*This is historical information from 1967 and should be taken as such. Proper research for current recommendations of herbicides in a pecan orchard should be done*
JACK A. NORTON. J. BENTON STOREY, AND GEORGE D. MADDEN¹
AN EFFECTIVE WEED CONTROL program in commercial pecan orchards would be extremely beneficial to Texas pecan growers. Weed competition usually reduces moisture and nutrient content which results in substantial yield reductions and poor quality. Weed control is also of paramount importance in establishing pecan orchards. Trees grown on weed-free soil come into production earlier, resulting in faster economic gain to the grower.
When mechanical methods of weed control are employed, the operation must be repeated often, and this is time consuming and costly. Physical damage to the trunks, caused by careless use of equipment, may occur, and an undetermined amount of the pecan tree's feeder roots may be destroyed, thereby limiting the uptake of water and nutrients.
Herbicides having label clearance for use in bearing pecan orchards are limited to dinoseb (2-sec-butyl-4, 6-dini-trophenol) and two amine salt formulations of the same dintro compound. These compounds (depending upon the formulation used) are recommended for both pre emergence and post-emergence weed control. The dinitro compounds are dependent upon critical environmental conditions for their activity. They provide 4 to 6 weeks residual activity at recommended pre emergence rates, and they have a high mammalian toxicity rating which makes their use more hazardous to man and livestock than several of the pre emergence herbicides currently available for crop use.
Recent research in California has met with success in pre emergence control of annual weeds in productive walnut orchards (4). Lange et al. (4), reported in 1967, that certain s-triazine and substituted urea herbicides were equally effective for pre emergence weed control in walnut trees, but that responses have differed, depending upon the soil type, organic matter content of the soil, amount of rainfall, and weed species present.
McKay and Berry (6), in 1958, reported excellent pre emergence weed control in nursery plantings of chestnut seedlings. In their study, DNBP was applied in solution after planting the seed. The herbicide seemed to have little or no effect on normal germination of the chestnut seed or phytotoxicity to the young seedlings.
Gordinier (3) has stated that the three most useful herbicides for use in nut tree plantings are dalapon (2, 2 dichloropropionic acid), amitrol (3-aminos-triazole) and simazine [2, chloro-4, 6-bis (ethylamino)-s-triazine]. He suggested the use of dalapon as a preplant treatment, and simazine for pre emergence weed control in existing and recently transplanted nut trees. Gordinier (3) continued that simazine may damage nut trees if a substantial amount leaches into the root zone. He pointed out that damage is more likely to occur on newly planted trees than to deep-rooted established trees.
Amizine (a combination of amitrole and simazine) applied at 7 pounds per acre was reported to have given excellent weed control throughout the growing season and well into the succeeding growing season without appreciable damage to nut trees (3).
Phytotoxic effects of simazine and diuron [3-(3, 4 dichlorophenyl) -1, 1- dimethyl urea] have been observed in peaches, cherries and apples by a number of researchers ( 1, 2, 5, 7). Phytotoxic symptoms occurred when trees were treated with these compounds at rates in excess of 12 pounds per acre and where they were used for 2 or more years in succession at these rates. Trees in plots which had received heavy rates of simazine showed interveinal chlorosis and necrosis, and were prematurely defoliated. Diuron toxicity was characterized by veinal chlorosis which was followed by an interveinal and marginal leaf necrosis and by premature leaf drop (7).
Materials and Methods
Research was initiated in Lamb and Brown counties during the Spring of 1967 to evaluate pre emergence herbicides in established pecan orchards. Herbicides were selected on a speculatory basis concerning their safety to pecan trees and their known residual weed killing properties relative to several annual weed species indigenous to these counties. In addition, the herbicides selected were known to possess a much lower mammalian toxicity rating than the dintro compounds. The herbicides selected for evaluation were simazine, diuron and bromacil (5-bromo-3-sec-butyl-6-methyl-uracil). Simazine was applied at 2.0 and 4.0 pounds active ingredient per acre. Diuron and bromacil were applied at 1.5 and
3.0 pounds a. i. per acre. Soil types in Lamb and Brown counties were an Amarillo fine sandy loam and a Frio silty clay soil, respectively.
Supplemental water was provided by furrow irrigation in Lamb County and by flood irrigation in Brown County at times when needed throughout the summer.
