The Chemical Control of Monopolizing Single Species of Perennial Weeds

Chapter 9

THE C H E M I C A L C O N T R O L OF M O N O P O L I Z I N G S I N G L E SPECIES OF P E R E N N I A L W E E D S

The perennial weeds that rely principally upon their underground organs for vegetative multiplication are especially adaptable to their association with sugarcane, because there are no cross-ploughings during the succeeding ratooning of this crop. So, a weed whose established system of rhizomes would be destroyed by crossploughing of the field for planting new cane, resumes multiplication and establishes itself, usually to monopolize a whole field, for many years until the ratooning of cane is interrupted. During its multiplication and establishment, the cane plant meets insurmountable competition, and suffers heavy losses in growth and yield. Thus, ratooning has to be replaced by new planting. As shown previously, the rhizomatous perennials, characteristically for their vegetative multiplication, absorb more soil water and nutrients (in excess of normal requirements) under environmental stress. Even fragmentation of their rhizomes by ploughing or cultivation has such a stimulative effect. Consequently, they can survive environmental adversities, including mechanical injury when ploughing the field, to regenerate themselves year after year. There are great differences, therefore, in controlling perennial species that tend to monopolize an area because of their powerful regeneration. Without consideration of any crop injury, when the field is fallowed, what should be emphasized is how to eradicate (chemically and effectively) the underground storage organs, rather than killing the aerial foliage (as with control of the annuals). Following new planting, there are selective applications of the translocative compounds for killing regrowths from remnant rhizomes, minimizing competition against the cane plants for at least the early, critical, growth period.

1. CHEMICAL ERADICATION OF TORPEDO G R A S S IN CANE FIELDS

In the warm and wet, southern parts of Taiwan, hardly any sugarcane fields escape infestation by a perennial weed, Panicum repens L. (called torpedo grass in Hawaii). Especially in low land with sandy soil, this weed with its robust and highly regenerative rhizomes propagates vegetatively year after year. Though it forms an inflorescence, it bears no fertile seeds. Where this grass has become established, other species are crowded out and sugarcane yield is greatly affected, particularly in ratoons (Fig. 45). In 1965, for example, over 200 ha of ratoon fields in Pingtung were so severely infested that one could not readily distinguish the cane plants from the grass. In most of such infested fields it was necessary to plough out the half-grown cane and replant. Checking encroachment by this weed is a major problem for cane growers, and cross-ploughing and harrowing have traditionally been employed to get rid of the

232

Fig. 4 5 . A field of ratoon cane seriously infested by the torpedo grass; suppressed cane shoots almost indistinguishable from the stout grass.

rhizomes before planting new cane. After years of this treatment, however, the weed has not been checked but has become more rampant, since cultivation served only to aid its spread. Heavy infestation generally recurred in ratoons when the field was subjected only to shallow intertillage (which was ineffective against even the superficial rhizomes). The pre-emergence mixture of diuron and 2,4-D commonly used against seed-propagated annuals was unsuccessful against P. repens. As pointed out by Crafts and Robbins (1962), eradication rather than control is desirable against a noxious weed whose infestation and spread are limited; this vegetatively propagated grass should, too, be dealt with in this way. Cane production would be greatly improved if a selective herbicidal method were found for eradicating the rhizomes of this weed from the fields. Although this perennial weed is present in most subtropical, cane-producing regions, such a method of selective eradication has not, as yet, been attempted. Several experiments, for this purpose were, therefore, conducted in the years from 1966 to 1971. Experimentation then followed the principle of Woodford (1950) that herbicides should be tested first with the weed, secondly with the crop and thirdly with the weed and crop together. Various chemicals, ranging from soil sterilants to contact and systemic herbicides, were tested for their effectiveness in different combinations; first with the weed alone at different growth stages. This was done either in weed-infested fields without cane, or after planting the weed in clay pots or flat beds. Then, a few promising compounds were tested in the fields for weed-killing effectiveness, and effect on cane. The results (Peng and Twu, 1974) are as follows:

233 ( 1 ) Ecological study of Panicum repens The growth habits of the weed under cultivation were studied for one year at Ailyau-chi, a heavily infested plantation in Pingtung District, southern Taiwan. The field was ploughed in October and prepared in December 1960, after which, samples of the grass plants were periodically measured. On each occasion a square metre of the area was sampled, the grass being dug out and cut into rhizomes and stems which were weighed separately. The results for 9 such observations are shown below. Date of observation

Count of stems 2 Wt. of stems ( g / m ) Wt. of rhizomes 2 (g/m ) Flowering

1966

1967

Dec. 7

Mar. 14

Mar. 30

Jun. 9

Jul. 30

Aug. 30

Sep. 30

Nov. 1

Nov. 30

223 43 420

349 92 390

485 113 313

738 700 1000

406 1000 1300

324 1200 1450

418 1380 1480

532 1000 2000

606 700 2400

Nil

Nil

Nil

Scarce

Scarce

Scarce

Full

Full

Full

After the field was ploughed in October 1966, in an attempt to destroy most of the grass plants, segments of the rhizomes remaining in the soil soon resumed growth, and 2 a density of 420 g/m was attained by December 7th, most of the rhizomes being within the top 10—15 cm. The density of rhizomes continued to increase, reaching 2 2400 g/m by the end of November 1967. Although the aerial stems showed maximum growth during the short rainy season from July to August, they still weighed 20% less than the corresponding rhizomes in the same period. Rhizomes of this weed exist in the soil all the year round, but the aerial parts may not be seen for some time after land preparation. To be effective, control of this weed should aim at the destruction of the underground rhizomes, rather than the superficial killing of the aerial parts. The destructive effect on cane of competition with this weed can be demonstrated by planting various quantities of its sprigs (6 densities of 5, 10, 25, 50, 75 and 100 g/pot) with single-budded cane cuttings in clay pots. The results of such an experiment have been shown in Table 2.6 (Treatment 7-13). A simple correlation coefficient (r) between the weights of weed and cane per pot, calculated as - 0 . 8 9 2 , significant at the 0.01 level of probability, showed that the growth of cane reduced proportionately as that of weed increased. However, it should be noted that the weight of cane plants decreased sharply at the start and levelled off to about 50% of the check pot when the weight of weeds increased to 60 g/pot. Beyond this, and in the range from 6 0 . 0 - 1 0 4 . 0 g/pot, the reduction of cane growth was slight. This suggests that a mild occurrence of this weed in a field may result in a loss in cane yield as severe as that caused by fully established weeds.

234 (2) Chemical eradication of grass by herbicides in a non-cropped

field

In the same Ai-lyau-chi plantation, heavily infested by torpedo grass, a field which had been thoroughly ploughed and prepared in October 1966, and left without a cane crop, was used for this experiment. The plot size was four rows 4 m long in 1.20 m row spacing. There were 3 replicated blocks in a randomised complete block design, and the treatments consisted of spraying with one of the following 10 combinations of herbicides: Rate (kg ai/ha)

Type

Application date

Repeat date

1 diuron + 2,4-D*

8 + 8

Pre

2 bromacil + 2,4-D

8 + 8

Pre

Feb. 8th 1967 Feb. 8th 1967

16 + 16 + 16

3-mon late post 3-mon. late post 1-mon. late post 1-mon. late post 1-mon. late post 1-mon late post 1-mon. late post 1-mon. late post

Oct. 29th 1966 Oct. 29th 1966 Mar. 30th 1967 Mar. 30th 1967 Dec. 6th 1966 Dec. 6th 1966 Dec. 6th 1966 Dec. 6th 1966 Dec. 6th 1966 Dec. 6th 1966

Treatment

3 paraquat + terbacil + Pesco 1 8 - 1 5 * * 4 paraquat -1- bromacil + Pesco 18-15 5 dalapon + paraquat + 2,4-D 6 dalapon + linuron + 2,4-D 7 dalapon + bromacil + Pesco 18-15 8 dalapon + paraquat + 2,3,6-TCA 9 dalapon 4- linuron + 2,3,6-TCA 10 dalapon + bromacil + 2,3,6-TCA

16 + 16 + 16 12 + cI + 8 12 + cI + 8 12 + cI + 8 12 + i * + 8 12 + c5 + 8 12 + I* + 8

Feb.8th 1967 Feb. 8th 1967 Feb. 8th 1967 Feb. 8th 1967 Feb. 8th 1967 Feb. 8th 1967

* Sodium salt used for pre-emergence and dimethyl amine salt for post-emergence treatments. **Consisting of MCPA 150 g/1 and 2,3,6-TCA 4 8 g/1, marketed by Fisons.

