Enhanced Biodegradation of Pesticides in the ... - ACS Publications

Chapter 16

Systems Allowing Continued Use of Carbamothioate Herbicides Despite Enhanced Biodegradation Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on July 15, 2018 at 20:44:17 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Robert G. Harvey Department of Agronomy, University of Wisconsin, Madison, WI 53706

Enhanced biodegradation of EPTC was reduced by applying the herbicide not more frequently than every other year. The extender dietholate did not prevent enhanced EPTC degradation when the two chemicals were used repeatedly together, possibly due to a demonstrated enhancement of dietholate biodegradation. Enhanced biodegradation of the experimental extender SC-0058 was not detected, and SC-0058 prevented enhanced EPTC biodegradation. Results suggest that by combining herbicide and crop rotation, use of chemical extenders, row cultivation, and late-season secondary herbicide applications, long-term control of weeds such as wild proso millet can be obtained.

S i n c e EPTC ( S - e t h y l d i p r o p y l c a r b a m o t h i o a t e ) was s y n t h e s i z e d i n 1954, i t has been used e x t e n s i v e l y to c o n t r o l annual g r a s s e s i n a l f a l f a (Medicago s a t i v a L . ) , e d i b l e beans ( P h a s e o l u s v u l g a r i s L . ) , p o t a t o e s (Solanum tuberosum L.) and o t h e r c r o p s ( 1 ) . C o m m e r c i a l i z a t i o n o f the c r o p p r o t e c t a n t d i c h l o r m i d ( 2 , 2 - d i c h l o r o N,N-di-2-propenylacetamide) i n the e a r l y 1970's a l l o w e d EPTC to be s a f e l y a p p l i e d i n f i e l d and sweet c o r n (Zea mays L.) to c o n t r o l h a r d - t o - k i l l weed s p e c i e s such as s h a t t e r c a n e [Sorghum b i c o l o r (L.) Moench.], w i l d p r o s o m i l l e t [Panicum m i l i a c e u m L. spp. r u d e r a l e (Kitagawa) T z e v e l e v . ] , and y e l l o w nutsedge (Cyperus e s c u l e n t u s L.) ( i l l ) .

D i s c o v e r y o f enhanced EPTC biodégradation i n s o i l s p r e v i o u s l y t r e a t e d w i t h the h e r b i c i d e (2.4-7) l e d to the c o m m e r c i a l i z a t i o n o f a f o r m u l a t e d m i x t u r e o f EPTC p l u s d i e t h o l a t e ( 0 , 0 - d i e t h y l - O - p h e n o l p h o s p h o r o t h i o a t e ) f o r c o n t r o l o f h a r d - t o - k i l l weeds i n c o r n . D i e t h o l a t e extended the l e n g t h o f time EPTC c o n t r o l l e d weeds by i n h i b i t i n g enhanced EPTC biodégradation. D i e t h o l a t e and c h e m i c a l s w i t h s i m i l a r a c t i v i t y are thus r e f e r r e d to as e x t e n d e r s . However, EPTC a p p l i e d w i t h d i e t h o l a t e was a l s o d i s c o v e r e d to be v u l n e r a b l e to

0097-6156/90/0426-Ό214$06.00/0 © 1990 American Chemical Society

Racke and Coats; Enhanced Biodegradation of Pesticides in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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Systems Allowing Use of Carbamothioate Herbicides