Weed counts were made at intervals during the summer to determine which weed species were being controlled by the various herbicides. The numbers of each individual weed species were recorded, to provide a basis for determining possible increases of tolerant weed species in future years. Three counts of 1 square yard each were taken at random near each tree in each plot.
Percent weed coverage estimations were made to determine the residual properties of each herbicide. In addition, soil samples were collected 166 days following herbicide application for use in an oat bioassay to detect active herbicide residues. Soil samples were collected from the top 4 inches and also at 8 inches to determine possible herbicide movement down into the pecan tree's feeder root zone.
Yield data and trunk diameter measurements were collected in the fall to determine the effect of the herbicides on pecan tree growth and production.
Laboratory analyses were conducted to determine the effect of the herbicides on pecan nut quality.
All data were analyzed by Duncan's Multiple Range Test with significance at the .05 level.
The herbicides used and experimental procedures are discussed separately for each location.
Lamb County
Simazine and diuron were applied around productive Western pecan trees of fairly uniform diameter in replicated, four tree plots in Lamb County. Treatment rates for simazine were 0.0, 2.0 and 4.0 pounds a. i. per acre. Diuron was applied at 0.0, 1.5 and 3.0 pounds a.i. per acre. Each treatment was replicated four times in a randomized block design which provided a total of 96 trees in the test. The herbicides were applied in 50 gallons of water at 40 psi with a conventional pto operated farm sprayer. One quart of paraquat (1, 1'dimethyl-4, 4'-dipyridinium bis (methyl sulfate) was applied per acre, with the simazine and diuron, to kill a sparse population of small annual weeds which were growing at time of application. A band 10 feet wide was sprayed on each side of the trees and was left undisturbed following application.
Brown County
In addition to simazine and diuron, bromacil was included in the study in Brown County. Treatment rates for simazine and diuron were the same as those used in Lamb County. Treatments for bromacil were 0.0, 1.5 and 3.0 pounds a. i. per acre. Treatments in Brown County consisted of surface treatments in which the herbicides were applied to the soil surface and left undisturbed and split application treatments which had half of the herbicide disked into the soil and the other half applied to the surface following disking. The herbicides were applied around Riverside trees of uniform size in three tree plots. Each treatment was replicated three times which provided a total of 162 trees for observation.
Results
Lamb County
Weed species controlled. Tables 1 and 2 list the weed species present 99 and 166 days following herbicide treatment. Pigweed (Amaranthus spp.) was controlled by both herbicides 99 and 166 days following treatment. The lower treatment rate of each herbicide provided control of pigweed statistically equal to the higher treatment rates. Green foxtail (Setaria viridis) and barnyard grass (Echinochola crusqualli) were controlled equally well by the higher treatments of both herbicides. Simazine was more effective in controlling pitcher sage (Salvia pitcheri) than diuron.
Residual weed control. Table 3 shows that simazine, applied at 4.0 pounds per acre maintained persistent weed control throughout the summer. Simazine, applied at 2.0 pounds per acre, was equal to 3.0 pounds per acre of diuron in residual activity. The oat bioassay substantiates this observation, Table 4. Table 4 shows that both herbicides, in concentrations sufficient to retard oat growth, moved down into the pecan tree's feeder root zone.
TABLE 1. The effect of pre emergence applications of diuron and simazine in controlling several weed species 99 days
following application in Lamb County, Texas---1967.
==========================================================================================
Treatment (Lb./A)
Diuron---0 Diuron---1.5 Diuron---3.0 Simazine---0 Simazine---2.0 Simazine---4.0
----------------------------------------------------------------------------------------------------------------------------------------------------------------
Kochia,
Kochia scoparia 3.63 ab ¹ 0.38 b 0.31 b 3.63 ab 3.38 ab 5.88 a
Pigweed,
Amaranthus spp. 41.00 a 3.13 b 0.38 b 41.00 a 3.50 b 1.06 b
Green foxtail
Setaria viridis 11.13 a 5.38 ab 0.06 b 11.13 a 2.63 b 0.63 b
Pitcher sage,
Salvia pitcher 13.19 ab 19.31 a 6.25 bc 13.19 ab 1.50 c 0.88 c
==================================================================================================
¹Treatment means for each weed species are arranged horizontally for statistical significance.
Different letters indicate significant differences at the .05 level with (a) being the least effective treatment by Duncan's Multiple Range Test.