Treatment of half the area of each set of plots (except 3 and 4) was repeated about 2 months later, at the dates shown above. It was thus possible to compare the effect of the herbicide combinations on torpedo grass in single and double applications. Combinations 5—10 were also used to spray other plots, 2 and 3 months later (on 8th February and 14th March 1967) as 2- and 3-month late post-emergence single applications. Again, each was repeated on half the area of the treated plots, on 30th March and 7th June 1967, so that a similar comparison between single and double applications could be made. A comparison of the herbicidal effects on the different growth stages of grass could also be obtained in this way. One square metre of all the treated and control plots was sampled, and the surviving weed plants were removed from the soil on 30th November 1967, approximately one year after the first pre-emergence treatment. Measurements of vegetative parts of the weed for different treatments are in Table 9.1.

235 TABLE 9.1 Measurements of torpedo grass 1 year after treatment Type of treatment

Single application Count of

%

Wt. of stems

stems

g/m

2

Wt. of rhizomes

%

g/m

2

%

Pre

1 2

81.0 65.8

66.4 53.9

350 495

53.8 76.1

650 895

31.3 43.1

3-mon. late post

3 4

15.5 6.8

12.7 5.6

30 105

4.6 16.1

90 205

4.3 9.8

1-mon. late post

5 6 7 8 9 10

112.0 104.3 91.5 90.0 105.8 111.8

91.8 85.5 75.0 73.8 86.7 91.6

1000 590 595 460 490 700

153.8 90.7 91.5 70.7 75.4 107.7

1975 1250 1025 1295 1245 990

95.2 60.2 49.4 62.4 60.0 47.7

2-mon. late post

5 6 7 8 9 10

120.3 79.5 54.3 123.0 111.5 116.3

98.6 65.2 44.5 100.8 91.4 95.3

990 615 300 780 605 800

152.3 94.6 46.1 120.0 93.0 123.0

1220 1345 665 1445 1250 1395

58.8 64.8 32.0 69.6 60.2 67.2

3-mon. late post

5 6 7 8 9 10

84.8 122.3 29.5 115.5 117.3 94.0

69.5 100.2 24.2 94.7 96.2 77.1

450 770 115 935 705 545

69.2 118.4 17.7 143.8 108.4 83.8

350 2000 200 1780 1550 1085

16.9 96.4 9.6 85.8 74.7 52.3

CK

122.0

100.0

650

100.0

2075

100.0

F test LSD ( 0 . 0 5 ) (0.01)

5.63** 327.68 446.41

50.4 68.7

100 160

15.4 24.6

2.78** 954.68 1300.58

46.0 62.7

double application Pre

1 2

59.3 49.0

41.6 34.4

3-mon. late post

3 4

_

_

—

_

-

-

—

-

1-mon. late post

5 6 7 8 9 10

80.0 87.8 50.3 20.8 69.0 25.5

56.2 47.6 35.3 14.6 48.5 17.9

700 590 375 205 500 255

107.7 90.7 57.7 31.5 76.9 39.2

95 425

4.6 20.5

_ -

1100 1250 645 240 645 365

53.0 60.2 31.3 11.6 31.1 17.6

236 TABLE 9.1

(Continued) Double application

Type of treatment

Wt. of rhizomes

Wt. of stems

Count of stems

g/m

2

%

g/m

2

%

2-mon. late post

5 6 7 8 9 10

120.3 79.5 11.3 107.3 111.5 51.0

84.5 55.9 7.9 75.4 78.4 35.8

880 605 30 477 482 375

135.4 93.0 4.6 73.4 74.1 57.7

1235 1255 65 1345 1200 520

59.5 60.5 3.1 64.8 57.8 25.1

3-mon. late post

5 6 7

84.8 122.3 7.3

59.6 85.9 5.1

565 645 30

86.9 99.2 4.6

860 1500 50

41.4 72.3 2.4

Ö 9 10

96.5 4.8

67.8 3.4

655 65

100.7 10.0

1425 120

68.7 5.8

CK

142.3

100.0

650

100.0

2075

100.0

ο

F test LSD (0.05) (0.01)

4.02** 429.93 585.43

66.1 90.0

3.18** 911.84 1242.21

43.9 59.9

* * D e n o t e s significance at 0.01 level of probability.

Table 9.1 reveals the following facts: (a) Regardless of treatment with any combination of herbicides, regrowth of the rhizomes was about twice as heavy as that of the stems, emphasizing the importance of effective destruction of the rhizomes of this weed, (b) The combination involving dalapon, bromacil and Pesco 18-15 (Treatment 7) showed the greatest killing effect. Compared with the unsprayed plots, it killed 90.4% of the rhizomes with a single overall application, as indicated by observations 7 months after spraying. When this treatment was repeated on the same plots 2 months later, representing a double application of herbicide, most of the rhizomes were killed and a remnant amounting to only 2.4% of the quantities was found in control plots 4 months after spraying. When single and double applications of this treatment were conducted at earlier growth stages of the weed, less effective kills resulted and more rhizomes were found to survive, (c) Treatments 3 and 4, in which terbacil and bromacil were each combined with Pesco 18-15, also produced the greatest kills of rhizomes (95.7% and 90.2%) when used in 3-month late post-emergence single applications, (d) Used in single pre-emergence applications, the group involving diuron and bromacil in combination with 2,4-D produced the best results, killing 68.7% and 56.9% of the rhizomes. When repeated about 3 months later (this time in overall sprays) they again killed most rhizomes, with only 4.6% and 20.5% remaining. Apparently diuron, with its lower water solubility, remained as residue in the soil much longer than bromacil, and was therefore more effective. From this and other experiments it appears that these herbicides, used for total control of weeds, are, in order of decreasing potency, terbacil, bromacil, diuron,

237 linuron, dalapon, Pesco 18-15, 2,3,6-TCA and 2,4-D. The exceptionally high doses of terbacil and bromacil, applied at an earlier growth stage of the weed, caused much less reduction of rhizomes finally than did the late application, because of the ability of the rhizomes to regenerate in the soil. This emphasises the difficulty of chemical eradication of this grass from an infested area, for remnants of the rhizomes surviving can multiply and form again a dense sod. (3) Field test of herbicide combinations for total control of torpedo grass in sugarcane Whether the herbicide combinations for total control could be safely used in sugarcane was investigated on the same plantation later in the same year. The cane (variety F 156, planted in February as the 1967—68 spring-planted crop) was treated with the herbicides either singly or in combination, at the same high rates as before. The sprays were directed onto the interrow weeds alone, to avoid the severe damage which the young cane might suffer under contact spraying. A pre-emergence dose of diuron plus 2,4-D, at 1.6 plus 1.6 kg ai/ha, was hand sprayed on the cane rows to control some of the weeds before the sprays directed at the interrows were applied. The results showed that clean hand-hoeing 6 times, to remove aerial parts of torpedo grass during the early growth of cane, exhausted the rhizomes and left a remnant of 1.10 t/ha of them, which was only 15.7% of that observed in non-weeded plots at harvest, 10 months later. This manual operation also led to a 45 t/ha yield of cane, which was 31.2% better than the yield from non-weeded plots. The reduction of grass rhizomes and the increase in cane yield resulting from clean hand-hoeing were both statistically significant. If the hoeing was done only twice, the grass rhizomes were reduced merely to 64.7%, and an insignificant 14.0% increase in yield of cane resulted. This is at variance with the control of annual weeds, where clean hand-weeding produced somewhat detrimental effects on cane growth and a smaller cane yield than did fewer weedings (Peng and Sze, 1969a). Among the herbicide treatments, only the mixture of dalapon and 2,4-D at 6.4 kg ai/ha, used as a directed spray one month after planting, effectively killed the grass so that only 1.33 t/ha of rhizomes survived in the soil near the time of harvest. This represented 19.1% of those in non-weeded plots. The plots that received this treatment produced 30.3% more cane than did non-weeded plots, and the treatment was, therefore, comparable with clean hand-hoeing. Bromacil, in directed post-emergence application 2 months after planting, in another treatment, killed the grass rhizomes even more effectively, by 94.3%, but also caused severe phytotoxicity of cane, causing 81.6% reduction compared with non-weeded plots. Summarising the results of this experiment, it seems that in contrast with such soil sterilants as terbacil and bromacil the compound dalapon should be used at a fairly high dosage, and as early as possible, for more effective control of this grass. As dalapon readily decomposes in soil, it may cause no adverse effect on cane plants, even at high rates, if the spray is directed only at interrow weeds. In contrast, 2,4-D is less effective against this weed, and injury to cane may be caused by its residual activity if it is used at too high a rate. A mixture of the two compounds showed a much higher killing effect, without significantly affecting the cane plant adversely.