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enhanced biodégradation (2.5.8). Problems w i t h enhanced s o i l biodégradation o f EPTC f o r m u l a t i o n s made c o n t r o l l i n g h a r d - t o - k i l l weeds i n f i e l d and sweet c o r n more d i f f i c u l t . The f o l l o w i n g r e s e a r c h and l i t e r a t u r e summary d e s c r i b e s how enhanced biodégradation has a f f e c t e d w i l d p r o s o m i l l e t c o n t r o l i n sweet c o r n and proposes systems t o r e s o l v e the problem. Methodology W i l d p r o s o m i l l e t c o n t r o l s t u d i e s were conducted i n s o u t h e r n W i s c o n s i n a t v a r i o u s l o c a t i o n s and w i t h s m a l l - p l o t p r o c e d u r e s d e s c r i b e d elsewhere (3.5.8) (Harvey, R. G.; McNevin, G. R. Weed T e c h n o l . . i n p r e s s ) (Harvey, R. G. Weed S c i . . i n p r e s s ) . EPTC was a p p l i e d i n a commercial f o r m u l a t i o n c o n t a i n i n g e i t h e r d i c h l o r m i d o r R-29148 [ 3 - ( d i c h l o r o a c e t y l ) - 2 , 2 , 5 - t r i m e t h y l o x a z o l i d i n e ] t o p r e v e n t sweet c o r n i n j u r y . Other h e r b i c i d e s i n c l u d e d i n f i e l d s t u d i e s were commercial f o r m u l a t i o n s o f a l a c h l o r [ 2 - c h l o r o - N - ( 2 , 6 - d i e t h y l p h e n y l ) N - ( m e t h o x y m e t h y l ) a c e t a m i d e ] , ametryn [ N - e t h y l - N ' - ( 1 - m e t h y l e t h y l ) - 6 (methylthio)-1,3,5-triazine-2,4-diamine], cyanazine {2-[[4-chloro-6(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methyl p r o p a n e n i t r i l e } , c y c l o a t e ( S - e t h y l c y c l o h e x y l e t h y l c a r b a m o t h i o a t e ) , and p e n d i m e t h a l i n [ N - ( 1 - e t h y l p r o p y l ) - 3 , 4 - d i m e t h y l - 2 , 6 - d i n i t r o benzenamine]. EPTC was a l s o a p p l i e d w i t h the e x p e r i m e n t a l e x t e n d e r SC-0058 [S-ethyl-N,Nbis(3-chloroallyl)carbamothioate]. H e r b i c i d e and e x t e n d e r d i s s i p a t i o n s t u d i e s were conducted i n t h e l a b o r a t o r y by m i x i n g 6 mg/kg w/w EPTC and 1 mg/kg w/w d i e t h o l a t e o r SC-0058 w i t h s o i l samples c o l l e c t e d from f i e l d p l o t s w i t h known h i s t o r i e s o f p r i o r c a r b a m o t h i o a t e h e r b i c i d e u s e . A f t e r i n c u b a t i o n a t 25 C f o r time p e r i o d s d e t e r m i n e d by p r e l i m i n a r y t e s t s w i t h the same s o i l , h e r b i c i d e r e s i d u e s were e x t r a c t e d from s o i l by steam d i s t i l l a t i o n , and a n a l y z e d by g a s - l i q u i d chromatography. R e f e r e n c e s t a n d a r d s were p r e p a r e d s i m i l a r l y , b u t were f r o z e n immediately a f t e r t r e a t m e n t . D e t a i l s o f these p r o c e d u r e s a r e d e s c r i b e d elsewhere ( 5 . 8 ) . Percent h e r b i c i d e o r e x t e n d e r r e c o v e r e d was e s t i m a t e d by comparing r e s i d u e s i n samples o f the same s o i l i n c u b a t e d as d e s c r i b e d above o r f r o z e n immediately a f t e r treatment. Enhanced d e g r a d a t i o n was presumed t o o c c u r when p e r c e n t r e c o v e r y was lower i n s o i l s w i t h h i s t o r i e s o f p r i o r h e r b i c i d e o r extender use than those w i t h o u t . Data were s u b j e c t e d t o a n a l y s i s o f v a r i a n c e and means compared u s i n g Duncan's m u l t i p l e range t e s t . In the t a b l e s which f o l l o w , a l l means w i t h i n the same column f o l l o w e d by the same l e t t e r a r e n o t d i f f e r e n t a t the 0.05 a l p h a l e v e l . Field

Studies

L o c a t i o n Not P r e v i o u s l y T r e a t e d With EPTC. Studies conducted i n 1978 and 1979 demonstrated t h a t EPTC a t b o t h 4.5 and 6.7 kg a i / h a a p p l i c a t i o n r a t e s provided e f f e c t i v e w i l d proso m i l l e t c o n t r o l i n sweet c o r n grown on s o i l s n o t p r e v i o u s l y t r e a t e d w i t h t h a t h e r b i c i d e ( T a b l e I ) . Because o f the p r o l o n g e d p e r i o d o f m i l l e t g e r m i n a t i o n and t h e s h o r t s o i l p e r s i s t e n c e o f EPTC, m i l l e t c o n t r o l 95 days a f t e r t r e a t m e n t (DAT) was s u b s t a n t i a l l y l e s s than 45 DAT. However, t h e s e l a t e - s e a s o n m i l l e t escapes d i d n o t cause sweet c o r n y i e l d reductions. A p p l y i n g EPTC w i t h c y a n a z i n e s l i g h t l y improved l a t e -


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season m i l l e t c o n t r o l compared t o a p p l y i n g EPTC a l o n e . When r e s u l t s o f a l l 44 t r e a t m e n t s i n t h e s t u d y were summarized, e a r l y - and l a t e season m i l l e t c o n t r o l and sweet c o r n y i e l d s i n t h e 19 t r e a t m e n t s i n c l u d i n g EPTC were s u b s t a n t i a l l y g r e a t e r than i n t h e 25 t r e a t m e n t s not i n c l u d i n g EPTC.

