Agronomic and economic feasibility of growing corn (Zea mays L.) with different levels of tillage and dairy manure in Quebec

Soil & Tillage Research, 14 (1989) 311-325 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

311

Agronomic and Economic Feasibility of Growing Corn (Zea m a y s L.) with Different Levels of Tillage and Dairy Manure in Quebec A.N. WEILL', E. McKYES 1 and G.R. MEHUYS 2

1Department of Agricultural Engineering and 2Department of Renewable Resources, Macdonald College, McGill University, 21111 Lakeshore Road, Ste-Anne de Bellevue, Qudbec H9X IC0 (Canada) (Accepted for publication 25 January 1989 )

ABSTRACT Weill, A.N., McKyes, E. and Mehuys, G.R., 1989. Agronomic and economic feasibility of growing corn (Zea mays L. ) with different levels of tillage and dairy manure in Quebec. Soil Tillage Res., 14: 311-325. In order to assess the agronomic and economic feasibility of growing corn under reduced tillage using manure as a source of fertilizer, a study of silage and grain corn (Zea mays L. ) production using three levels of tillage {conventional, reduced and zero till) and two types of fertilizer (inorganic fertilizer and dairy manure) was initiated on a clay soil and a sandy loam soil in 1981 at Macdonald College. Results obtained between 1983 and 1986 showed that good yields could be obtained with zero and reduced tillage in combination with inorganic fertilizers. The use of dairy manure in combination with conventional and reduced tillage resulted in good plant yields at the sandy loam site but decreased plant yield at the clay site. The practice of zero till in combination with manure resulted in difficulties with weed control, poor seed emergence and a greater risk of frost damage in the spring. The costs of production were lowest when zero till was used in association with inorganic fertilizers.

INTRODUCTION

The change from conventional tillage to a zero till farming system can lead to drastic changes in soil physical conditions. Decreases in plant yield have been attributed to lower soil temperatures (Griffith et al., 1973), higher bulk densities and resistance to penetration (Cannell et al., 1978; Ketcheson, 1980 ), phytotoxicity problems (Yakle and Cruse, 1983), and an increased weed population (Maurya, 1985). On the other hand, increased soil water content (Blevins et al., 1971; Hill and Blevins, 1973; Taylor et al., 1984) and better fertilizer-use efficiency (Moschler and Martens, 1975; Blevins et al., 1983; Fox and Bandel, 1986) have increased plant yields. Climate and soil type essen0167-1987/89/$03.50

© 1989 Elsevier Science Publishers B.V.

312

tially determine whether the changes in soil conditions due to zero and reduced tillage are favourable to plant growth. In cold and wet soils, corn yield (Zea mays L.) under zero till systems may be reduced (Cannell et al., 1978) while opposite results are likely to occur on well-drained soils, as well as on coarsetextured soils (Brar et al., 1983). In fine-textured soils variable results have been obtained (Ketcheson, 1980; Vyn et al., 1983 ), and testing may be required for each soil type and location. Provided that adequate yields can be maintained, zero till can be very economical since slightly lower yields are usually more than offset by the cost savings (Raghavan et al., 1980; McKyes et al., 1986). The use of manure as the main nitrogen and potassium source may further decrease the costs of production. Little information exists, however, on the use of manure in a zero till system and its effect on the plant environment and yield. The use of manure as a fertilizer in zero tilled soil may provide a heavy mulch, changing the plant environment and thus affecting plant growth. This study was designed to assess the agronomic and economic feasibility of producing silage and grain corn in Qudbec under zero and reduced tillage systems when either inorganic fertilizers or manure are used. The study began in the fall of 1981, and identical treatments have been applied yearly since 1982. This paper will report results obtained from 1983 to 1986, except for grain yield which was not available for 1983. MATERIALS AND METHODS