238 Paraquat achieves quicker kills of the aerial parts than of the rhizomes, but is of little value as the rhizomes will not be significantly affected. Although bromacil causes the greatest mortality of grass plants, its effectiveness against this weed in cane is doubtful because its high residual activity in the soil may cause severe injury to the current cane crop and, very likely, to the following crop as well. (4) Synergistic activity of herbicides against torpedo grass Another experiment was conducted at this Institute to determine whether some promising herbicide combinations were successful because of synergistic activity. At the same time, the effect on the grass of different placements of the toxicants was examined. The grass plants (both stems and rhizomes) were cut into sprigs of equal length, each containing 1 or 2 nodes, thoroughly mixed, and planted at 500 g/plot. The plots were flat beds 0.5 x 0.5 m in size, with a border of buried bricks; the grass was planted in January 1968. After about two months, when the grass had grown to the 3-node stage, the plots were treated with herbicides. The compounds tested were diuron, paraquat, dalapon and 2,4-D, which were used either singly at 10 kg ai/ha, or 2, 3 or 4 of them were combined in proportion making up a total of 10 kg ai/ha, in equal amounts of water. For comparison of the synergistic effects of the herbicides, the observations of the fresh weight of the surviving stems and rhizomes of the grass, 4 months after treatment, are in Table 9.2. It is seen that a combination of dalapon and 2,4-D (Treatment 10) killed more rhizomes than did each used singly at the same total application rate. The synergistic activity of the compounds was, thus, demonstrated, and the use of the mixtures in previous experiments was justified. On the other hand, the combination of three compounds paraquat, dalapon and 2,4-D (Treatment 13) showed no significant difference from the use of any two of them in combination. However, the combination of the three compounds was much cheaper, as the component doses of the expensive compounds were less. After extensive experiments (Peng, 1969) this formula was adopted for commercial use for the control of emerged annual weeds in sugarcane, in Taiwan. The mixture of diuron and 2,4-D showed greater effect than either ingredient alone, but placing diuron so as to attack different parts of the grass plant showed no significant effective difference. In spite of apparent tissue deterioration following the treatment with herbicides, the specific gravity of stems and rhizomes was not significantly affected. (5) Eradication of grass rhizomes by dalapon combinations in repeated during the fallow period, and effect on subsequent cane yield

applications

In field practice, if it is decided to discontinue ratooning in a field, there is usually a fallow period of 6—8 months between the harvest of a ratoon in the winter, and the planting of a new crop in the fall. In Taiwan, in recent years, deterioration of the yield of ratoon cane has become pronounced, and the cultivation of ratoons rarely exceeds two harvests. During the fallow period, green manures are grown in an attempt to restore soil productivity. When ratooning has to be abandoned because of

ntact gence s nts 1 Treat. No. Combinations of herbicides 2 Rates in proportion (kg ai/ha)

pray e grass 1 2 3 4 5 6 7 8 9 10 11 12 13 14

diuron paraquat dalapon 2,4-D diuron + paraquat diuron + dalapon diuron + 2,4-D paraquat + dalapon paraquat + 2,4-D dalapon + 2,4-D diuron + paraquat + dalapon diuron + paraquat + 2,4-D paraquat + dalapon + 2,4-D diuron + paraquat + dalapon + 2,4-D diuron + 2,4-D diuron + 2,4-D diuron + 2,4-D

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

15 16 17

18

yl amine salt of 2,4-D was used. tes non-significance. es significance at the 0.01 level of probability.

F test LSD ( 0 . 0 5 ) (0.01) 432.39 581.49

9 4**

151.0

5.7 78.8 0.5

6 + 4 6 + 4 6 + 4

16.0 480.0 453.3 643.3 16.3 17.7 56.7 533.3 540.0 385.0 28.5 41.7 450.0 551.7 46.7 76.7 41.7 863.3

0.4 5.2 0.0 100.0

g/plot

0.3 25.4 25.2 70.0 0.2 0.2 1.0 42.8 54.4 36.8 1.5 0.2 58.9 52.0

%

28.6 38.5

6.3** 313.83 422.05

%

Wt. of rhizomes

R o o t contact: grass plants were dug out of flat beds, b o t t o m soil was sprayed and grass replanted in beds. Pre-emergence: aerial parts of grass were removed before spraying on soil surface of flat beds. Rhizomes soaked: whole grass plants were dug out, soil removed, rhizomes dipped for 1 s in herbicide and replanted.

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

1

5.0 383.3 380.0 1056.7 3.3 3.3 15.3 646.7 821.7 555.0 23.3 3.0 890.0 785.0

g/plot

Wt. of stems

Observation at July 9, 1 9 6 8 for

Synergistic effects of herbicides and effects of different placements (average of 4 replicates)

9.2

1.9 55.6 52.5 74.5

1.9 2.1 6.6 61.8 62.5 44.6 3.3 4.8 52.1 63.9 5.4 8.9 4.8

100.0 36.4 48.9

240 heavy infestation by torpedo grass, cross-ploughing is practised during a fallow period to destroy the established rhizomes. It was decided, therefore, to study the possibility of the total eradication of the grass with herbicides during the fallow period, and to investigate the response of subsequent cane crops, possibly without any residual influence of the toxicants. A field experiment was initiated at Liu-kwi-chu, another plantation in Pingtung district heavily infested by this species. After harvesting the last ratoon crop, which was estimated to yield less than 30 tons cane per ha, or 65% lower than the average, the field was cross-ploughed in March 1968. On April 2nd, regrowth of torpedo grass was treated with 5 different dalapon combinations, in applications which were repeated at intervals of 2 weeks. With unsprayed plots as control, there were 6 treatments each replicated 4 times in a randomised complete block with a plot size of 20 rows, 20 m long x 1.25 m row spacing. After 5 applications all the emerged grass seemed to have been killed, and there was no regrowth. The field was again crossploughed on July 2nd, approximately in the middle of the fallow period, to stimulate the regrowth of grass from deeply buried rhizomes. Two additional sprays of each herbicidal treatment were used to knock down the few scattered grass plants which subsequently emerged. In early September, the field was cross-ploughed for a third time, in preparation for planting the new cane (variety F 156) on September 19th. In addition to the autumn-planted crop, 2 successive ratoons were harvested to study the effects on the cane yield of the grass rhizomes which survived the treatments. At the outset of the growing period of each of the subsequent crops, overall preemergence herbicides diuron and 2,4-D in a mixture (each at 1.6 kg ai/ha) were used for control of the annual weeds. To determine the killing effect on grass of the dalapon combinations in repeated applications, a square metre of each plot was sampled, and the rhizomes were dug out and weighed by treatments. This was done twice during the fallow, and once at each harvest of the autumn-planted crop and the two ratoons. The results are in Table 9.3. It is seen that in general all the dalapon combinations in repeated applications during the fallow, effectively reduced the density of grass rhizomes in soil. After seven sprays, with one cross-ploughing interposed, there was almost complete mortality of rhizomes, whereas only about 50% mortality was found towards the end of five sprays, before interposing of the cross-ploughings. The remnants of the rhizomes were further reduced by cultivation of the autumn-planted crop. They gradually recrudesced in the first ratoon, and recovered almost completely towards the end of the second ratoon because cultivation, and particularly cross-ploughing, could not be practised during the ratooning season. With the exception of treatment 3, in which the dosage of 2,4-D ester was too low, all the dalapon combinations promoted improved cane yields in the subsequent second ratoon crop. The treated, autumn-planted crop gave a cane yield 62—98% higher than the control plots, since competition from the grass had been reduced to a minimum. In addition, 2 0 - 5 3 % , and 1 4 - 3 2 % increments were maintained in the first and second ratoons, respectively, although these differences from the control plots were not statistically significant. Comparison among treatments showed that dalapon used on the autumn-planted