TABLE 2.The effect of pre emergence applications of diuron and simazine in controlling several weed species 166 days
following application in Lamb County, Texas---1967.
==================================================================================================
Treatment (Lb./A)
Diuron---0 Diuron---1.5 Diuron---3.0 Simazine---0 Simazine---2.0 Simazine---4.0
---------------------------------------------------------------------------------------------------------------------------------------------------------------
Kochia,
Kochia scoparia 1.27 ab ¹ 1.06 bc 0.94 bc 1.27 ab 4.38 a 0.31 c
Pigweed,
Amaranthus spp. 22.88 a 0.25 b 0.81 b 22.88 a 7.38 b 1.88 b
Green foxtail
Setaria viridis 5.06 a 0.63 b 0.31 b 5.06 a 1.25 b 0.88 b
Pitcher sage,
Salvia pitcher 3.94 bc 10.25 a 6.69 ab 3.94 bc 1.25 c 0.88 c
Lovegrass,.
Eragrostis spp. 0.50 a 0.06 ab 0.00 b 0.50 a 0.13 ab 0.06 ab
Barnyard grass,
Echinochlaoa crusgalli 1.81 ab 4.06 a 0.06 b 1.81 ab 0.88 ab 0.06 b
==================================================================================================
¹Treatment means for each weed species are arranged horizontally for statistical significance.
Different letters indicate significant differences at the .05 level with (a) being the least effective treatment by Duncan's Multiple Range Test.
Table 3. Effect of diuron and simazine on residual weed control and yield of western pecans in Lamb County, Texas---1967.
====================================================================
Percent weed coverage*
Treatment Days following application Pecans harvested
(Lb. actual per acre) 99 Days 166 Days (Lb. per tree)
--------------------------------------------------------------------------------------------------------------
Diuron ---0.0 67.50 a 66.25 a 15.15 c
Diuron ---1.5 15.00 bc 38.25 b 20.75 a
Diuron ---3.0 7.75 cd 21.00 c 19.30 ab
Simazine ---0.0 67.50 a 66.25 a 15.15 c
Simazine ---2.0 21.25 b 25.00 c 17.03 abc
Simazine ---4.0 5.00 d 6.00 d 19.61 a
====================================================================
Means followed by different letters are significantly different at the .05 level by Duncan's Multiple Range Test.
*Percent weed coverage is an estimate of the soil surface covered with weeds on the days indicated.
Table 4. Oat bioassay indicating residual activity 166 days following herbicide treatment in Lamb County, Texas---1967.
====================================================================
Fresh oat seedling weight
(Percent of control)
Treatment 1 to 4-inch.soil depth 4 to 8-inch.soil depth
--------------------------------------------------------------------------------------------------------------
Diuron ---0.0 100.0 a 100.0 a
Diuron ---1.5 95.2 a 95.4 a
Diuron ---3.0 69.4 b 76.9 ab
Simazine ---0.0 100.0 a 100.0 a
Simazine ---2.0 55.6 c 83.1 b
Simazine ---4.0 40.3 d 73.1 c
====================================================================
Means followed by different letters are significantly different at the .05 level by
Duncan's Multiple Range Test.
Pecan production and quality. Increases in yield amounting to more than 4 pounds per tree were obtained in all plots which had received herbicide treatment.
This data is presented in Table 3.Table 5 shows that pecan nut quality was not affected by any herbicide treatment.
No phytotoxic symptoms were observed in this test.
Brown County
Weed species controlled. Tables 6, 7 and 8 list the weed species present 64, 99 and 166 days following herbicide application. Johnsongrass (Sorghum halepense) was prevalent in all plots but is not listed in the tables since no treatment controlled this species. Excellent control of pigweed was obtained from each herbicide tested. Bromacil and simazine were superior to diuron in controlling most of the other weed species.
Residual weed control. Table 9 shows that excellent weed control was maintained with simazine and bromacil for 64 days following application. Diuron, applied at 3.0 pounds per acre, provided good weed control over the same period but 1.5 pounds per acre was insufficient for this soil type. Good control was maintained for 166 days with 3.0 pounds per acre of bromacil. Incorporating half of
Table 5. Effect of diuron and simazine on western pecan nut quality in Lamb County, Texas---1967.