Experimental design A 2 × 3 factorial experiment was established on a Macdonald clay (51% clay, 36% silt, 13% sand) and a St-Benoit sandy loam (16% clay, 15% silt, 69% sand) in the fall of 1981 (Kelly et al., 1984). Previously the sandy loam site had been under grain corn and the clay in timothy hay (Phleum pratense L. ) for 5 years. Three levels of tillage and two fertilizer sources were combined in a randomized complete block design. Each t r e a t m e n t combination was replicated three times on 10 × 12 m plots in each soil. The tillage treatments were conventional, fall moldboard plowing followed by two spring diskings (C); reduced, fall chisel plowing followed by one spring disking (R); zero till, the corn being planted directly into the previous year's stubble (Z). The fertilizer treatments were inorganic, 170 kg ha -1 of N, 80 kg ha -1 of P20~ and 75 kg ha -1 of K20 (I); organic, which consisted of 40 Mg h a - 1 of dairy manure plus inorganic phosphorus, supplying 170 kg ha -1 of N and 200 kg ha -1 of K from the manure and 80 kg h a - ~of P20~ from the chemical fertilizer ( 0 ) . The inorganic nitrogen sources were ammonium nitrate for the zero till plots in order to reduce ammonia volatilization and urea for the others. Phosphorus, in the form of superphosphate, was banded in both the inorganic and the manure plots at

313

5 cm below and 5 cm beside the seeds during seeding. Muriate of potash was applied as the K source in the inorganic plots. Fertilizers and manure were incorporated by one or two diskings in the C and R plots, while they were left on the surface in the Z plots.

Weed control In 1983, all plots received the same herbicide treatment which consisted of 1.5 kg h a - 1 of atrazine (6-chloro-N-ethyl-N' - (-methylethyl)-l,3,5-triazine2,4-diamine ) and 2.5 kg h a - 1 of alachor (2-chloro-N- (2,6-diethylphenyl) -N(methoxymethyl)acetamide) before seeding, followed by two spray applications 8 days apart of 0.84 kg h a - 1of bentazon (3- ( 1-methylethyl ) - ( 1H ) -2,1,3benzothiadiazin-4 (3H) -one, 2,2-dioxide ) mixed with Citowet (a surfactant ). In addition, 2 kg h a - 1 of atrazine mixed with the surfactant Kornoil was applied to the zero till plots receiving manure (ZO plots) in order to control volunteer weed infestations (Kelly, 1985). Spot spraying of Killex, a mixture of mecoprop ( ( + ) -2- ( 4-chloro-2- methylyphenoxy ) propanoic acid ), dicamba (3,6-dichloro-2-methoxybenzoic acid) and 2,4-D ((2,4-dichlorophenoxy)acetic acid) and Roundup (glyphosate) (N- (phosphonomethyl) glycine ) was also necessary to control dandelions (Taraxacum officinale), fescue (Festuca eliator L. ) and volunteer weed infestations on these plots at the clay site, while spot spraying with Roundup was necessary on the ZO plots at the sandy loam site to control fescue, milkweed (Asclepias syriaca L.) and volunteer cereal infestations. In 1984, bentazon applications were not necessary, and the additional herbicide spraying of atrazine on the ZO plots was replaced by spot spraying with Killex and Roundup. In 1985 and 1986, alachlor was replaced by metoalachore ( 2-chloro-N- ( 2-ethyl-6-methylphenyl ) -N- ( 2 -methoxy- 1-methylethyl) acetamide) at a rate of 2.1 kg ha-1 and atrazine was decreased to 1.3 kg ha-1. Owing to a yellow-nutsedge infestation (Cyperus esculentus L. ) after corn emergence in 1985, all plots at the sandy loam site had to be sprayed with bentazon. Atrazine and alachlor or metolachlor were always applied before planting and incorporated by one disking in the C and R plots.

Seeding Seeding of Warwick (Trojan) 844 silage corn took place on 22 May 1983 (Kelly, 1985 ) and 16 May 1984, and that of 'Pride 1169' grain corn took place on 11 May 1985 and 10 May 1986 at a rate of 80 000 seeds ha -1. An International Harvester four-row conservation air planter with heavier than normal sprung frame and coulters, each made of a double-disk opener mounted on the frame, was used to seed 12 rows of corn per plot spaced at 75 cm with a 16.5cm plant spacing within the row. To ensure seeding in the harder surface layer

314

of the zero till clay plots and through the mulch or stubble, the down-pressure springs were set to their maximum force.