Jan. 14, 1972

Jan. 2 8 , 1970

Dec. 2 6 , 1970

Jan. 14, 1972 t/ha

%

t/ha

%

% t/ha

t/ha

% 5.1 44.0

des were applied 7 times at intervals of 2 weeks.

es significance at the 0.01 level of probability.

5.2 64.9

115.2 173.1

107.8 162.0

62.8 109.0

73.3 127.2 55.1 95.7

49.4 130.0

45.0 118.4 38.4 101.0

110.9 166.7 2.5 21.3

127.0 190.9

88.2 153.1

0.8 NS

Treatment N o . 1 4

0.1 1.7

Yields of cane crops harvested

Treatments were (1) Dalapon + 2,4-D sodium (5 + 5 kg ai/ha). (2) Dalapon + 2,4-D sodium (10 + 5 kg ai/ha). (3) Dalapon + 2,4-D ester (5 + 0.7 kg ai/ha). (4) Paraquat + terbacil + dalapon (0.4 + 2 + 5 kg ai/ha). (5) Paraquat + bromacil + dalapon (0.4 + 2 + 5 kg ai/ha). (6) Check (untreated). 2 Observed on May 30 after 5 applications and on August 20 after 7 applications with 1 cross-ploughing between the 5th and 6th. NS Denotes non-significance.

50.5 132.9

38.0 100.0

0.1 0.8

2.2 NS

5.5 43.1 0.1 0.9

43.3 114.2

0.6 9.3

57.6 100.0

0.7 10.8

6.0**

0.4 7.3

66.5 100.0

22.5 NS

7.6 60.1 1.3 8.7

69.4 120.5

5.8 49.8

4.7**

104.9**

3

132.2 198.7

0.5 7.8 0.5 7.6

11.6 100.0

t/ha 2

5.8 49.8

% 0.8 12.6

6.6 100.0

t/ha 7.3 57.7 0.5 3.2

0.4 5.8

%

6.0 100.0

t/ha

6.9 54.5 1.2 8.0

2110.2**

1

0.3 5.3

%

5.6**

%

12.7 100.0 15.1 100.0

Dec. 2 6 . 1970 t/ha

6.4 50.2 0.1 0.8

Date

Yields of grass rhizomes measured during fallow and at harvest of cane crops

Jan. 2 8 , 1970 F

Aug. 2 0 , 1968

6 (CK)

May 30, 1968

5

combinations in repeated applications during fallow

0.05

LSD

0.40 3.15 0.04 0.26 0.07 1.15 0.39 5.92

28.50 42.83

242 crop at 10 kg ai/ha, in combination with 2,4-D sodium, reduced the yield even more than it did at 5 kg ai/ha. Obviously, the repeated application of dalapon at too high a dosage is an uneconomic way to control the rhizomes. For treatment during the fallow period, the results of these investigations favour the use of the highly potent residual compounds terbacil and bromacil, which might cause severe injury to cane if applied during the growing season. Cross-ploughing three times, at the beginning, middle and end of the fallow period, reduced the rhizomes from approximately 12.5 t/ha before the first ploughing to 6.04 t/ha at harvest of the autumn-planted crop. Without further treatment, this should improve the yield. Treated plots produced an impressive crop, with an average yield almost four times as high as that of the last, infested crop (Fig. 46). In the second ratoon, the density of regenerated rhizomes in the treated plots was about 5 t/ha, and the effect on yield was similar to that of the 11.55 t/ha of rhizomes in the control plots. This confirmed observations in pot tests mentioned in the second section. (6) Regrowths of torpedo grass in ratoon crops to be eliminated with dalapon Since the young plants of ratoon cane (as has been shown) tolerate at the 6-leaf stage the foliage-absorbed dalapon, even at such a high application rate as 10 kg/ha, and suffer only slight, transient growth reduction (Peng, 1972), the use of this herb-

Fig. 4 6 . A bumper crop of autumn-planted sugarcane with stalk yield of 132.2 t/ha, as a response to fallow eradication of rhizomes of torpedo grass with herbicides, while competition from untreated grass reduced the yield to only 66.5 t/ha.

243 icide in overall sprays for the selective killing of regrowths of torpedo grass among the plants of ratoon cane is feasible. As described in preceding sections, repeated treatments of dalapon plus 2,4-D, coupled with several cross-ploughings on a field heavily infested by torpedo grass during fallow, destroyed more than 80% of rhizomes of the grass when the 1974-75 autumn cane was planted. In the first ratoon following, the few surviving rhizomes that remained, mostly near the cane stubbles, sprouted together with the germinating cane shoots. By spraying 5 kg/ha of dalapon over cane rows twice, two weeks apart, the regrowths of grass and their rhizomes were completely eliminated, as observed 5 months later, leaving the cane plants growing without any toxic effect. This method of eliminating surviving remnants is, therefore, quite useful, supplementing the eradication of established rhizomes in fallow in a recycling practice, in order to get rid of this noxious grass eventually from an infested area. Equally safe for the growing cane plants are metribuzin or asulam, which can be used as a substitute for dalapon in over-the-rows application to control this grass in ratoons, as mentioned previously.

2. CHEMICAL CONTROL OF B E R M U D A G R A S S

Bermuda grass (Cynodon dactylon) is a perennial plant, vegetatively propagated by both stolons and rhizomes that spread extensively in the soil to monopolize an area. When heavily infested the field looks as though carpeted with a thick green slender pile, formed by the weed's extensively interwoven stolons sustaining numerous narrow leaves. Eradication of this grass by hand is costly, difficult, and unsatisfactory, while ploughing serves only to transport the cut portions of rhizomes and stolons, and to extend the infestation. In cane fields, dalapon and TCA are the chemicals normally used to eradicate this grass, but some biotypes of C dactylon are highly tolerant to these two herbicides, as observed in Mauritius (Rochecouste, 1962a; 1962b). There, susceptible and moderately susceptible bio types can be controlled by dalapon applied at rates of 1 0 - 2 0 lb. and by TCA of 4 0 - 6 0 lb. It has been shown that the best results are obtained when these chemicals are applied at the time of rhizome formation, when there is a rapid downward flow of metabolites towards the underground parts of the plant. Ploughing, together with herbicide treatment, may give more satisfactory results. This technique requires repeated sprays at small dosages, usually about 15 lb. for TCA and 5 lb. for dalapon. It must be emphasized, however, that herbicide application should be made at least two to three weeks after cultivation, when the young shoots have exhausted the food reserves contained in the portions of rhizomes from which they arise. At this particular growth phase the young plants are very susceptible to herbicide activity. Later studies in Mauritius (Rochecouste, 1967) on the control of a C dactylon biotype that is tolerant to both TCA and dalapon, have shown that the two uracils, bromacil and isocil, can give good results. In fact, with two applications of bromacil or isocil at 6.5 lb. ai per acre, sprayed one year apart, gave more than 95% control of this grass. Obviously, the use of such high rates of bromacil and isocil should be restricted to controlling the grass on industrial sites only. Biotpyes of C dactylon