====================================================================
Specific
Treatment gravity Percent Percent Nuts per
(Lb./A) (g/cc) fill kernel pound
--------------------------------------------------------------------------------------------------------------
Diuron---0 .815a 90.00a 61.02a 78.45a
Diuron---1.5 .808a 88.40a 60.17a 78.61a
Diuron---3.0 .810a 89.35a 60.89a 81.77a
Simazine---0 .815a 90.00a 61.02a 78.45a
Simazine---2.0 .815a 89.08a 59.99a 78.41a
Simazine---4.0 .813a 89.13a
====================================================================
Means followed by different letters are significantly different at the.05 level by
Duncan's Multiple Range Test.
TABLE 6. Effect of diuron, simazine, and bromacil in controlling several weed species 64 days following application in
Brown County, Texas---1967.
==================================================================================================
Treatment Colorado Western Silver
(Lb./A) Pigweed Goathead grass yarrow N. shade Spurge
----------------------------------------------------------------------------------------------------------------------------------------------------------------
Surface application
Diuron---0 2.56 b ¹ 50.89 a 24.00 ab 8.55 a 2.44 a 6.11 a-d
Diuron---1.5 0.22 c 33.56 ab 8.33 c 3.22 b 0.44 bc 0.00 cd
Diuron---3.0 0.11 c 17.78 bcd 1.78 c 1.00 b 0.00 c 0.11d
Simazine---0 2.56 b 50.89 a 24.00 ab 8.55 a 2.44 a 6.11 a-d
Simazine---2.0 0.00 c 0.89 e 9.22 c 0.00 b 0.00 c 0.33 d
Simazine---4.0 0.00 c 3.89 de 2.78 c 0.00 b 0.00 c 0.00 d
Bromacil---0 2.56 b 50.89 a 24.00 ab 8.55 a 2.44 a 6.11 a-d
Bromacil---1.5 0.00 c 1.44 e 4.11 c 0.00 b 0.00 c 0.00 d
Bromacil---3.0 0.00 c 1.56 e 0.78 c 0.00 b 0.00 c 0.00 d
Split application with one-half disked in
Diuron---0 5.44 a 23.11 bc 37.22 a 12.33 a 1.56 ab 10.00 a
Diuron---1.5 0.00 c 19.33 bcd 10.89 bc 0.67 b 0.11 c 0.44 d
Diuron---3.0 0.00 c 6.11 de 12.66 bc 0.00 b 0.00 c 0.00 d
Simazine---0 5.44 a 23.11 bc 37.22 a 12.33 a 1.56 ab 10.00 a
Simazine---2.0 0.00 c 2.44 e 9.00 c 0.00 b 0.00 c 0.00 d
Simazine---4.0 0.00 c 0.00 e 0.44 c 0.00 b 0.00 c 0.00 d
Bromacil---0 5.44 a 23.11 bc 37.22 a 12.33 a 1.56 ab 10.00 a
Bromacil---1.5 0.00 c 0.00 e 3.00 c 0.00 b 0.00 c 0.00 d
Bromacil---3.0 0.00 c 0.89 e 2.33 c 0.00 b 0.00 c 0.00 d ==========================================================================================================
¹Each weed species analyzed separately with treatment means arranged in vertical columns for statistical significance.
Different letters indicate significant difference at the.05 level with (a) being the least effective treatment for a given weed species by Duncan's Multiple Range Test.
TABLE 7. Effect of diuron, simazine, and bromacil in controlling several weed species 99 days following application in
Brown County, Texas---1967.
