Weed Control, Plant Physiology and Biochemistry PREEMERGENCE AND POSTEMERGENCE HERBICIDE TREATMENTS OF SUGARCANE FIELDS IN MAURITIUSl

J. Cyril Mongelard Experiment Station, Hawaiian Sugar Planters' Association Honolulu, Hawaii ABSTRACT

Preemergence and postemergence herbicide treatments were compared in a series of trials in Mauritius. More efficient weed control was obtained with early postemergence treatments than with preemergence treatments. In the humid zone, atrazine at 4 Ib/A applied preemergence was less effective on weeds than a postemergence application of atrazine 3 Ib + Actril-O 1 Ib/A applied 7 weeks after planting. In the superhumid zone, diuron at 4 Ib/A applied preemergence was also less effective thana postemergenceapplication, 1 month after planting, of diuron 3 Ib + Actril-O 1 Ib/A or diuron2 Ib + Actril-O lib/A. Several other herbicides with both preemergence and postemergence properties were applied postemergence and were found superior to the preemergence treatments. Postemergence treatments were more efficient because of delayed application with herbicide mixtures which controlled both emerged weeds and germinating weed seeds and provided residual preemergence control. Under cool or dry conditions which were not conducive to weed growth, postponement of treatments for 1-2 months was found to be of economic advantage because it allows for a reduction in the rate of the residual herbicide in the mixture. INTRODUCTION

The excellent weed control obtained with preemergence applications of substituted ureasand triazines has greatly reduced the use of hormone herbicides. Despite the high costs of these preemergent herbicides in the 1960's, they were used economically in Mauritius in regions where high rainfall, humidity, and temperature stimulated weed growth. On equal expenditure, preemergence treatments, provided they give satisfactory control of the weed population, are preferred over postemergence treatments. Preemergence treatments are essential during the wet summer months in the humid and superhumid zones. The concentration of herbicides should be sufficient to give adequate weed control for at least 3 months without affecting cane germination and growth. In the superhumid zone, a second treatment can rarely be avoided. However, postemergence treatments would prove advantageous if they satisfy the following conditions: (a) they are less expensive than preemergence treatments with equally good weed control; (b) they can be applied before 1 Research conducted while the author was with the Mauritius Sugar Industry Research Institute.

1161

(:"1

Il62

WEED CONTROL, ETC.

compention between weeds and crop is likely to affect the returns on a crop, in other words, when weeds are small; (c) the treatments effectively kill established weeds and check further weed growth for a required period of time; and (d) they have no deleterious effects on cane growth and yield. The best advantages of postemergence treatments, provided satisfactory control can be obtained, are: (a) delayed application until weeds start to grow -in winter or when dry, treatments can be delayed for 1-2 months; and (b) the possibility of knowing the weed population that needs to be checked; hence, the possibility of choosing the most effective residual herbicide to be used in the mixture. The economic advantage of using expensive preemergence herbicides during the cooler and drier months from May to November was questioned, and a series of trials was, therefore, carried out to compare .weed control and crop' tolerance from various preemergence and early postemergence treatments. Only 2 trials will be discussed in this paper since results for all trials were similar. They are representative of several experiments carried out in the humid zone (50-100 in. annual rainfall) and in the superhumid zone (> 100 in. annual rainfall) . MATERIALS AND METHODS

The following herbicides were evaluated: 3- (3,4-dichlorophenyl) -1,1dimethylurea (diuron); 3-tert-butyl-5-chloro-6-methyluracil (terbacil); mixture of 1 part 3,5-diiodo-4~hydroxybenzonitrile(ioxynil) + 6 parts 2;4-D (Actril-D); methyl-4-aminobenzenesulphonyl carbamate (asulam); N-[l- or 2- (3a, 4, 5, 6, 7, 7a-hexahydro-4, 7 methanoindanyl) J-N/-dimethylurea '(BAS 2103); I-phenyl-4amino-5-bromo-pyridazon-6 + mixture of isomers of BAS 2103 (BAS 2440); N- (4-trifluoromethylphenyl) -N'N/-dimethylurea (C 15935) ; N- (4-bromo-3-chlorophenyl) -N'.methoxy-N'-methylurea (C 6313); I,I-dimethyl-3- (a,a,a,-trifluoro-mtolyl) urea (fluometuron); 2-chloro-4- (ethylamino) -6- (isopropylamino) -s-triazine (atrazine); 2- (ethylamino) -4- (isopropylamino) -6- (methylthio) -s-triazine (ametryne); and 2-tertiobutyl-4- (2,4-dichloro-5-isopropylphenyl)-5-oxo-I,3,4-oxadiazoline (RP 17623). The experiments were carried out in a randomized complete block design, with 3 replications. The herbicides were applied in water at the rate of 73 gallA with hand-pump knapsack sprayers. In the humid zone, the cane variety used was M 13-56 planted in a Low Humic Latosol soil under fairly dry conditions. There were 20 treatments including control plots which were not sprayed. The check treatment was a preemergence application of atrazine at 4 lb I A, sprayed () days after planting in February. All the other plots did not receive herbicide treatment until weed growth was sufficient to warrant spraying, 7 weeks after planting. In the superhumid zone, 23 treatments were included in the experiment carried out from April-August in a Low Humic Latosol soil on the cane variety M 93-48. Control plots received no herbicide and check plots received a preemergence application of diuron at 4 Ib I A, 6 days after planting. All other treatments were applied 1 month after planting.

J.