Table

I . E a r l y - and l a t e - s e a s o n w i l d p r o s o m i l l e t c o n t r o l and sweet c o r n y i e l d s r e s u l t i n g from f i r s t - t i m e h e r b i c i d e t r e a t m e n t s

Herbicide(s) No h e r b i c i d e Handweeded EPTC EPTC + c y a n a z i n e

Rate (kz/ha)

4.5 6.7 6.7+2.2

Average r e s u l t s o f 19 c o m b i n a t i o n s which i n c l u d e d EPTC Average r e s u l t s o f 25 c o m b i n a t i o n s w i t h o u t EPTC

Percent m i l l e t c o n t r o l 45 DAT 95 DAT 0 e 0 c 94 a 97 a 94 a 57 c 96 a 63 c 76 b 97 a

97 a

70 be

68 b

20 d

Sweet c o r n yield (Mg/ha) 0.2 c 12.1 a 10.1 a 10.8 a 11.9 a

11.2 a 2.5 b

L o c a t i o n P r e v i o u s l y T r e a t e d With EPTC A l o n e . When a p p l i e d i n 1980 to a f i e l d t r e a t e d the f o u r p r e v i o u s y e a r s w i t h EPTC, EPTC a t 6.7 kg/ha a l o n e o r f o l l o w e d by ( f b ) preemergence c y a n a z i n e p r o v i d e d s a t i s f a c t o r y m i l l e t c o n t r o l 30 DAT, b u t n o t a t 57 and 110 DAT ( T a b l e II). EPTC p l u s d i e t h o l a t e p r o v i d e d b e t t e r s e a s o n - l o n g m i l l e t c o n t r o l and s i g n i f i c a n t l y g r e a t e r sweet c o r n y i e l d s than d i d EPTC a p p l i e d a l o n e o r f o l l o w e d by c y a n a z i n e . Dietholate obviously r e d u c e d the enhanced biodégradation o f EPTC i n t h e s o i l . Following an a p p l i c a t i o n o f EPTC w i t h e i t h e r a s i n g l e row c u l t i v a t i o n o r a p o s t e m e r g e n c e - d i r e c t e d a p p l i c a t i o n o f ametryn p r o v i d e d s e a s o n - l o n g m i l l e t c o n t r o l and r e s u l t e d i n sweet c o r n y i e l d s comparable t o EPTC p l u s d i e t h o l a t e , t h e r e b y overcoming t h e e f f e c t s o f enhanced EPTC degradation.

T a b l e I I . W i l d p r o s o m i l l e t c o n t r o l and sweet c o r n y i e l d s from h e r b i c i d e t r e a t m e n t s on s o i l p r e v i o u s l y t r e a t e d w i t h EPTC

Herbicide(s) No h e r b i c i d e Handweeded EPTC EPTC f b c y a n a z i n e EPTC p l u s d i e t h o l a t e EPTC f b c u l t i v a t i o n EPTC f b ametrvn

Rate (kg/ha)

6.7 6.7+2.2 6.7 6.7 6.7+2.2

Percent 30 DAT 0 b 96 a 73 a 85 a 95 a 80 a 84 a

millet 57 DAT 0 d 95 a 8 d 23 c 69 b 64 b 92 a

control 110 DAT 0 e 95 a 8 e 21 d 61 c 50 b 79 b

Sweet c o r n yield (Me/ha) 0.2 d 18.4 a 1.3 d 4.7 c 11.6 b 11.4 b 12.3 b


16.