==================================================================================================
Treatment Colorado Western Silver
(Lb./A) Pigweed Goathead grass yarrow N. shade
----------------------------------------------------------------------------------------------------------------------------------------------------------------
Surface application
Diuron---0 2.89 a ¹ 17.00 a 13.00 a 2.33 a 2.33 a
Diuron---1.5 0.77 b 14.78 ab 11.78 ab 0.00 b 0.00 b
Diuron---3.0 0.22 b 11.44 bc 3.11 de 0.00 b 0.11 b
Simazine---0 2.89 a 17.00 a 13.00 a 2.33 a 2.33 a
Simazine---2.0 0.00 b 4.56 de 7.00 bcd 0.00 b 0.00 b
Simazine---4.0 0.00 b 2.56 de 3.00 de 0.00 b 0.00 b
Bromacil---0 2.89 a 17.00 a 13.00 a 2.33 a 2.33 a
Bromacil---1.5 0.11 b 4.00 de 3.22 de 0.00 b 0.00 b
Bromacil---3.0 0.00 b 1.11 e 0.56 e 0.00 b 0.11 b
Split application with one-half disked in
Diuron---0 3.89 a 14.67 ab 11.78 ab 3.44 a 0.67 b
Diuron---1.5 0.11 b 13.22 ab 8.89 abc 0.11 b 0.00 b
Diuron---3.0 0.11 b 7.33 cd 4.56 cde 0.00 b 0.00 b
Simazine---0 3.89 a 14.67 ab 11.78 ab 3.44 a 0.67 b
Simazine---2.0 0.00 b 3.33 de 8.22 a-d 0.00 b 0.00 b
Simazine---4.0 0.00 b 0.00 e 0.11 e 0.00 b 0.00 b
Bromacil---0 3.89 a 14.67 11.78 ab 3.44 a 0.67 b
Bromacil---1.5 0.00 b 0.89 1.22 e 0.00 b 0.00 b
Bromacil---3.0 0.77 b 2.00 e 1.22 e 0.00 b 0.00 b ===================================================================================================
¹Each weed species analyzed separately with treatment means arranged in vertical columns for statistical significance.
Different letters indicate significant difference at the.05 level with (a) being the least effective treatment for a given weed species by Duncan's Multiple Range Test.
TABLE 8. Effect of diuron, simazine, and bromacil in controlling several weed species 166 days following application in
Brown County, Texas---1967.
==================================================================================================
Treatment Colorado Western Silver
(Lb./A) Pigweed Goathead grass yarrow N. shade
----------------------------------------------------------------------------------------------------------------------------------------------------------------
Surface application
Diuron---0 1.89 a ¹ 5.44 abc 16.11 a 1.56 a 0.33 a
Diuron---1.5 0.00 b 9.00 a 6.78 b-e 0.11 a 0.22 a
Diuron---3.0 0.00 b 4.44 b-c 0.33 e 0.22 a 0.33 a
Simazine---0 1.89 a 5.44 abc 16.11 a 1.56 a 0.33 a
Simazine---2.0 0.11 b 0.78 ef 6.78 b-e 0.00 a 1.00 a
Simazine---4.0 0.11 b 0.67 ef 1.78 de 0.00 a 0.33 a
Bromacil---0 1.89 a 5.44 abc 16.11 a 1.52 a 0.33 a
Bromacil---1.5 0.56 b 2.56 b-f 13.11 ab 0.00 a 0.33 a
Bromacil---3.0 0.33 b 1.11 d-f 0.11 e 0.00 a 0.33 a
Split application with one-half disked in
Diuron---0 2.11 a 5.22 a-d 10.33 abc 1.11 a 0.56 a
Diuron---1.5 0.00 b 5.56 ab 9.00 a-d 0.00 a 0.33 a
Diuron---3.0 0.22 b 2.11 b-f 2.89 cde 0.11 a 0.22 a
Simazine---0 2.11 a 5.22 a-d 10.33 abc 1.11 a 0.56 a
Simazine---2.0 0.11 b 1.33 b-f 5.22 cde 0.00 a 0.89 a
Simazine---4.0 0.00 b 0.22 f 0.56 e 0.00 a 0.33 a
Bromacil---0 2.11 a 5.22 a-d 10.33 abc 1.11 a 0.56 a
Bromacil---1.5 0.00 b 1.22 c-f 5.89 b-e 0.00 a 0.22 a
Bromacil---3.0 1.11 ab 1.56 b-f 1.22 e 0.00 a 0.22 a ===================================================================================================
¹Each weed species analyzed separately with treatment means arranged in vertical columns for statistical significance.
Different letters indicate significant difference at the.05 level with (a) being the least effective treatment for a given weed species by Duncan's Multiple Range Test.
Table 9. The effect of pre emergence herbicides and methods of application on residual weed control in Brown County, Texas---1967.
