A COMPARISON BETWEEN MCPA AND PESCO is-I5 FOR THE ...

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to the disappearance of herbicides in the field. Both MCPA and Pesco 18-15 are highly soluble in water and are likely to leach out of the top soil very quickly. Unde.r the flooded conditions of irrigated rice fields, leaching is likely to be even more intensive and the residual effect of these herbicides will not be. long. The residual effect of such herbicides in the soil will generally be related to their quantity of active ingredient per unit land surface. The evidence from this study confirms this. At. the higher dosage rate of 5ptsjacre the length of the weed control period was longer with the pre-emergence application of both MCPA and Pesco 18...:15than at the lower dosage rate of 3ptsjacre. For longer residual effects, perhaps larger dosage rates may be used if the crop tolerance is high. A delayed application of the herbicide relative to the life of the crop also affords an opportunity of extending weed control into the latter part of the crops life. This TABLE 7 was attempted with the post-emergence application but the stage of application in relation to the weeds was perhaps so late that effective kit! was slow to achieve. The effect of the herbicide on the composition of the weed flora was assessed rather empirically and note was only made of the most striking species in the plots. The results of this assessment cannot therefore, be very conclusive. lschaemum rugosum, a mono cot, was significant in all plots; Sphenoclea zeylanica was significant in all pre-emergence plots and also· in the unweeded plots; Cyperus difformis was particularly significant in the 'unweeded' plots. Yields.-The yields obtained for the different treatments were subjected to an analysis of variance. This showed significant differences due to treatments. Table 7 below shows the mean yield of treatments arranged in ranked order for comparison, using Duncan's multiple range test. EFFECTS OF TREATMENT ON THE YIELD OF MAIZE (a) Standard Error of the mean - ± 4.01 (b) Shortest significantRange (Rp) p: (2) (3) (4) (5) (6) (7) (8) (9) (10) Rp 2.97 3.12 3.21 3.27 3.32 3.35 3.37 3.39 4.41 (c) Ranked Means of Treatment Yields Pre-em Pre-em Pre-em Pre-em No Post-em Post-em Clean Post-em Post-em Treatment: MCPA Pesco Pesco MCPA Weed- Pesco MCPA Weed- Pesco MCPA 18-15 18-15 ing ing 5pt/ac 5pt/ac 3pt/ac 3pt/ac 5pt/ac 3pt/ac 3pt/ac 5pt/ac Yield/plot 1.8 6.8 7.5 8.7 11.7 21.3 23.2 25.7 27.2 28.2 lb./acre 101.5 387.5 430.5 498.0 669.9 1224.3 1330.0 1473.5 1559.6 1617.0 This comparison showed clearly significant differences in yields between the pre-emergence application on the one side and the post-emergence application on the other side. Yield from the unweeded treatment was, however, not significantly different from those with post-emergence application of Pesco 18-15 at 5ptsjacre and those with post-emergence application of MCPA at 3ptsjacre. Two principal factors accounted for yield differences of the different treatments. These are (a) the relative crop tolerance to the herbicide and (b) the degree of effective weed control. The effect of weed control without any interference from herbicide can be obtained by direct comparison between the unweeded and the clean weeded treatments. These showed a remarkably significant difference due to competition from weeds. The highest mean yields were obtained from the post-emergence application of Pesco 18-15, at 3ptsjacre, and MCPA, at 5ptsjacre, although these were not significantly better than the mean yield of the clean weeded treatment. It was quite evident that yields were significantly affected by crop tolerance. In this respect time of application was a more significant factor. The most significant
evidence for this came from MCPA. While MCPA at 5ptsjacre produced the highest yield among all treatments when applied post-emergence, the same dosage produced the least at pre-emergence. Within the preemergence application there was also an indication that the crop's tolerance of both herbicides was lower at the higher dosage rate of 5ptsjacre than at 3pts/acre. With post-emergence application there was evidence of increasing toxicity with Pesco 18-15 at higher dosage (5ptsjacre) than at the lower dosage rate (3ptsjacre) in terms of yields. MCPA, however, showed greater efficiency with increasing dosage, and may be tried at higher rates for more effective weed control. References LE CLERG,B.L., LEONARD,W.H. & CLARK, A. G. (1962) Field Plot Technique, Burgess Publishing Company, Minnesota. DUNCAN, D.B. (1955) Multiple F-tests. Biometrics, 11, 1-42. GREEN, D.H. & KALoGERIS,B (1963) A preliminary Herbicide screening trial on Ground. Miscellaneous Report No. 418. Tropical Pesticides Research Institute, Arusha.
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