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217

L o c a t i o n P r e v i o u s l y T r e a t e d With EPTC P l u s D i e t h o l a t e . When a p p l i e d i n 1983 to a f i e l d t r e a t e d the p r e v i o u s y e a r w i t h EPTC p l u s d i e t h o l a t e , 6.7 kg/ha EPTC w i t h and w i t h o u t d i e t h o l a t e r e s u l t e d i n poor m i l l e t c o n t r o l 40, 70 and 100 DAT, and e x t r e m e l y low sweet c o r n y i e l d s ( T a b l e I I I ) . In t h i s s o i l , enhanced EPTC biodégradation o c c u r r e d whether a p p l i e d w i t h or w i t h o u t d i e t h o l a t e ( 8 ) . Combining c y a n a z i n e w i t h EPTC p l u s d i e t h o l a t e o n l y i n c r e a s e d m i l l e t c o n t r o l 40 DAT compared to EPTC p l u s d i e t h o l a t e w i t h o u t c y a n a z i n e , and d i d not improve sweet c o r n y i e l d . F o l l o w i n g EPTC p l u s d i e t h o l a t e w i t h an e a r l y postemergence a p p l i c a t i o n o f p e n d i m e t h a l i n p l u s c y a n a z i n e r e s u l t e d i n m i l l e t c o n t r o l 40 DAT comparable to the handweeded t r e a t m e n t , and m i l l e t c o n t r o l 70 and 100 DAT and sweet c o r n y i e l d s g r e a t e r than those r e s u l t i n g from EPTC p l u s d i e t h o l a t e p l u s cyanazine treatment. The c o n t r i b u t i o n from the EPTC p l u s d i e t h o l a t e component o f t h i s s e q u e n t i a l treatment was a p p a r e n t l y s m a l l , however, s i n c e r e s u l t s were s i m i l a r to those from p e n d i m e t h a l i n p l u s cyanazine a p p l i e d alone.

T a b l e I I I . W i l d proso m i l l e t c o n t r o l and sweet c o r n y i e l d s from h e r b i c i d e t r e a t m e n t s on s o i l p r e v i o u s l y t r e a t e d w i t h EPTC + dietholate

Herbicide(s) No h e r b i c i d e Handweeded EPTC EPTC p l u s d i e t h o l a t e EPTC p l u s d i e t h o l a t e + cyanazine EPTC p l u s d i e t h o l a t e fb pendimethalin + cyanazine Pendimethalin + cyanazine

Rate (Wha)

6.7 6.7

Percent 40 DAT 0 c 95 a 52 b 56 b

millet 70 DAT 0 c 96 a 11 c 15 c

control 100 DAT 0 c 98 a 0 c 6 c

Sweet c o r n yield (Mp/ha) 0.2 c 11.4 a 0.2 c 0.2 c

6.7+2.2

77

a

25 c

8 c

0.2

c

6.7 f b 1.7+2.2

94

a

67 b

50 b

7.4

b

1.7+2.2

95 a

67 b

36 b

5.2

b

S i x - y e a r H e r b i c i d e R o t a t i o n Study. Mean w i l d p r o s o m i l l e t c o n t r o l o b t a i n e d i n a s t u d y conducted from 1984 to 1989 i s summnrized i n T a b l e IV. S i n c e the f i e l d had been t r e a t e d w i t h 6.7 kg/ha EPTC i n 1983, enhanced EPTC biodégradation presumably was p r e s e n t from the b e g i n n i n g o f the s t u d y . When a p p l i e d w i t h c y a n a z i n e a n n u a l l y over the s i x - y e a r p e r i o d , EPTC and EPTC p l u s d i e t h o l a t e p r o v i d e d o n l y 46 and 64% m i l l e t c o n t r o l , r e s p e c t i v e l y . Laboratory studies confirmed t h a t t h i s poor m i l l e t c o n t r o l was due t o enhanced biodégradation o f the a c t i v e i n g r e d i e n t (Harvey, R. G. Weed S c i . . i n p r e s s ) . Every o t h e r y e a r and e v e r y t h i r d y e a r a p p l i c a t i o n s o f EPTC p l u s c y a n a z i n e and EPTC w i t h d i e t h o l a t e p l u s c y a n a z i n e p r o v i d e d 83 and 86, and 88 and 91% m i l l e t c o n t r o l , r e s p e c t i v e l y . R o t a t i n g EPTC p l u s c y a n a z i n e or EPTC p l u s d i e t h o l a t e p l u s c y a n a z i n e t r e a t m e n t s w i t h a p p l i c a t i o n s o f c y c l o a t e p l u s c y a n a z i n e , or a l a c h l o r p l u s c y a n a z i n e r e d u c e d the