====================================================================
Percent weed coverage
Treatment Days following application
(Lb./A) 64 days 117 days 166 days
--------------------------------------------------------------------------------------------------------------
Surface application
Diuron---0.0 66.6 a 87.0 abc 95.0 a
Diuron---1.5 35.0 b 90.7 ab 86.7 ab
Diuron---3.0 7.3 cd 49.0 de 78.3 bc
Simazine---0.0 66.6 a 87.0 abc 95.0 a
Simazine---2.0 1.3 d 36.3 fg 83.3 abc
Simazine---4.0 1.0 d 21.0 h 55.0 ef
Bromacil---0.0 66.0 a 87.0 abc 95.0 a
Bromacil---1.5 0.0 d 24.7 gh 41.7 f
Bromacil---3.0 0.0 d 7.0 I 8.7 gh
Split application with one-half disked in
Diuron---0.0 73.3 a 92.6 a 91.7 ab
Diuron---1.5 14.0 c 77.3 c 70.0 cd
Diuron-3.0 2.6 d 60.0 d 61.0 de
Simazine-0.0 73.3 a 92.6 a 91.7 ab
Simazine-2.0 1.2 d 39.3 ef 54.3 ef
Simazine-4.0 0.2 d 3.0 I 5.67 h
Bromacil-0.0 73.3 a 92.6 a 91.7 ab
Bromacil-1.5 0.0 d 4.7 I 21.0 g
Bromacil-3.0 0.0 d 12.3 hi 17.7 gh
====================================================================
Averages in columns not followed by the same letter are statistically different at the 5% level of probability by Duncan's Multiple Range Test.
the treatment rate increased the persistence of each herbicide. This effect was most pronounced with the 4.0 pound per acre simazine treatment. The same effect was observed in an oat bioassay which is given in Table 10.
Pecan tree growth and phytotoxicity. Table 11 shows that significant increases in trunk diameter were obtained in plots which had received simazine at 2.0 and 4.0 pounds per acre and bromacil at 1.5 pounds per acre when the full treatment rate had been applied to the soil surface and left undisturbed. No increase was obtained in plots which had received diuron at the 1.5 or 3.0 pound rate. However, when half of the diruon was incorporated, increases were obtained at both rates.
Interveinal necrotic spots developed on leaves of trees which had been treated with 3.0 pounds of bromacil per acre. Four trees were killed by this treatment. This accounts for the low average increase in trunk diameter value for the bromacil, 3.0 pounds per acre, treatment in Table 11.
Summary
The first year’s results indicate that diuron [3,(3,4 dichloro---1, 1---dimethyl ureal] and simazine (2---chloro---4,6---bis (ethyl amino) -s-triazine) are suitable for pre-emergence weed control in pecan orchards. Neither diuron up to 3 pounds,
TABLE 10. Oat bioassay indicating residual activity 166 days following herbicide treatment at Brownwood Pecan Re- search Station, Brownwood, Texas, in 1967.
===============================================================================================
Split application with
Surface application one-half disked in
(Lb./A) (Lb./A)
-----------------------------------------------------------------------------------------------------------------------------------------------------------
0 1.5 3.0 0 1.5 3.0
------------- --------------- ------------- ------------ --------------- ----------------
Diuron, 1 to 4"1 100.0 bc 124.9 a ² 98.7 bc 100.0 ab 91.1 ab 67.6 cd
Diuron, 4 to 8" 100.0 cde 138.0 ab 112.4 bcd 100.0 a 77.0 be 89.3 abc
0 2.0 4.0 0 2.0 4.0
------------- --------------- ------------- ------------ --------------- ----------------
Simazine, 1 to 4" 100.0 bc 66.0 c-f 48.1 f-h 100.0 ab 52.0 efg 30.2 h
Simazine, 4 to 8" 100.0 cde 80.6e 100.7 cde 100.0 a 84.0 abc 55.1 e
0 1.5 3.0 0 1.5 3.0
------------- --------------- ------------- ------------ --------------- ----------------
Bromacil, 1 to 4” 100.0 bc 94.9 bc 80.8 cd 100.0 ab 38.0 gh 69.3 cd
Bromacil, 4 to 8" 100.0 cde 99.2 cde 116.2 abc 100.0 a 57.2 de 69.6 cde
===============================================================================================
Means followed by different letters are significantly different at the .05 level by Duncan's Multiple Range Test.
1Soil depth from which the soil samples were taken.
2Fresh oat seedling weights expressed as percent of control.
Table 11. The effect of pre emergence herbicides and methods of application on pecan tree growth in Brown County, Texas---1967.
