-weeding followed by diuron Hand-weeding followed by fenac Hand-weeding followed by fenac Hand-weeding followed by fenac dalapon + Tordon 22K dalapon + Tordon 22K Daxtron + Tordon 22K Daxtron + Tordon 22K Daxtron + Tordon 22K

25 26 27 28 29 30 31 32 33 34

Product dosage rate (kg or 1/ha)

Days of residual persistence

4 1 + 4 kg 4 1 + 8 kg 4 1 + 16 kg 8 1 + 4 kg 8 1 + 8 kg 8 1 + 16kg 2 kg 4 kg 8 kg 16 kg 2 + 10 kg 4 + 10 kg 4 + 10 kg 8 + 10 kg 4 kg + 1% tank mix 8 kg + 1% tank mix 2 kg 4 kg 8 kg 16 kg 6 + 4 kg 12 + 4 kg 18 + 4 kg 4 kg

% Weed control against Perennials*

Annuals*

95 125 133 95 103 185 40 73 120 145 93 83 100 93 83

14 23 31 10 27 34 5 6 28 32 13 15 20 13 9

26 65 61 30 54 86 8 11 57 62 25 28 38 25 17

98

17

33

30 48 100 115 76 86 86 96

4 5 23 27 10 14 15 15

6 8 47 53 10 14 17 30

8 kg

130

30

60

16 kg

151

32

64

4 kg

91

18

17

8 kg

96

15

30

101

27

52

88 91 86 91 93

10 18 14 12 13

20 17 14 23 25

16 kg 9 kg + 4.5 1 18 k g + 4.5 1 5 1 + 4.5 1 10 1 + 4.5 1 15 1 + 4.5 1

*The main species of weeds which were only limitedly controlled and which re-emerged were: perennials - Panicum repens, Cynodon dactylon, Imperata cylindrica, Cyperus rotundus; annuals - Elusine indica, Digitaria sanguinalis, and Euphorbia hirta.

203 for dilution of the products, and knapsack sprayers were used for applications, there were monthly assessments of weed mortality, until the regrowth of weeds totally re-occupied the plots. (b) Results of experiment The different temperatures and rainfall, as well as the weed biology, at the three locations, markedly influenced the performance of each herbicide treatment. In the southern area of the island, around Pingtung, the monsoon is longer and the rain is more intense, so the growth of weeds was much more luxuriant than in the northern areas. The applications, made in the beginning, or at the end of the monsoon, showed pronounced differences in the residual effects of the herbicides against the resprouting of the weeds. In Pingtung, for example, a sudden heavy rain during the night immediately following the October application rendered the chemicals almost inoperative. All but a few of the applications gave satisfactory control only for one month. The May application in Pingtung and both applications in the other two locations performed normally. Generally, the applications made at the beginning of the rainy season showed less residual effect, against both the annual and perennial weed species, because the high rainfall, coupled with the hot summer temperature, had a more debilitating effect on the herbicidal activity. The average results of the six replicates of the experiment are in Table 7.6. Among the 34 treatments, only those plots treated with paraquat 4-diuron 4- 2,4-D at 8 1 + 1.6 kg 4- 5 kg/ha showed a complete kill of all weedy topgrowths shortly after spraying. This application also maintained the longest residual effect against the subsequent re-emergence of weeds. The same combination at lower rates of application produced proportionately less effect. The second best result was obtained from the treatment in which a combination of diuron 4- 2,4-D at 1.6 kg + 5 kg/ha was used to spray the bare roadbeds. Next in effectiveness was a combination of Hyvar X 4- 2,4-D at 1.6 kg + 5 kg/ha, and the Hyvar X-WS combination at the same rates. Other applications did not show a comparable, practical, performance. It appeared, therefore, that the paraquat 4- diuron 4- 2,4-D treatment was the best formula for total chemical control of railway weeds in this region. Not only did it give an immediate 'knock down' of the dense vegetation, but it seemed to be able to resist leaching and washing by the intensive rains during monsoon. Many experiments showed that paraquat, enhanced by the activity of diuron, functioned perfectly to knock down luxuriant brushes within a few hours of application. Diuron, in the presence of paraquat, is also more readily absorbed by soil particles, which accounts for its greater resistance to leaching and washing by rains. This was further confirmed by the reduced effectiveness of Hyvar X mixtures at the same dosage rates. However, Hyvar X, when compared to diuron alone, on other occasions, was superior to diuron in herbicidal effects. It was also interesting to note that a mulching effect, preventing leaching and decomposition of the herbicide elements from rain and sunlight, was obtained from