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biodégradation r a t e o f EPTC a p p l i e d a l o n e o r w i t h d i e t h o l a t e (unpublished data). Continuous use o f c y c l o a t e p l u s c y a n a z i n e over the s i x - y e a r p e r i o d p r o v i d e d an average o f 89% m i l l e t c o n t r o l even though c y c l o a t e i s a carbamothioate h e r b i c i d e (Table I V ) . Previous s t u d i e s have shown l i t t l e p o t e n t i a l f o r enhanced biodégradation o f c y c l o a t e (8). A l a c h l o r plus cyanazine or pendimethalin plus cyanazine t r e a t m e n t s , o f t e n c o n s i d e r e d as a l t e r n a t i v e s to EPTC f o r m i l l e t c o n t r o l , d i d n o t p r o v i d e b e t t e r m i l l e t c o n t r o l than c o m b i n a t i o n s o f c y a n a z i n e p l u s EPTC o r EPTC p l u s d i e t h o l a t e when the l a t t e r t r e a t m e n t s were used no more f r e q u e n t l y than e v e r y o t h e r y e a r .

T a b l e IV. W i l d p r o s o m i l l e t c o n t r o l w i t h EPTC and EPTC w i t h d i e t h o l a t e i n a s i x - y e a r h e r b i c i d e r o t a t i o n study Average p e r c e n t m i l l e t Treatment c o n t r o l over 6 year study EPTC + c y a n a z i n e a t 6.7 + 2.2 kg/ha: S i x r e p e a t e d annual a p p l i c a t i o n s 46 d A l t e r n a t e year a p p l i c a t i o n s 83 ab Every t h i r d year a p p l i c a t i o n s 86 ab EPTC p l u s d i e t h o l a t e + c y a n a z i n e a t 6.7 + 2.2 kg/ha: S i x r e p e a t e d annual a p p l i c a t i o n s 64 cd A l t e r n a t e year a p p l i c a t i o n s 88 ab Every t h i r d year a p p l i c a t i o n s 91 a Other h e r b i c i d e s a p p l i e d a n n u a l l y over 6 y r : C y c l o a t e + c y a n a z i n e a t 6.7 + 2.2 kg/ha 89 a A l a c h l o r + c y a n a z i n e a t 4.5 + 2.2 kg/ha 80 abc P e n d i m e t h a l i n + c y a n a z i n e a t 1.7 + 2.2 kg/ha 70 be

T a b l e V. W i l d p r o s o m i l l e t c o n t r o l and sweet c o r n y i e l d s f o l l o w i n g t h r e e annual a p p l i c a t i o n s o f EPTC a l o n e and w i t h e x t e n d e r s Treatment used over 3 y r p e r i o d Herbicide(s) Rate ( l b / A ) EPTC p l u s d i e t h o l a t e 6.7+1.1 EPTC O I U S SC-0058 6.7+1.1

Percent m i l l e t c o n t r o l 40 DAT 73 b 85 a

Sweet c o r n y i e l d (Mg/ha) 7 .6 b 11 .0 a

D i e t h o l a t e V e r s u s SC-0058. E f f e c t i v e n e s s o f the e x t e n d e r s d i e t h o l a t e and SC-0058 ( e x p e r i m e n t a l e x t e n d e r c u r r e n t l y b e i n g i n v e s t i g a t e d by ICI Americas I n c . ) to p r e v e n t r e d u c t i o n s i n w i l d p r o s o m i l l e t c o n t r o l due to enhanced biodégradation was e v a l u a t e d . The combined r e s u l t s o f t h r e e s e p a r a t e t e s t s i n c l u d i n g p l o t s t r e a t e d a n n u a l l y f o r t h r e e - y e a r p e r i o d s w i t h the r e s p e c t i v e h e r b i c i d e e x t e n d e r c o m b i n a t i o n s a r e p r e s e n t e d i n T a b l e V. Even a f t e r t h r e e c o n s e c u t i v e a p p l i c a t i o n s , 6.7 kg/ha EPTC p l u s 1.1 kg a i / h a SC-0058 p r o v i d e d 85% m i l l e t c o n t r o l 40 DAT. The same r a t e s o f EPTC p l u s d i e t h o l a t e p r o v i d e d o n l y 73% m i l l e t c o n t r o l 40 DAT. Use o f SC-0058 a l s o r e s u l t e d i n 45% g r e a t e r sweet c o r n y i e l d s . Thus SC-0058 i s more e f f e c t i v e than d i e t h o l a t e i n p r e v e n t i n g enhanced EPTC


16.