--------------------------------------------------------------------------------------------------------------
Mean
Treatment Percent increase
(Lb./A) in trunk diameter
--------------------------------------------------------------------------------------------------------------
Surface application
Diuron---0.0 13.28 bc
Diuron---1.5 14.03 bc
Diuron---3.0 17.83 bc
Simazine---0.0 13.28 bc
Simazine---2.0 23.11 ab
Simazine---4.0 29.80 a
Bromacil---0.0 13.28 be
Bromacil---1.5 23.95 ab
Bromacil---3.0 5.88c
Split application with one-half disked in
--------------------------------------------------------------------------------------------------------------
Diuron---0.0 17.03 bc
Diuron---1.5 20.66 ab Diuron---3.0 20.99 ab
Simazine---0.0 17.03 bc
Simazine---2.0 24.29 ab
Simazine---4.0 19.00 ab
Bromacil---0.0 17.03 bc
Bromacil---1.5 17.42 bc
Bromacil---3.0 12.51 bc
--------------------------------------------------------------------------------------------------------------
Averagers not followed by the same letter are statistically different at the 5% level of probability by Duncan's Multiple Range Test.
nor simazine up to 4 pounds active per acre were found to be injurious to pecan trees in either location. However, bromacil [5---bromo- 3---sec---butyl---6-6 methyl-uracil] was phytotoxic to pecan trees at 3 pounds active per acre. Thus, bromacil has been dropped from the test even though it exhibited excellent weed control.
Simazine on an equal dosage basis was more persistent and controlled a wider spectrum of annual weeds than diuron. Perennial weed species were not controlled in either location, regardless of treatment.
Significant increases in yield, amounting to more than 4 pounds of pecans per tree, were obtained from plots which had received diuron at 1.5 and 3.0 pounds active per acre and simazine at 4.0 pounds per acre. Pecan nut quality was not affected by any treatment.
Tree growth, based on percent increase in trunk diameter, was significantly increased in plots treated with 2.0 and 4.0 pounds simazine and 3.0 pounds diuron. No increase in tree size was obtained in plots which had received the lower treatment rate of diuron.
Percent weed coverage estimations taken periodically throughout the summer, and also bioassay analyses indicate that the persistence of each herbicide tested in Brown County was increased by incorporating one-half of the treatment rate. Control of perennial weed species was not obtained, however, and this may limit the feasibility of the added operation of mechanical incorporation.
Trade names are used in this report solely to provide specific information. Mention of a trade name does not constitute a guarantee or warranty of the products' names, and does not signify that this product is recommended to the exclusion of other comparable products.
ACKNOWLEDGMENTS
We wish to express our appreciation to Mr. John Speck, Plainview, Tex., for permitting us to conduct part of our research in his orchard.
Also, we wish to thank the Geigy Chemical Corporation for furnishing simazine, and E. I. DuPont DeNemours and Company, Incorporated, for supplying experimental quantities of diuron and bromacil.
Recognition also is given to Mr. Pat Westfall, Mr. Morris Smith, Mr. Grady Wadsworth, and Mr. J. Dan Hanna for their combined efforts in assisting with the research.
LITERATURE CITED
- Benson, N. R., and E. S. Degman. 1961. The use of herbicides around non-bearing pome fruit trees.Proc.Amer.Soc.Hort.Sci.78:46-52.
- Gilbert, F. A., L. Holm, and E. Haltvick. 1959. The control of weeds in newly established sour cherries. Weeds. 7 (1) :223-229.
- Gordinier, E. J. 1964. Chemical weed control in established nut plantings. 55th Annual Report, Northern Nut Growers Assn. 50-53.
- Lange, A. H., et al. 1967. Pre-emergence herbicides for weed control in walnuts. Calif. Agri. 21 (1) :2-4.
- Larsen, R. P., and S. K. Ries. 1960. Simazine for controlling weeds in fruit tree and grape plantings. Weeds. 8:671-677.
- McKay, J. W., and F. H. Berry. 1958. Herbicides for weed control in nut plantings. 49th Annual Report, Northern Nut Growers Assn. 75-78.
7. Saidak, W. J. and W. M. Rutherford, 1963. The tolerance of young apple trees to amitrol, diuron and simazine. Can. Plant. Sci. 43:113-118.
₁Respectively, graduate research assistant and associate professor, Horticulture Section, Department of Soil and Crop Sciences, Texas, A&M University, College Station, Texas; and research horticulturist, U. S. Pecan Field Station Brownwood, Texas.