244 tolerant to TCA or dalapon have also been effectively controlled with amitrol at 10—20 lb. per acre. The eradication of C. dactylon in ratoon cane is difficult because a certain proportion of the rhizomes lie dormant underneath the cane stool and resume activity only after harvest, when environmental conditions become favourable. At such a time, however, the developing cane shoots are too young to tolerate the rates of application that would kill the grass. The herbicide treatment of C. dactylon in plant cane is certainly the most effective way of eradicating the grass, and the following recommendations from Mauritius are given: (a) The grass should be treated when the cane is five months old, shortly before 'close in', since the subsequent shading of the interrow prevents the weed from renewing its activity. (b) TCA may be used at the rate of about 50 lb. per 100 gallons of water, and the grass foliage must be thoroughly wet. Dalapon may also be used at lower rates of 5—10 lb., depending on the tolerance of sugarcane varieties to this chemical. (c) C. dactylon growing in cane stools should be treated with a directional spray so as to avoid wetting cane foliage. When dalapon is applied, a shield-sprayer should preferably be used. (d) Regrowth should be treated at intervals of 6 weeks for complete eradication. Bermuda grass has not been as tenacious and troublesome as torpedo grass in the infestation of sugarcane in Taiwan. Having a similar regenerative capability, however, this grass perpetuates vegetative reproduction to survive fragmentation by cultivation and injury by herbicides (it tolerates more than 5 kg/ha of diuron in soil application) in the growing of each new crop. When a large planted area was infested, only a directed foliar application with the mixture of paraquat, dalapon and 2,4-D at 2 + 8 4- 2 kg ai/ha on this weed one month after planting could cause a 4-month-long clean weeding, cf. Chapter 7 and Figs. 35 and 36. Fortunately, the prostrate growth habit of this grass makes easy an almost blanket spraying (horizontally directed) under the erect cane leaves, to kill the weed in cane rows too. Localized patches of this grass that have escaped a pre-emergence herbicide treatment for the general control of annuals, can be effectively controlled by spot-treatment with dalapon, TCA or the new compounds asulam (in combination with 2,4-D) and metribuzin at 2—4 kg ai/ha, repeated 2—3 times until a complete kill is accomplished. The selective control measures in growing cane (described above) destroy aerial parts of this grass to relieve the pressure of competition against the cane plants. They are, however, far from successful in eliminating the underground rhizomes that are the source of reinfestation. Maroder (1973) showed that aerial organs of this grass, when foliar-treated twice with 5 kg/ha of dalapon, were completely killed 3 weeks after. However, the underground rhizomes were only partially affected, showing dead apices and small, necrotic, malformed shoots. The roots apparently were not affected and showed a normal appearance. Eventually, most rhizomes were able to recover. By means of a radioactive tracer, he showed that dalapon was freely translocated in the aerial organs and, from these, to the rhizomes. In rhizomes and in stolons, dalapon caused growth inhibition and necrosis of buds; root growth was apparently normal.

245 Bingham (1967) reported similar results by testing Bermuda grass for the effects of DCPA, DMPA, Bensulide, Diphenamid, etc., which are used for the control of crabgrass (Digitaria spp.) in its turf. In a greenhouse, sprigs of Bermuda grass were planted in pots to develop stolons, and alongside were other pots which were not planted, but were treated with each of the herbicides on its soil surface. When letting the stolons trail to the herbicide-treated pots, he found that all the surface applications of these herbicides prevented the rooting of a variety of this grass from the stolon nodes. These herbicides did not appear to have a direct influence on the growth rate of stolons. Placement of the herbicides at various soil depths also reduced the number of roots developing below the treated layer. Under field conditions, the surface application of these herbicides on Bermuda grass turf appeared to have a temporary effect on the established root systems. In Taiwan, according to experimental results recently published, Bermuda grass, with similarities to torpedo grass, responds to such environmental adversities as mechanical fragmentation, soil moisture stress, and herbicidal toxicity, by absorbing extra soil nutrients to enhance its regeneration. With weekly irrigation of 4 1 per pot during the dry season, there was 257.0 g/pot rhizomes in dry weight after growing for about 6 months, whereas the unirrigated pots yielded 125.5 g/pot, or 48.8% less rhizomes. However, the unirrigated rhizomes contained at harvest 35.2% more Ν and 38.0% more K. Moreover, when the harvested parent rhizomes were replanted to produce their clonal grass, the low-yielding rhizomes, that contained the higher levels of Ν and K, gave 25% more aerial parts and 19% more rhizomes, after 2 months. In other treatments the parent grass that was foliar-treated with 4 kg ai/ha dalapon yielded 40% less rhizomes, but contained 14% more Ν and 34% more P. The clonal grass from the treated parent grass produced 42% more aerial parts and rhizomes. These facts show how this weed can readily reinfest fields after cross-ploughings, in growing each cane crop (Peng and Twu, 1982).

3. CHEMICAL CONTROL O F THE YELLOW A N D PURPLE NUTSEDGES

Belonging to the tuberous perennials, there are two important members of the Cyperaceae, the Cyperus esculentus and Cyperus rotundus. C. esculentus, known as yellow nutsedge, is a perennial sedge which reproduces by tubers and seeds. Its flowering stem is erect, triangular, yellowish green in colour, and bears at its top umbrellalike leaves, which subtend the inflorescence and its characteristic yellowish-brown to straw-coloured spikelets. The underground tubers or nutlets are globose in shape and, depending on soil types, may penetrate deeply into the soil. Each tuber contains food reserves and is made up of thickened and greatly shortened internodes borne at the end of thread-like rhizomes. The tubers are persistent and remain viable in the soil for months until growth conditions become favourable when they give rise to shoots which develop into new plants. Although the plant has been reported to occur in many sugar producing countries, it is particularly troublesome in certain areas of South Africa. In Mauritius, yellow nutsedge may be suppressed by treating with esters of 2,4-D

246 and 2,4,5-T at about 2 - 3 lb. a.e. per acre, for about a month. However, even with repeated applications of this treatment, the sedge is not completely eradicated. Soil incorporation of EPTC has also been reported to have a good suppressing effect on the growth of this weed. C. esculentus, known as water grass in South Africa, is controlled by paraquat and bromacil. Recommendations by the South African Sugar Association Experiment Station (Anon., 1965) are: paraquat applied at 0.5 lb. ai per acre about 4 weeks after planting, when the sedge is coming into flower, gives a control of one to two months depending on the season at which the treatment is made. To obtain residual control of the weed, bromacil at about 1 lb. ai per acre is added to the paraquat spray when used in light sandy soil. The effectiveness of this mixture is claimed to last until canopy formation. Although damage is caused to the cane, it is reported to be negligible in the final yield. However, the rate of bromacil that can be used without seriously affecting cane growth has been found to be related to soil organic matter content. Consequently, the rate of bromacil 1.2-3.6 lb. ai per acre is regulated according to soil types from light sandy soils to heavy soils. Cypenis rotundus (purple nutsedge) in general appearance is very similar to yellow nutsedge, but the leaves that subtend the inflorescence are not as long as those of that species. Moreover, its spikelets are usually dark reddish, or chocolate brown, while those of C. esculentus are yellowish brown to straw-coloured. This sedge (also commonly called nutgrass) is one of the most troublesome weeds of arable land, and is of widespread distribution in most sugarcane growing areas, where, owing to the rapid and efficient method of its vegetative propagation, it presents a serious agricultural problem. The control of nutgrass by cultivation is laborious and unsatisfactory because new plants readily arise from disconnected tubers left in the soil. In Mauritius (Roche couste, 1967), use of 2,4-D amine at about 4 lb. ae and the esters at 2—3 lb. ae per acre give temporary control of four or six weeks in killing the foliage and the nut from which the foliage arises. Soil incorporation of EPTC has given some success, but this use has not been found attractive enough on a field scale. Fumigation with methyl 2 bromide at 1 — 1.5 lb ./ft. for 48 h is undoubtedly a very effective method of controlling this sedge, but this technique cannot be applied on a field scale. This sedge is susceptible to bromacil and can be satisfactorily eradicated by this chemical at rates of between 4—6 lb. ai per acre, depending upon soil type. Purple nutsedge, together with most broad-leaf species, is very susceptible to translocative action of the hormone-like 2,4-D which happens to be among the earliest organic herbicides available to world agriculture. Therefore the history of chemical weed control for sugarcane in Taiwan begins with using this compound, as early as 40 years ago, when the sedge was among the dominant species in the cane fields. With one overall post-emergence application of 2,4-D sodium at 1.6 kg ai per ha when cane is about one month old, the broad-leaves are all killed, but the nutsedge needs two more additional applications to kill both its foliage and the nuts in the upper soil layer from which the treated foliage arises. A weed-free condition can thus be maintained until the formation of the canopy by the cane leaves (Chang and Sze, 1963). In the crop land, as well as in greenhouses, the purple nutsedge has received by far