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degradation. S i m i l a r r e s u l t s have been r e p o r t e d elsewhere R. G. Weed S c i . . i n p r e s s ) . Laboratory

219

(Harvey,

Studies

Biodégradation Of EPTC And D i e t h o l a t e . L a b o r a t o r y d i s s i p a t i o n s t u d i e s c o n d u c t e d i n s o i l s o b t a i n e d from the h e r b i c i d e r o t a t i o n s t u d y d e s c r i b e d above demonstrated enhanced biodégradation o f EPTC a p p l i e d w i t h and w i t h o u t d i e t h o l a t e , and a l s o enhanced biodégradation o f d i e t h o l a t e i n s o i l s p r e v i o u s l y t r e a t e d w i t h EPTC p l u s d i e t h o l a t e ( T a b l e V I ) . A more complete r e p o r t o f enhanced d i e t h o l a t e biodégradation appears elsewhere (Harvey, R. G. Weed S c i . , i n p r e s s ) . Enhanced d i e t h o l a t e d e g r a d a t i o n o c c u r s when the e x t e n d e r i s a p p l i e d a l o n e , w i t h EPTC, o r w i t h b u t y l a t e [ S - e t h y l bis(2-methylpropyl)carbamothioate]. S t u d i e s a l s o show t h a t enhanced d i e t h o l a t e d e g r a d a t i o n p e r s i s t s two y e a r s f o l l o w i n g a p r e v i o u s d i e t h o l a t e a p p l i c a t i o n (unpublished data). I t has n o t y e t been d e t e r m i n e d whether enhanced biodégradation o f d i e t h o l a t e i s r e s p o n s i b l e f o r poor weed c o n t r o l o c c u r r i n g when EPTC p l u s d i e t h o l a t e i s a p p l i e d t o s o i l s p r e v i o u s l y t r e a t e d w i t h the same c o m b i n a t i o n o r whether s o i l microbes r e s p o n s i b l e f o r EPTC d e g r a d a t i o n c a n c i r c u m v e n t the i n h i b i t o r y e f f e c t s v i a a n o t h e r mechanism. P r e v i o u s s t u d i e s i n Nebraska (9) f a i l e d t o show enhanced d i e t h o l a t e d e g r a d a t i o n even when enhanced d e g r a d a t i o n o f EPTC a p p l i e d w i t h d i e t h o l a t e was d e t e c t e d . S o i l s u t i l i z e d i n the Nebraska study o r i g i n a t e d i n an a r e a o f lower annual r a i n f a l l and had s h o r t e r h i s t o r i e s o f p r e v i o u s d i e t h o l a t e use than the s o i l s used i n W i s c o n s i n s t u d i e s . S o i l d r y i n g has been shown t o e l i m i n a t e enhanced EPTC d e g r a d a t i o n ( 2 ) , and a s h o r t e r h i s t o r y o f d i e t h o l a t e use may have m i n i m i z e d the e x p r e s s i o n o f enhanced d i e t h o l a t e degradation.

T a b l e V I . L a b o r a t o r y d i s s i p a t i o n o f EPTC a l o n e o r EPTC and d i e t h o l a t e i n s o i l s w i t h d i f f e r e n t h e r b i c i d e use h i s t o r i e s Prior herbicide use h i s t o r y No h e r b i c i d e EPTC EPTC w/ d i e t h o l a t e

P e r c e n t EPTC and d i e t h o l a t e r e c o v e r e d EPTC Dietholate EPTC 46 a 42 a 55 a 6 b 39 a 6 b 15 b 17 b 9 b

Biodégradation Of EPTC. D i e t h o l a t e And SC-0058. The p e r c e n t a g e s o f EPTC and e x t e n d e r s p e r s i s t i n g f i v e days a f t e r a p p l i c a t i o n t o s o i l s w i t h d i f f e r e n t h i s t o r i e s o f h e r b i c i d e and e x t e n d e r a p p l i c a t i o n over the p r e v i o u s t h r e e y e a r s a r e shown i n T a b l e V I I . Enhanced biodégradation o f EPTC o c c u r r e d when a p p l i e d a l o n e t o s o i l s p r e v i o u s l y t r e a t e d w i t h EPTC a l o n e , w i t h d i e t h o l a t e , and w i t h SC-0058, and when a p p l i e d w i t h d i e t h o l a t e t o s o i l p r e v i o u s l y t r e a t e d w i t h EPTC p l u s SC-0058. Enhanced d i e t h o l a t e biodégradation o c c u r r e d when a p p l i e d w i t h EPTC t o s o i l p r e v i o u s l y t r e a t e d w i t h EPTC p l u s dietholate. Enhanced EPTC biodégradation was p r e v e n t e d by a p p l y i n g