204 the killed topgrowths of weeds when paraquat was used in combination with diuron, as against applications of diuron alone to the bare roadbeds. The addition of 2,4-D was important in every case, in order to control a prominent perennial species, Cyperus rotundus. It appeared, however, that the best formula was less efficient for controlling the subsequent regrowth of some perennial species, such as Panicum repens, Cynodon dactylon and Imperata cylindrica, even though it provided an efficient control of most annuals. All these perennial species, which usually form localized populations, needed repeated spot-treatments with dalapon and 2,4-D for better results. Repeated applications with 2,4-D alone, however, were enough to kill the Cyperus species. (c) Demonstration with screened formula The formula for a foliar application with diuron + paraquat + 2,4-D at 1.6 kg + 8 1 + 5 kg/ha was put into a year-long demonstration following the completion of the experiments. Five locations of one-half hectare each in the railway yards were chosen to be the demonstration sites for the May 3 and September 3 applications of this formula. In the demonstration, the dosage of the component diuron was classified into 24 kg, 20 kg and 16 kg/ha, in order to get more information on the residual persistence of the combined herbicides. To get a balanced control of some perennials, 5 kg/ha of dalapon was added to the preparation. Depending on the condition of the weed regrowths, repeated sprays with dalapon and 2,4-D, each at 5 kg/ha, were made to keep the railway yards as clean as possible during the demonstration period. The following facts and conclusions were reached: The applications with the screened formula at the end of the first monsoon did not remain effective through the following monsoon, even with the 24 kg/ha application of the component diuron. With the first shower in the next rainy season, all the annual weeds suddenly germinated and grew quickly. The residual effect of the herbicides may have faded away long before the end of the first dry season. The prolonged dryness from October to the following May greatly helped to prevent the germination of annual weeds, once their established plants had been killed and the soil sterilized by the foliar-sprayed herbicides. A sufficient volume of diluting water was used, so as to rinse the soil surface. In the applications made at the beginning of the monsoon, the residual activity of the herbicides seemed to be less effective, particularly in the warmer, southern areas of the island. During the rainy season, weeds re-emerged freely, although at a much slower pace. In fact, the high rainfall and hot summer temperature often seemed to be the main cause of the inactivation of the residual herbicides in the soil. In sugar cane, for example, diuron sprayed in the dry and cold spring was much more toxic to cane seedlings than it was in the hot and wet summer. For this reason, the applications made in May in the southern locations needed more than three supplementary broadcast sprays with dalapon and 2,4-D, each at 5 kg/ha, to check the resprouting of weeds throughout the season. Generally, the September applications, made at the end of the first monsoon, maintained a longer and more highly active residual effect against all annual species, through to the following May. Only one supplementary spray with dalapon and 2,4-D

205 was needed to spot-treat regrowths of the perennial species. Figures 39 and 40 show respectively the railway yards before, and nine months after, this herbicide treatment. Since the sugar mills in Taiwan start operations in November each year, and busy transportation of materials begins at that time, the September application with this formula appears to be the most appropriate method of weed control for the railroads. For those railways used solely for the transportation of raw materials for grinding, reduced dosage rates of the more expensive diuron and paraquat, in foliar applications, can be prepared, and so achieve the shorter period of weed control needed during the milling season. For those railways with all-year-round traffic, in rural districts, full dosages of the herbicides are used, and supplementary sprays with dalapon and 2,4-D are necessary to spot-treat regrowths of perennials. As the unit cost of this chemical method is well within the range of that which is spent on hand-weeding for the railroads, this formula, of September application, has been recommended for general use on all the lines of the Taiwan sugar industry's railways. In addition, a training class for the foremen and maintenance engineers was held, to give them the necessary knowledge about chemical weed control. Spraying equipment suitable for the railways has been designed, in order to make use of this method.