247 the widest studies on its ecology and control. This can be dated as far back as 1925 when Ranade and Burns reported that control of this weed in India could be achieved by two successive ploughings during the hot season. Similar success was later reported by Andrews (1940) and Smith and Mayton (1942). Deep ploughing of the soil to expose the tubers to the sun was essential to kill the propagative structures. Hollingsworth and Ennis (1956) noted that cultivation alone gave as good a control as (and sometimes better than) the use of herbicides. Thus, where practical, deep tillage seems to be an economical control measure. Herbicides like 2,4-D, amitrole, methylarsinic acid (MSMA), dichlobenil, the substituted uracils and the thiocarbamates were reported to give varying degrees of control. Two applications of amitrole were effective on nutsedge clones from a single tuber, but for an established stand, owing to the varied growth stages, success was limited (Hauser, 1963a; 1963b). Nutsedge was most susceptible to amitrole applied 4 weeks after initial emergence, according to the worker. In Arizona, repeated applications (4 to 8 times a year) at 5.6 to 16.8 kg/ha of methylarsinic acid destroyed most established space-planted purple nutsedge (Hamilton, 1971). Dichlobenil and terbacil at 6.7 to 9 kg/ha, when incorporated in the soil, gave excellent control for 12 to 18 months. But these herbicides were highly persistent in the soil, and enough residues could remain, for as long as 24 months, to be toxic to subsequent crop growth (Walters and Burgis, 1968). The thiocarbamates may be the most effective group of compounds for nutsedge control; this includes such herbicides as EPTC, butylate and vernolate (Antognini et a l , 1959; Jordon et al., 1960; Kasasian, 1971). Soil incorporation of EPTC (3.7 kg/ha) gave good seasonal control of purple nutsedge in western U.S.A. At such a rate, EPTC was reported to cause bud inhibition of the tuber (Antognini et al., 1959). However, the tubers were reported to be killed and suppressed for 8 to 12 weeks when exposed to soil-incorporated EPTC at 13.4 to 17.9 kg/ha (Holt et al., 1962). In Tanzania, Magambo and Terry (1973) reported testing glyphosate for the control of purple nutsedge growing in a mature coffee plantation. Using 2, 4 and 6 kg/ha in single applications and 2 + 2 kg/ha in split applications, they obtained 95 — 100% control of the nutsedge foliage within 4—6 weeks after application. Effectiveness lasted for 26 weeks. Excavated from the upper 10 cm soil, the numbers of tubers were found to be reduced by all treatments. The dry weights of tubers appeared not to be affected by glyphosate, but the sprouting of these treated tubers was inhibited. However, under greenhouse conditions, soil moisture and relative humidity play important roles in affecting the activity of glyphosate in nutsedge, as reported very recently by Moosavi-nia and Dore (1979a), and by Chase and Appleby (1979). They found that this herbicide was more effective when applied to the shoots of the weed under field capacity watering, and at 90% relative humidity, than when applied under moderate and extreme soil moisture stress and at 50% relative humidity. The former authors also found that glyphosate toxicity to nutsedge was enhanced by shading (Moosavi-nia and Dore, 1979b). As a physical method of controlling nutsedge in vegetable culture, ornamental areas or other areas of interest economically, Swarbrick and Dominiak (1973) in Queensland, Australia, suggested mulching with the standard 0.2 mm black poly-

248 thene film to suppress nutsedge penetration. Teo et al. (1973) proposed a new approach to purple nutsedge control by using a plant growth regulating substance to induce sprouting of all dormant buds on the tubers, followed by foliage killing with a herbicide. In this way the tubers would be depleted of their viable buds and eventually there would be substantial reduction of viable tubers in soil. They drenched the soil in an aluminium foil tray with 50 ppm benzyl adenine solution, and planted nutsedge tubers under greenhouse conditions. The synthetic cytokinin did enhance sprouting of tubers significantly, and paraquat at 2.2 kg/ha was applied to kill the sprouts. However, a practical usage on a field-scale cannot be developed before the expensive cytokinins have been synthesized at an economic, low price.

4. CHEMICAL CONTROL OF COGONGRASS

Imperata cylindrica (cogongrass) is listed by Holm (1969) as one of the ten worst weeds in developing countries. However, it has not received such comprehensive studies of its biology as has purple nutsedge, being only superficially investigated in 4 this respect in Indonesia, where it is called Alang-alang' (Soerjani and Soemarwoto, 1969). This perennial grass has an erect habit of growth and a well developed system of stout rhizomes. When in flower, it is characterized by a dense, cylindrical, whitish panicle. It is a very troublesome weed, for its rhizomes are able to reach depths of up to four feet in the soil. It is, however, of restricted distribution in sugarcane producing countries. In Mauritius, this grass is very resistant to herbicide treatment and requires fairly high rates of TCA ( 1 0 0 - 2 0 0 lb ./acre); even then, a second application may be required. Repeated applications of dalapon at about 10 lb. per acre may also give satisfactory results. The combination of mechanical cultivation with spraying of dalapon at 5 —10 lb. or TCA at 25 lb. has also been reported to be effective. Bromacil at 5 - 1 0 lb. ai/acre plus paraquat at 0.5—1.0 lb. ai per acre, or bromacil used alone at about 10 lb. ai/acre may be used on a tentative basis (Rochecouste, 1967). In Malaysia, /. cylindrica is called lalang', and rapidly invades land cleared from jungle, cultivated land being neglected, roadsides, and other disturbed sites, to form 'sheet lalang'. An old lalang stand is prone to catch fire. As 'sporadic lalang', it occurs in planted rubber and other tree crops where it is not completely shaded. Apart from toilsome cross-ploughing and harrowing to clear the bush formed by this weed, repeated applications of sodium arsenite or the more effective dalapon have been hitherto relied upon for its chemical control in Malaysia. None of these herbicides alone provides long-lasting control. Moreover, high rates, of up to 20 lb./acre, of dalapon in high volume application (up to 100 gal/acre) are required for effective (though expensive) control of /. cylindrica. The sequential applications of paraquat, or dalapon followed by paraquat, were suggested by Seth (1970; 1971b) for a more economical control of this weed. In the first formula, an initial application of 0.5 lb./acre paraquat, followed by two further