220

ENHANCED BIODEGRADATION OF PESTICIDES IN T H E ENVIRONMENT

the h e r b i c i d e w i t h SC-0058. not o c c u r .

Enhanced biodégradation o f SC-0058 d i d

T a b l e V I I . D i s s i p a t i o n o f EPTC and e x t e n d e r s a p p l i e d to s o i l s w i t h d i f f e r e n t h i s t o r i e s o f p r e v i o u s h e r b i c i d e and e x t e n d e r use H e r b i c i d e and e x t e n d e r s applied for 3 yr previously and i n d i s s i p a t i o n study S o i l previously untreated: EPTC a l o n e EPTC w/ d i e t h o l a t e EPTC w/ SC-0058 Soil previously treated w i t h EPTC: EPTC a l o n e EPTC w/ d i e t h o l a t e EPTC w/ SC-0058 S o i l p r e v i o u s l y t r e a t e d with EPTC w/ d i e t h o l a t e : EPTC a l o n e EPTC w/ d i e t h o l a t e EPTC w/ SC-0058 S o i l p r e v i o u s l y treated with EPTC w/ SC-0058: EPTC a l o n e EPTC w/ d i e t h o l a t e EPTC w/ SC-0058

Percent h e r b i c i d e or extender p e r s i s t i n g 5 DAT EPTC Dietholate SC-0058 64 a 72 a 66 a

57 a

21 de 57 a 48 abc

80 a

12 e 7 e 35 bed

8 b

26 cde 63 a 50 ab

82 a

74 a

65 a

53 a

76 a

Discussion Even when n o t s u b j e c t to enhanced biodégradation, EPTC does n o t always p r o v i d e s e a s o n - l o n g c o n t r o l o f w i l d p r o s o m i l l e t i n sweet c o r n , b u t the h e r b i c i d e i s one o f the b e t t e r c h e m i c a l o p t i o n s f o r c o n t r o l l i n g the weed. Enhanced biodégradation, however, l i m i t s use of the h e r b i c i d e i n c o n s e c u t i v e seasons i f a c c e p t a b l e m i l l e t c o n t r o l i s d e s i r e d . Use o f EPTC o n l y e v e r y o t h e r y e a r w i l l h e l p m a i n t a i n e f f e c t i v e m i l l e t c o n t r o l w i t h t h a t h e r b i c i d e . Such a l t e r n a t e y e a r use may n o t s o l v e m i l l e t problems i f sweet c o r n i s grown r e p e a t e d l y i n the same f i e l d u n l e s s e f f e c t i v e a l t e r n a t i v e c o n t r o l t o o l s a r e available. In the absence o f a c c e p t a b l e a l t e r n a t i v e s , sweet c o r n c o u l d be r o t a t e d w i t h o t h e r c r o p s . Crops such as a l f a l f a , wheat ( T r i t i c u m a e s t i v u m L . ) , and peas (Pisum s a t i v u m L.) u s u a l l y compete w i t h , and shade out w i l d p r o s o m i l l e t i n the absence o f h e r b i c i d e s (10). E f f e c t i v e and e c o n o m i c a l postemergence h e r b i c i d e s a r e a v a i l a b l e to c o n t r o l m i l l e t i n soybeans ( 1 0 ) . In a d d i t i o n to r e d u c i n g enhanced EPTC biodégradation, c r o p r o t a t i o n s can a l s o reduce m i l l e t p o p u l a t i o n s by r e d u c i n g the number o f p l a n t s which must be c o n t r o l l e d w i t h the h e r b i c i d e ( 3 ) . D e s p i t e c r o p and h e r b i c i d e r o t a t i o n s , s e a s o n - l o n g m i l l e t c o n t r o l won't always be p r o v i d e d by EPTC. Combining EPTC t r e a t m e n t s w i t h c y a n a z i n e , o r f o l l o w i n g w i t h e a r l y postemergence p e n d i m e t h a l i n p l u s c y a n a z i n e o r


16.