Field measurements

Following seeding, the number of days to 80% emergence of the seed planted and the final populations were recorded. Above-ground plant yield (hereafter referred to as silage yield) and grain yield were measured on a dry-matter basis. The middle four rows in each plot were harvested. Orthogonal comparisons on the treatment main effects were performed. When an interaction between fertilizer and tillage occurred, orthogonal comparisons were performed on treatment simple effects.

Economic analysis

A simple economic analysis was performed for both silage and grain corn production. Costs of production were calculated in 1988 Canadian dollars as outlined by McKyes et al. (1986). The units of production were chosen to be 25 ha for silage production and 80 ha for grain production. For both cases, the same fertilizer applications, herbicide applications and machinery used for these operations were chosen. Harvesting equipment was not included in the calculations as it would have been equal for all treatments. Labour was assumed to cost $8.00 h -1 and fuel was priced at $0.45 1-1 (McKyes et al., 1986). Machinery costs were calculated using the average of the prices given by five dealers in eastern Ontario, and the yearly depreciation of the machinery was calculated as outlined by the ASAE (1983). A 14% interest rate and a 10% salvage value were assumed while repair and maintenance costs were calculated using the method of Kepner et al. (1978). The working life of each tractor was assumed to be 10 000 h, that of the moldboard plow, chisel plow and disk harrow 2500 h, that of the sprayer 2000 h and that of the broadcaster and manure spreader 1200 h (Kepner et al., 1978). The average cost of herbicides per hectare for the 1983-1986 period was used for the calculations. In order to conform to normal farm practice, the cost of spot sprayings with Killex and Roundup on the ZO plots was replaced by the cost of applying 0.28 kg ha-1 of dicamba on the clay and 2 kg ha-1 of atrazine and Kornoil on the sand. Plant yields were accounted for by calculating the cost to produce 1 Mg of either silage or grain corn. This cost will be referred to as the specific cost of production (CDN$ Mg- 1).

315 R E S U L T S AND D I S C U S S I O N

Plant emergence

Plant emergence (Figs. 1 and 2 ) was not affected by any treatment in 1983. In 1984 and 1985, 80% emergence was never reached in the ZO plots at the sandy loam site. This may have resulted from poor seed placement since the planter performance was affected by the existence of a mulch, and some seeds could have been placed at the soil-manure interface especially where thick pockets of manure occurred. The burying by the planter of some of the mulch together with the seed results in a poor soil to seed contact and an inadequate moisture environment. In England, the poor establishment of direct-drilled cereals in wet seasons has been attributed to toxic substances produced by the previous year's crop residues which were incorporated with the seed during the drilling process (Lynch, 1977 ). The use of a conservation planter with coulters that could effectively clear the seeding rows of any existing trash would solve this problem. In 1985, poor emergence was also observed in the ZO plots at the clay site. A serious infestation of dandelions may have contributed to the reduction in plant emergence as up to 32% of the plot area was covered by this weed at emergence time (data not shown).

Clay site 50

40

{u

E ea

I

I

30

C} co o

20

10

/ 1983

ZI

~

1984

RI

~

CI

1986

985

~

ZO

~

RO

~

CO

Fig. 1. Number of days to 80% emergence of seed planted at the clay site. Vertical bars indicate the LSD between treatment combinations within each year.

316

Sandy

loam

site

50

\\

40

I

30 121 El

2O

0

0 121

o

i

1984

1 983

~]

ZI

~

RI

1985

[7-/]~ CI

~

70

1986

~

RO

~

CO

Fig. 2. Number of days to 80% emergence of seed planted at the sandy loam site. Vertical bars indicate the LSD between t r e a t m e n t combinations within each year.

Clay site 1O0

9o %JE

8o

E

70

I1

~o

~

5o

~

~o

~

~o

~

g

[

I

~

/\ /\ /\ /\ /\ /\ /\ /\ /\ /\ /\ /\ /\ /\ /\ /N /\

o

g o I1

o 0_

lO /]x,/lx'~ 0

,

~

\

Rt

r/-//~l ct

1986

1985

984

1983

IZ~Z] zt

\

~

zo

~

RO

~

CO

Fig. 3. Plant populations at the clay site. Vertical bars indicate the LSD between treatment combinations within each year.