249 applications of this herbicide at 0.25 lb./acre when regeneration of aerial parts from underground storage organs (the rhizomes) reached 50%, gave a duration of control comparable to what would have been given by a single application of dalapon at 16.8 kg/ha. In the second method, application of 6 - 8 lb./acre dalapon followed by 0.25 lb./acre paraquat 2—4 weeks later also gave control as good as the conventional treatment of dalapon alone at 15—20 lb./acre. Also, these methods were supposed to be more acceptable for use in plantation crops, due to their being less toxic than the conventional application of high rates of dalapon and arsenic compounds. Paraquat is a contact herbicide with rapid desiccant action and quick inactivation on contact with soil. A single application of the chemical on such a perennial weed as /. cylindrica rapidly desiccates the foliage; this is followed by a rapid regeneration from the underground rhizomes. In the first case, therefore, repeated applications of paraquat aimed at exhausting the weed's carbohydrate reserves provide an approach to effective control of this difficult weed. However, the author argued, the timing of the repeated applications is critical, as foliage must be killed before it starts to contribute to the metabolic reserves of the rhizome system. In the second case, it was postulated that dalapon is a slow-acting and freely-translocated herbicide and tends to accumulate in the root/rhizome system of plants. The rapid and timely destruction of foliage by paraquat may create conditions under which the dalapon that has already accumulated in emerging shoot-buds is able to suppress their growth (Seth, 1971a). A similar example, of controlling Mikania cordata, was also given by the author. This weed infests tea, rubber and oil-palm plantations in Malaysia. Helped by its ability to form adventitious roots at each node of a branch in contact with the soil, this plant quickly establishes itself on open patches and among cover crops. Under fully open conditions, if it is allowed to grow unchecked, it rapidly covers the ground, swamping all other growths. A very effective treatment for control of this species was found: two applications of 0.28 kg/ha of paraquat at a volume rate of only 112.5 1/ha, with the second application made 2—3 weeks afterwards — when about 50% regeneration of weed foliage from the protected axillary buds on the branches had been attained. The author also demonstrated that equal or better results were obtained with herbicide mixtures of paraquat + diuron 4- MSMA at 0.28 4- 0.28 + 1.68 kg/ha, or paraquat + diuron at 0.84 + 0.84 kg/ha, for the control of the perennial species, Paspalum conjugatum (Seth, 1971b), than an amitrole (0.42 kg/ha)/paraquat ( 0 . 2 8 0.56 kg/ha) sequential treatment (interval 3 weeks) that Headford initiated (1966). In greenhouses, glyphosate was tested and found quite effective to control /. cylindrica. Moosavi-nia and Dore (1979a; 1979b) found, furthermore that after the grass was sprayed with glyphosate the effectiveness of this herbicide was reduced by soil moisture stress, but enhanced by the shading of the treated grass. In Taiwan, /. cylindrica is of restricted distribution on ditch-banks, river shores, bush land, and other, non-cultivated areas, by natural spreading. Very rarely does it occur in cultivated fields. Unlike torpedo grass, cut portions of its stems or rhizomes are difficult to plant for germinating into new young grass. Only through separating and transplanting individual seedlings from 'bunches' of the naturally-grown grass, is its artificial propagation possible. It also lacks the characteristic of being able to be

250 stimulated to absorb extra soil nutrients to survive environmental adversities, and to enhance its regenerative ability, cf. torpedo grass and Bermuda grass. This explains why there is no occurrence of it in cane fields, where extreme changes in the environment under cultivation do not allow the cut-portions of either stems or rhizomes to become established (Peng and Twu, 1982). For control of /. cylindrica, as a noncropped-land weed in Taiwan's cane plantations, the use of the mixture of paraquat + diuron, or paraquat + dalapon, in a proportion of 0.8 + 5.0 kg ai/ha each, sprayed over the established foliage, or dalapon (better with surfactant) alone at 5 kg ai/ha repeated 2 - 3 times over the young regrowths, are all quite effective.

5. CHEMICAL CONTROL OF JOHNSON GRASS

Jonhson grass (Sorghum halepense) is a stout, perennial plant that reproduces both by seeds and by extensively creeping rhizomes. In cane fields the primary rhizomes of this grass are alive in the ground at the beginning of a growing season and die at the end. The secondary rhizomes are produced from the primaries and come to the surface; from the base of these, tertiary rhizomes are sent out at about flowering time. These are usually large, and penetrate more deeply into the soil. The primary rhizomes decay each year, while the other two types are persistent and produce new plants the following year. The grass is very troublesome in many sugar producing countries, particularly U.S.A. (Louisiana), Fiji, Australia and India, not only because of its aggressive vegetative methods of propagation, but also because its fertile seeds may remain viable in the soil for up to seven years, depending upon the depth up to which they occur. The methods used for controlling S. halepense are based on a combination of cultivation operations and herbicide application. In Louisiana, most of the rhizomes are first destroyed by ploughing infested fields as often as necessary before herbicide application; better results are obtained when the ploughing is carried out during the warmer months of the year (Stamper, 1965). In plant cane, a mixture consisting of 4 lb. TCA plus 1 lb. ae of an amine salt of 2,4-D is usually applied, and this is followed by two other similar applications at about one month interval. However, about one month after the first application a mixture consisting of TCA 2 lb. plus 1 lb. ae of 2,4-D amine is also often used. In ratoon cane, the combination of TCA at 11 lb. plus 1 lb. 2,4-D amine is usually applied immediately after the first cultivation, and this is followed by dalapon at 4 lb. only in sugarcane varieties tolerant to this chemical. Fully adult plants of Johnson grass are very difficult to control even at high rates of TCA and dalapon. Fenac is showing promise for the control of the grass in pre-emergence sprays, while exploratory work with the new herbicide bromacil has given excellent results in both pre- and post-emergence treatments of the grass. Satisfactory control has also been reported with ametryne, diuron plus surfactant, or combinations of these herbicides with dalapon (Jones, 1964). In non-cropped lands, repeated applications of sodium chlorate or bromacil are also claimed to be effective.

251 More recently, the use of monosodium methanearsonate (MSMA) has been claimed to be more successful than the traditional treatment of dalapon for the control of rhizomes of Johnson grass in ratoon cane in Louisiana (Millhollon, 1970). Two overhead applications of MSMA at either 4 + 4 or 4 + 2.5 lb./acre were made, the first in the middle of April to kill the grass foliage when the grass and cane was already 10—20 in. tall, and a second made approximately 4 weeks later to kill the regrowth of the grass; this gave 95% control of the weed. The standard treatment of 2 applications of 4.5 lb./acre dalapon made at the same time for comparison, controlled only 70%. Sugarcane that received the MSMA sprays also yielded more than that treated by the standard dalapon. However, the ratoon cane plants tolerated MSMA applied before June, without arsenic residue in the plants, though temporary leaf chlorosis and stunting occurred. When applied in June or August, the chemical caused a concentration of arsenic residues up to 0.45 ppm in juice, and 1.68 in bagasse. For the control of established Johnson grass on drainage ditchbanks in cane fields in Louisiana, Millhollon (1969) proposed 5 applications of MSMA at 3.6 lb./acre or dalapon at 7.4 lb./acre in the first year to kill the grass foliage; this was to be followed, for annual control of new growth (particularly the seedlings from surviving buds on rhizomes and stems), with mixtures of fenac + bromacil, or TCA + MSMA as preemergence treatments, or MSMA, dalapon and sodium chlorate as post-emergence treatments. Bermuda grass, a desirable species against soil erosion, could rapidly vegetate ditchbanks treated with MSMA, whereas other herbicide treatments suppressed vegetation (desirable and undesirable) to the point where soil erosion could be a problem. In Louisiana, sugarcane is regularly grown in a 3-year cycle of one plant cane and two ratoons, and the land is fallow-ploughed for a growing season before replanting. The new crop must be planted within a month after harvest has begun in October to avoid possible freeze damage to the seed cane. The fallow ploughing helps to reduce Johnson grass rhizomes and seed in soil and, consequently, competition from this weed in the new crop. When there is no major problem of Johnson grass, some cane growers abolish fallow ploughing and replant a new crop immediately, thus carrying on a system of continuous cropping. Recently, Millhollon (1980a) reported that even in fields heavily infested with Johnson grass, continuous cropping of sugarcane resulting in crop yields no less than normal was possible if an effective herbicide program was used. The best results obtained from such an experiment were: at the beginning of cropping, in the autumn of each year, picloram at 1.7 kg/ha was sprayed pre-emergence to control effectively the Johnson grass seedlings and suppress the development of rhizome plants, and this was followed by post-emergence application in the spring, with either dalapon at 5.0 kg/ha or MSMA at 4.5 kg/ha, to kill the regrowths. In Taiwan, fortunately, Johnson grass has not been found to escape (and spread to other areas) from a location at the southernmost tip of the island, where an experiment station 50 years ago introduced it for growing as a forage crop and soon discarded it after learning of its noxious properties as a weed in America. Perhaps due to some ecological difficulties this grass remained then only as a few scattered, natural patches in the bush land of its original introduction.