HARVEY


221

p o s t e m e r g e n c e - d i r e c t e d ametryn t r e a t m e n t s w i l l improve s e a s o n - l o n g millet control. F o l l o w i n g EPTC a p p l i c a t i o n s w i t h one o r more row c u l t i v a t i o n s w i l l a l s o g r e a t l y improved EPTC performance even i n the p r e s e n c e o f enhanced d e g r a d a t i o n , and i s c o n s i s t e n t w i t h c u r r e n t i n t e r e s t s i n p r o t e c t i n g groundwater and i n c r e a s i n g s u s t a i n a b i l i t y o f farm o p e r a t i o n s . The d a t a p r e s e n t e d from sweet c o r n r e s e a r c h a l s o a r e a p p l i c a b l e to weed c o n t r o l problems i n f i e l d c o r n . H e r b i c i d e s u s e d i n sweet c o r n may a l s o be used i n f i e l d c o r n . Sweet c o r n i s t y p i c a l l y p l a n t e d i n w i d e r rows and i n lower p o p u l a t i o n s t h a n i s f i e l d c o r n Q), and sweet c o r n h y b r i d s a r e t y p i c a l l y s h o r t e r t h a n f i e l d c o r n hybrids. These f a c t o r s cause sweet c o r n t o be l e s s c o m p e t i t i v e w i t h weeds t h a n f i e l d c o r n , and weed problems u s u a l l y a r e more s e v e r e i n sweet c o r n t h a n f i e l d c o r n . Thus, systems e f f e c t i v e f o r c o n t r o l l i n g weeds i n sweet c o r n a r e u s u a l l y even more e f f e c t i v e i n f i e l d c o r n . Data p r e s e n t e d do n o t d i r e c t l y a d d r e s s enhanced biodégradation of c a r b a m o t h i o a t e h e r b i c i d e s o t h e r t h a n EPTC. However, r e s e a r c h i n d i c a t e s t h a t b o t h d i e t h o l a t e and SC-0058 reduce enhanced b u t y l a t e biodégradation problems (Harvey, R. G. Weed S c i . . i n p r e s s ) . Repeated a n n u a l a p p l i c a t i o n s o f c y c l o a t e d i d n o t l e a d t o enhanced biodégradation o f t h a t h e r b i c i d e . R e g i s t r a t i o n o f c y c l o a t e f o r use i n f i e l d and sweet c o r n c o u l d improve weed c o n t r o l o p t i o n s . Acknowledgments The a u t h o r e x p r e s s e s h i s a p p r e c i a t i o n t o John A l b r i g h t and Tim Anthon f o r t h e i r a s s i s t a n c e w i t h f i e l d t r i a l s ; t o John K u t i l , Tamara Harvey, S h e l i a S t e e l e , and K e v i n B u h l e r f o r t h e i r h e l p w i t h biodégradation s t u d i e s ; and t o I C I Americas I n c . and the Midwest Food P r o c e s s o r s A s s o c i a t i o n f o r t h e i r f i n a n c i a l s u p p o r t o f t h i s research.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Gray, R. A. Abstr. Weed Sci. Soc. Am., 1975,p145. Harvey, R. G.; Dekker, J. H.; Fawcett, R. S.; Roeth, F. W.; Wilson, R. G. Weed Technol. 1987, 1, 341-349. McNevin, G. R. Ph.D. Thesis, University of Wisconsin, Madison, 1982. Rahman, Α.; James, T. K. Weed Sci. 1983, 31,783-789. Harvey, R. G.; McNevin, G. R.; Albright, J. W.; Kozak, M. E. Weed Sci. 1986, 34, 773-780. Gunsolus, J. L.; Fawcett, R. S. Proc. North Cent. Weed Control Conf., 1980, 35, 18. Obrigawitch, T.; Wilson, R. G.; Martin, A. R.; Roeth, F. W. Weed Sci. 1982, 30, 175-181. Harvey, R. G. Weed Sci. 1987, 35, 583-589. Wilson, R. G.; Rodebush, J. E. Weed Sci. 1987, 35, 289-294. Doersch, R. E.; Fischer, H. L.; Harvey, R. G. North Cent. Reg. Ext. Public. 1987, No. 265, 4 p.

RECEIVED January 22, 1990


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