317

Plant population As a result of the poor emergence of the corn planted in the ZO plots, significantly lower plant populations occurred for this t r e a t m e n t as compared to all others in 1985 at the clay site (Fig. 3) and in 1984 and 1985 at the sandy loam site (Fig. 4 ). Also plant population on the ZI plots tended to be lower than that on RI and CI plots. Although 80% of the seeds emerged on the ZO plots at the sandy loam site in 1986, the subsequent plant population on these plots was only about 25% of the number of seeds planted. A severe June frost damaged 73% of the young ZO corn plants but only 31% on the other treatments. The manure mulch may have acted as a barrier to overnight soil radiation, leaving a colder layer of air just above the soil.

Silage yield When inorganic fertilizers were used, tillage did not significantly affect silage yields (Figs. 5 and 6). W h e n averaged over the 4 years, the RI t r e a t m e n t resulted in the highest dry-matter production, followed by ZI. Differences were, however, small and not significant. W h e n manure was used as a source of fertilizer, reduced tillage usually resulted in a yield decrease. Over a period of 4 SGndy

loom

site

100

[

90

%JE

BO

c

70

[ 7~7;

o

\%

TI

g

60

o

g

/ /

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50

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S

0 1983

ZI

IN-'K]

1984

RI

r///,l

o

198!

kN--Nx~ z o

IXX]

1986

RO

~g~

CO

Fig. 4. Plant populations at the sandy loam site. Vertical bars indicate the LSD between treatment combinations within each year.

318

Clay site 22 20 18

16 \

JE

\ \ \ \ x \ \ \ \ \ \ \ \ \ \ \ \

14 12

(o •~

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10

8 6

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/\

/ N ,.4, /N / N ,/, / \ ".4,

RI

/ / \~

/\ /\ /\ /\ /\

¢,, 1984

1983

[771 zl

7-~

gz/-A cJ

198.5

kN-N~ zo

IX~

1986

RO

~

co

Fig. 5. Mean values of silage yield (on a dry-matter basis) at the clay site. Vertical bars indicate the LSD between treatment combinations within each year.

Sandy loam site 22 20

la 16

7V, /\ /\

14 12

/ /\ /\ /\ /\ /\ /\ /\

l:J

10

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8 6

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1983

1771 zt

G

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1984

RI

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1986

1985

kN-N~ z o

G2~] a o

~

co

Fig. 6. Mean values of silage yield (on a dry-matter basis) at the sandy loam site. Vertical bars indicate the LSD between treatment combinations within each year.

319 TABLE 1 M e a n c o r n p l a n t m a s s (g, on a d r y - m a t t e r b a s i s ) a t h a r v e s t C l a y site

Tillage Z R C Fertilizer I O

Sandy loam site ~

1983

1984

1985

1986

1983

1985

1986

167 ~ 174 a 167 a

194 ~ 205 a 211 ~

2235` 218 ~ 214 a

217 ~ 205 ~ 194 ~

140 a 151a 152 a

181 a 171 a 162 a

209 " 20(}~ 184 a

172 a 167 a

224 a 183 h

238 ~ 199 b

204 ~ 207 ~

20F 194 ~

164 a 179 a

201" 194 ~'

~For 1984 v a l u e s were: ZI, 222; ZO, 335; RI, 230; RO, 212; CI, 209; CO, 212. T h e r e w a s s i g n i f i c a n t i n t e r a c t i o n a t t h e 5% level. Z I - Z O , Z O - R O , Z O - C O are d i f f e r e n t u s i n g o r t h o g o n a l c o m p a r i s o n s on t r e a t m e n t s i m p l e effects. W i t h i n e a c h c o l u m n a n d t r e a t m e n t , m e a n s w h i c h do n o t h a v e c o m m o n s u p e r s c r i p t differ s t a t i s t i c a l l y a t t h e 5% level.