252 McWhorter (1972b) showed some success in controlling Johnson grass in Mississippi by flooding experiments in greenhouse and field. Best results were obtained after 8 days of flooding at 40°C, when all the rhizomes planted, together with the 4-weekold seedlings, were killed. Decrease in temperature required more days for a complete kill (16 days at 30°C). Immersing the rhizomes in water and soil provided a better control than immersing them in water without soil. Johnson grass is difficult to control because it produces large quantities of seeds and rhizomes. To control its growth from rhizomes is also difficult because many rhizome buds fail to accumulate translocated herbicides, e.g. dalapon (Hull, 1969). As this grass does not translocate toxic quantities of herbicides to the dormant buds, these will eventually germinate after the existing culms have been killed by herbicides or cultivation. Beasley (1970) demonstrated that apical dominance of rhizome buds is very marked in Johnson grass. It is emphasized that adquate control of this grass from foliar treatments requires the movement of toxic quantities of phloem-mobil herbicides through the plant and into the areas of vegetative reproduction (Hull, 1969; 1970; Oyer et al., 1959; Foy, 1964).

6. USE OF SURFACTANTS TO ENHANCE HERBICIDAL ACTIVITY OF DALAPON

Dalapon is the most important herbicide used for the control of perennial gramineous weeds which infest plantation crops such as sugarcane. Considerable work in the study of external and internal factors that influence its activity in the plants is found in the literature. In recent years, the addition of some surfactants to spray solutions of dalapon in order to enhance its activity or improve certain characters, has been widely investigated and put into practice. Jordan et al. (1963) proved that the addition of a combination of 3 kinds of additives at certain concentrations to spray solutions of dalapon had the effects: (1) gradual hardening on the plant foliage so as to retain unabsorbed surface residues of dalapon, and prevent subsequent leaching into the soil by rain or sprinkler irrigation (to cause injury of crops) and (2) enhancing penetration of dalapon into the leaves of the plants (oats and Bermuda grass being tested), resulting in toxicity of the spray. McWhorter (1963) reported that the addition of a surfactant to solutions of dalapon at percentages from 0.03, 0.06, 0.12, 0.25 to 0.59 progressively enhanced the activity of this herbicide on Johnson grass. The increase in activity was influenced by the application rates which were M>, 1,2 and 4 lb./acre, by the formulation of dalapon (whether technical or formulated grades), and, in some instances, by the volume of water in which the treatments were applied, at 10, 20, 40, 80 and 120 gal/acre. Varying the volume of water, as diluent, affected control more with the technical, than with the formulated dalapon, probably because the commercial product had already had incorporated some surfactant during its formulation. Greater dalapon activity could be expected from the use of a surfactant with lower rates of formulated dalapon than with higher rates, since the surfactant level in formulated dalapon spray solutions was in direct proportion to the rate of dalapon applied. When a maximum level of surfactant is desired without additional surfactant, formulated dalapon should be applied

253 in the minimum volume of water that provides adequate plant coverage. The activity increase of dalapon per unit of surfactant, i.e. per percentage of surfactant in solution, generally decreased as the herbicide level increased, and excessive quantities of surfactant might reduce the herbicide effectiveness. Moreover, this investigation showed that Jonhson grass plants from short rhizomes were more readily controlled by dalapon than those that came from longer rhizomes. By adding 7 surfactants to dalapon solutions to obtain measurements of surface tension of solutions, contact angle of slution droplets with surfaces, and herbicidal toxicity of sprays on corn seedlings, Foy and Smith (1965) found that all surfactants markedly enhanced herbicidal activity; increased enhancement with increased rate. However, the surfactants differed considerably in their influence on surface tension and on the wetting of surfaces as measured by angle of contact. Minimum surface tensions and contact angles occurred at 0.1-0.5% concentrations for all surfactants. Maximum herbicidal activity was observed at 10 times these levels, or greater. In Taiwan, the addition of a laundry detergent is practised (for general pre-emergence control of weeds in sugarcane), usually at a level of 0.5-1.0% of the tank mixes, of diuron, atrazine, other chemicals of the wettable powder formulations, or 2,4-D sodium, which is a water-soluble powder. For control of perennial weeds, using dalapon, the mixing of a detergent in the spray solutions is more common. The addition of a detergent to a herbicide suspension, with water as the diluent, has the advantage of preventing precipitation of solid particles of the wettable powder chemicals in the mixing tanks, or in the sprayers during application. A uniform concentration of herbicides in the spray is thus maintained throughout the application. To determine the ability of a common laundry detergent to enhance dalapon activity, as compared to the commercial surfactants, an experiment was conducted at this institute in 1971 (Peng et al., 1974). In glassware containers, 9 cm in diameter and 5.5 cm in height, filled with washed sand, corn was planted and thinned to one seedling per container after sprouting. Surfactants Citowett (alkylaryl polyglycolether), WK (dodecylether of polethylene glycol), Multifilm (chemical name undisclosed), and a common laundry detergent (also chemically unknown) were used for testing their ability to enhance the herbicidal effect of dalapon on corn seedlings (which were placed under 24-hour illumination by fluorescent lamps). Commercial dalapon was dissolved in distilled water to make solutions of concentrations from 10, 20, 30, to 600 ppm, at intervals of 10 ppm, and each surfactant was added to each concentration at 0.5, 1.0, 5.0 and 10.0 percent of the solution. Another set of concentrations of dalapon were without surfactants. Corn seedlings which had expanded 2—3 leaves, 3—5 days after planting, were sprayed with the solutions. The treatments were each replicated 4 times. Untreated plants were the control for each of the ten concentrations. Fresh weights of both treated and control plants were taken for assessment of the herbicide effect about 10—15 days later, when the control plants had reached about 15 cm and had begun to lodge. The treatment effect was expressed by percent reduction of treated plants relative to the controls. The simple correlation (r) between each, ten, increasing concentrations of dalapon with or without surfactants, and the corresponding percent reductions in fresh plant weight, was calculated to determine whether there was a progressive phytotoxicity caused by treatments.

254 It was found that without the addition of any surfactants dalapon, sprayed at increasing concentrations up to 600 ppm, caused no progressive, significant reduction of corn growth, probably because of its slow activity within such a short growing period of the corn plants. A product 'Dowpon S', which is dalapon pre-incorporated during formulation with a chemically undisclosed surfactant 'Dalawet', showed the same results probably being insufficient (in the formulation) to enhance the activity of the dalapon under the test conditions. • The addition of 0.5% Citowett, a commercial surfactant, to the dalapon solutions began to cause toxicity of corn seedlings only when concentrations of dalapon were in the range of 210—300 ppm; no enhancement was observed below or above this range. However, when 5% Citowett was added, corn seedlings were evidently injured proportionately, from 10 to 100 ppm of dalapon, with a significant (at 0.01 level) r = —0.86, and the L D 5 0 (the dosage of a toxicant that causes 50% injury of plants) was shown in 3 0 - 4 0 ppm of the herbicide. The addition of 0.5% WK caused corn seedlings to be injured progressively when the dalapon concentration was in the range 10—200 ppm, with L D 5 0 in 30—40 ppm. When 0.5% of a common brand of laundry detergent was added to the herbicide solution, corn seedlings were injured immediately from 10 to 100 ppm, and injury reached 50% (value of LD 5 0) at 70—80 ppm. 63.4% and 79.4% injury of corn plants were recorded when 1% and 5% of detergent was added respectively to a 100 ppm solution. However, no injury of plants was found at all when 10% of the detergent was added to 10—100 ppm dalapon solution, probably due to the negative effect of an excess of this kind of surfactant blocking the penetration of dalapon into the corn foliage. The above results confirmed the findings of McWhorter (1963), that the activity of dalapon at a percentage of surfactant in solution generally decreases as the herbicide level increases, and that herbicide effectiveness may be reduced by excessive quantities of surfactant (at too high a percentage of solution).