1163

CYRIL· MONGELARD RESULTS AND DISCUSSION

Humid Zone Trials Weed ratings and cane growth measurements were made 3t months after planting when cane was closing in; results are presented in Table L'·' Table 1. Results 3!1llonths after planting of preemergence and postemergence herbicide treatments on weeds and cane. (Humid zone trial)

Herbicide

Dosage rate (lbJA active ingredient)

Weed infestation (% of untreated control)

Effects on cane growthi

Postemergence treatments (7 weeks after planting) diuron + terbacil + Actril-D BAS 2440 + Actril-D C 15935 + Actril-D C 15935 + Actril-D BAS 2440 + Actril-D atrazine + Actril-D fluometuron + Actril-D C 15935 + Actril-D fluometuron + Actril-D BAS 2103 + Actril-D C 6313 Actril-D C 15935 Actril-D BAS 2103 + Actril-D ametryne fluometuron C 15935

2:+ 0.75 + 4 + 1 2 + 1 3 + 1 3 + 1 3 + 1 4 + 1 1 + 1 2 + 1 4 + 1 4 3

2 2

3

+ 3 3 1

26 26 31 32 32 35 36 38 39 40 42 42 47 48 50 69 83 100

++ +

+ +

++

Preemergence treatment (6 days after planting) atrazine

4

100

- No effects on cane growth effects but no significant reduction in growth + + Harmful effects with significant reduction in growth

+ Toxic

The weed population consisted mainly of Cyperus rotundus, Oxalis spp., Solanum nigrum, Bothriospermum tenellum, and Amaranihus caudatus. Of special significance are the better results obtained with the postemergence application of a reduced rate of atrazine 3 11:>,;.,+ Actril-D lIb/A compared to the preemergence treatment of atrazine 4 lb/A. On the whole, there probably was no definite advantage in using a high rate of a residual herbicide in mixtures with Actril-D, Treatment was delayed long enough for cane to close in before weed competition became severe. Both ametryne and the mixture diuron + terbacil + Actril-D adversely affected cane growth, and to a lesser extent did C 6313, C 15935 3 lb/A, and Actril-D 3 lb/A.

1I64

WEED CONTROL, ETC.

Superhumid Zone Trial The low ambient temperatures during the experiment were not optimum for weed and cane growth, though very humid conditions prevailed. Two months after planting the weed infestations in the control plots were sparse, and perhaps the postemergence spraying could have been delayed. Cane growth, already adversely affected by the low temperature, was affected even more by some of the treatments. Weed ratings and cane growth measurements were made 4 months after planting. Results are presented in Table 2. Table 2. Results 4 months after planting of preemergence and postemergence herbicide treatments on weeds and cane. (Superhumid zone trial)

Herbicide

Dosage rate (lbjA active ingredient)

Postemergence treatments (1 month after planting) 4 + 1 C 15935 + Actril-D C 15935 + Actril-D 3 + 1 diuron + terbacil + Actril-D 2 +0.75 + ,,' 4 C 15935 fluometuron + Actril-D 4 + fluometuron + Actril-D 3 + diuron + Actril-D 3 + 3 C 15935 RP 17623 + Actril-D 2 + 3 ametryne diuron + Actril-D 2 + BAS 2440 + Actril-D 3 + C 15935 + Actril-D 2 + flnometuron + Actril-D 2 + C 15935 2 BAS 2440 + Actril-D 4 + 4 + BAS 2103 + Actril-D asulam + Actril-D 3 + BAS 2103 + Actril-D 3 + C 15935 + Actril-D 1 + Actril-D 2

Weed infestation ('10 of untreated control)

Effects on cane growthi

26 26

+

28 28

+

31 33 33 33 33 35 36 37 38 39 44

+

-1

46 49 51

55 62

77

Preemergence treatment (6 days after planting) diuron 1 I..

4

49

No effects on cane growth effects 'but no significant reduction in growth

+ Toxic

A broad weed spectrum including more than 20 species was encountered in this experiment. The most abundant weeds were Cynodon dactylon, Bothriospermum tenellum, Cyperus rotundus, Lobelia clifJortiana, Oxalis spp., Digitaria timorensis, Setaria pallide-fusca, and Youngia japonica. The results indicate that weeds can be controlled by postemergence spraying with a mixture of a reduced rate of a long residual herbicide .and an efficient postemergence herbicide. Although the postemergence treatments, were applied relatively early, it

f"'~""-~~.C".

." \

I

I

J.

CYRIL MONGELARn

1165

was found that reduced rates of diuron in the diuron + Actril-D mixtures gave better control than the preemergence treatment with diuron. It should be emphasized that in many other trials also the mixture diuron + Actril-D was very effective, indicating a possible synergistic effect between these 2 chemicals. Actril-D may enhance the postemergence activity of diuron. This enhancement of activity was not so striking with other long residual herbicides. C 15935 and ametryne have postemergence properties when used alone, which probably explains why the additon of Actril-D to these chemicals did not increase their activity. BAS 2440, BAS 2103, asulam in mixtures with Actril-D, and Actril-D alone were not very effective on the particular weed population of this experiment. Reducing the rate of application of C 15935 below 2 lbjA did not give satisfactory results. None of the treatments affected cane germination. Early postemergence treatments of cane fields may prove (under dry and cool conditions) to be more economic than the preemergence treatments because a shorter time of activity is expected in the postemergence treatments, since application can be delayed, The use of single herbicides having both preemergence and postemergence activities may give excellent results in the future, but they will have to be more efficient on weeds and less toxic to the crop than the ones that are presently available.