years, total silage yields on the ZO plots averaged 81 and 74% of the average yield at the clay and sandy loam sites, respectively. The exceptionally low yield of the ZO treatment at the sandy loam site in 1986 was mainly due to the low plant population caused by the late June frost. At the clay site the manured plants yielded significantly less dry matter than the inorganically fertilized plants in 1984 and 1985. Individual plant weights {dry-matter basis) were also significantly lower for the manure treatments in 1984 and 1985 (Table 1) and they were strongly correlated with silage yield (r= 0.86, P--0.0001 ). Manure was probably not as efficient as inorganic fertilizers in supplying nutrients to the plants. Plant population was the second factor affecting silage yield (r= 0.49, P= 0.0001 ). At the sandy loam site, a significant interaction between tillage and fertilizer occurred in 1984 and 1986. In 1984, ZO and RO yielded significantly less silage than ZI and RI, respectively. In 1986, the difference in yield due to fertilizer source was only significant on the Z plots. At both sites, the difference in yield due to fertilizer source was minimal when the soil was tilled conventionally. Individual plant weights were not affected by fertilizer treatment and they were not correlated with silage yield. Manure was as effective as the inorganic fertilizers in supplying nutrients and the variation in silage yield was mainly due to variation in plant population (r--0.51, P= 0.0001 ).

Grain yield Although the grain yield patterns were similar to those of silage yield, treatment effects were more significant. At the clay site in 1984, a significant inter-

320

Clay site 10

[

7 .E

6 5 4 L 3

1

0

i

i

[P'7l

zi

~

1986

1985

1984

RI

~

cI

~

zo

[~

RO

~

CO

Fig. 7. Mean values of grain yield (on a dry-matter basis) at the clay site. Vertical bars indicate the LSD between treatment combinations within each year.

S a n d y l o a m site 10

I .c

I

i

6F

v lo .E

E

2

\" 0

I

"/

\

i

z,

~

986

1985

lg84

RI

17-/-A cl

k~

zo

[K:D RO

~

CO

Fig. 8. Mean values of grain yield (on a dry-matter basis) at the sandy loam site. Vertical bars indicate the LSD between treatment combinations within each year.

321 TABLE 2 Monthly rainfall for the 1983-1986 growing seasons reported at the Macdonald College weather station Month

Rainfall (mm) 1983

1984

1985

1986

30-year average

May June July August September

127 33 66 48 82

113 89 6 141 29

40 110 63 78 68

82 129 134 131 121

66 82 90 92 88

Total

356

426

357

597

418

action between fertilizer and tillage occurred and grain yield was significantly higher for ZI as compared to RI and CI and for CO as compared with ZO and RO (Fig. 7). In addition, ZI and RI yielded significantly more grain than ZO and RO, respectively. The same trends occurred in 1985, and in 1986 differences in grain yield were small and not significant. Differences due to treatment were generally not significant at the sandy loam site in 1984 and 1985, but in 1986 a significant interaction between fertilizer and tillage occurred, resulting in the following significant yield trends: ZI < RI, ZI < CI, ZO < RO, ZO < CO, ZO < ZI (Fig. 8). The smaller grain yields on the ZI plots in 1986 as compared with RI and CI could have been due to the plant mulch left on the Z plots for the first time and therefore possibly lower temperatures in these plots (Burrows and Larson, 1962; Allmaras et al., 1964; Griffith et al., 1973). It is, however, difficult to draw a definite conclusion because of the exceptionally rainy summer season of 1986 (Table 2 ). No till has been reported to yield more t h a n conventional tillage in a year drier than normal, but less than conventional tillage in a wetter year (Eckert, 1984). In this experiment, such a trend was not observed until the exceptionally wet summer of 1986.

Economic analysis of the six production systems Within each fertilizer treatment, zero till resulted in the lowest cost per hectare (Table 3). The higher herbicide consumption required by the ZO system did not reverse this trend. However, satisfactory weed control was not always achieved for this t r e a t m e n t and herbicide costs may well be underestimated. The two major sources of difference in cost per hectare were ascribed to machinery depreciation and fertilizer type. The maximum difference in cost per hectare due to machinery depreciation was between ZI ($158 ha -1) which re-

322 TABLE 3 Component and total costs of production of silage or grain corn per hectare for the tillage and fertilizationsystems (CDN$ (1988) ha- 1 ) Production Fert. Herb. Labour Fuel Machine Silage Grain system repair Machine Total Machine Total deprec, deprec. ZI RI CI ZO RO CO