Fig. 39. The railway yards of a sugar mill almost suffocated with a luxuriant vegetation, just before being treated by the September foliar application with diuron + paraquat + 2,4-D at 1.6 k g + 8 1 + 5 k g .

206

Fig. 4 0 . The railway yards of a sugar mill, nine months after the herbicide treatment, showing a complete elimination of the weeds.

(2) Total control of weeds on farm-roads, farmsteads, ditchbanks and other noncropped fringe land In the sugar mill estates weed control has usually been neglected outside the fields. However, the rank shrubs and heavy brush undergrowth on the roadsides, field edges and other non-cropped fringe not only interfere with transportation, stand-by work for planting, and other operations like applying pesticides and fertilizers, but serve as a refuge for field rodents, which host insects that transmit plant diseases, and are a plentiful source of weed seeds for spreading to nearby fields. Undesirable woody and herbaceous plants mar the environments of the farmsteads and farm-yards, and are a fire hazard. The vegetation on the ditch-banks and in field canals block the water flow, and reduce the efficiency of irrigation and drainage. Therefore, effective weed control on such non-cropped areas should be as important as in the fields, if a clean environment and efficient management of the plantations are desired. As most weeds on the non-cultivated and non-cropped areas are deep-rooted annual and perennial grasses, the use of herbicides to kill their topgrowth without hurting their roots must be emphasized, because their roots, in consolidating the soil against erosion during the rainy season, are indispensable. The use of paraquat, in combination with some residual compound such as diuron, in high volume application to rinse both foliage and soil surface can achieve chemical control of the brush undergrowth. According to the period required for control (3—9 months or so), a spray mixture of paraquat at 4—8 1 and diuron at 4—16 kg/ha can be used. (The monsoon

207 season form June through August with high temperature and intense rains is a barrier to the persistence in soil of even the most le aching-resistant diuron, applied at even so high a dose as 24 kg ai/ha; so the maximum persistence is nine months, from September to May, with the optimal dose of 16 kg ai/ha of diuron). The time of application should be when the monsoon is just over and the drought season begins, so that once the topgrowth has been knocked down by the sprays, the soil still has sufficient moisture to make the soil-herbicide active, so preventing regrowth; and there will be no more lingering or intense rain to interrupt the herbicide's persistence, until the next monsoon. Alternatively, the commonly used oil-soluble, amine, brush killer (a commercial formulation of 2,4-D amine and 2,4,5-T in mixture, at 4 lbs. of acid equivalent per gallon) is recommended for foliage sprays for brush control. For controlling the woody plants are the following recommendations (Leonard, Harvey and Perry, 1963): the hardwood shrubs and undesirable trees are satisfactorily controlled by basal sprays using a mixture of brush killer and diesel fuel (0.56 + 13.5 1 in proportion). Spray the base of stems thoroughly, or pour the mixture uniformly on the bark around the base, using enough brush killer to saturate the bark and the soil close by. Larger stems should frilled or cut with hatchet or axe near the base, and the spray mixture should then be applied liberally to the cuts. Or, the cuts may be made near the ground, through the bark and well into the wood, making sure that the cuts are continuous around the tree. Then fill the cuts with undiluted 2,4-D amine. For much larger trees, they must be sawn off near the ground, and the stumps, together with their deeply-penetrated roots, can be removed by 'stump control'. The most common method is to use the basal spray mixture of brush killer and diesel fuel. Make sure that the top, sides and base of a stump are thoroughly covered. Where the bark is thick, make axe cuts through it near the ground. Spray any sprouts around the stump. Freshly cut stumps are also controlled by applying undiluted 2,4-D amine to their tops; water occasionally to carry the amine down into the stump. There is also a method of soil application with a fumigant (ethlene, dibromide, DD, Telone and SMDC), for killing the large woody plants. Pour or inject Vi cup of a soil fumigant into holes 1 5 - 2 0 cm deep, spaced 15 cm apart around the tree. Killing is most rapid during the period of active growth. If the kill is partial, additional application will be required.