2431 223 223 85 85 85

49 49 49 66 49 49

16 28 29 27 32 33

12 38 43 22 44 50

21 37 44 35 45 53

158 302 332 236 370 400

500 676 720 470 625 670

58 113 123 88 137 147

399 487 511 322 392 417

~Highercost because ammoniumnitrate was used instead of urea. quired no investment in cultivation implements and CO ($400 ha -1) which, in addition to the moldboard plow and the disk harrow, also required investment in manure-spreading equipment. For the 80-ha grain-corn unit of production, the machinery depreciation values were $58 h a - 1for ZI and $147 h a - 1 for CO. Fertilizer type accounted for a $138-158 ha-~ difference in cost, and was the most important source of difference for the grain production unit and the second most important source of difference for the silage production unit. In comparison, m a x i m u m differences between any two treatments due to herbicide, labour, fuel, or machinery repair were 17, 17, 38 and $30, respectively. W h e n an inorganic fertilizer was used, zero till resulted in the smallest specific cost of production (Table 4 ) and conventional tillage in the highest. W h e n manure was used, results were more variable. Production costs were usually lower for CO t h a n for CI. No trend was noted between RI and RO. The small specific costs calculated for the ZI system were due simultaneously to the good yields obtained with this system and the relatively low investment per hectare. The better efficiency of money invested per Mg of grain or silage grown under the ZO system was apparent only in some years. To make this system competitive would require additional amounts of land to produce the same a m o u n t of silage or grain as with the other techniques. Global costs of production would then rise since fixed land costs are around $67 ha-~ (Conseil des Productions V~g~tales due Quebec, 1985). The lower production costs of CO as compared to CI were mainly due to the lower cost of fertilizer, since yields between these two treatments did not differ much. The possibility of producing large areas of grain using manure is debatable because of the impracticality of applying 40 Mg h a - 1 of manure to 80 ha every year, especially when dealing with cash-crop farms having no animals. An al-

323 TABLE 4 Specific costs of producing i Mg of silage or of grain corn (CDN$ (1988) Mg- ' of corn ) Production system Clay soil ZI RI CI ZO RO CO Sandy loamsoil ZI RI CI ZO RO CO

Silage

Grain

1983

1984

1985

1 9 8 6 Average

1984

1985

1 9 8 6 Average

43 57 61 44 54 54

31 43 45 41 48 46

28 39 46 52 49 44

35 43 48 35 41 44

34 45 50 43 48 47

59 88 90 72 79 66

58 74 101 75 71 64

74 86 90 62 71 81

64 83 94 70 74 70

52 63 72 49 56 59

32 43 49 45 46 47

38 57 59 41 48 53

39 46 54 94 51 49

40 52 58 57 50 52

67 83 91 65 75 75

69 85 99 61 69 76

102 97 109 310 76 85

79 88 100 145 73 79

t e r n a t i v e would be to c o n s i d e r a r o t a t i o n of m a n u r e a p p l i c a t i o n on 2 0 - 2 5 h a p r o v i d e d t h a t a source was available. CONCLUSIONS G o o d c o r n yields a n d low p r o d u c t i o n costs u s i n g t h e zero till t e c h n i q u e c a n be o b t a i n e d w i t h i n o r g a n i c fertilizers in s o u t h e r n Quebec, despite a p o s s i b l y r e d u c e d p l a n t p o p u l a t i o n . H e r b i c i d e a p p l i c a t i o n s s i m i l a r to t h o s e r e q u i r e d b y c o n v e n t i o n a l l y tilled soil w e r e u s u a l l y sufficient to c o n t r o l weeds. H o w e v e r , a special seeder w i t h a h e a v y f r a m e a n d h e a v y - d u t y c o u l t e r s is required. W h e n m a n u r e is u s e d as a source of fertilizer, zero till c a n result in p o o r e r e m e r g e n c e , weed c o n t r o l p r o b l e m s a n d i n c r e a s e d risks of f r o s t d a m a g e . Clearing t h e t r a s h f r o m t h e seeding rows would p r o b a b l y e l i m i n a t e s o m e of t h e s e p r o b l e m s . H o w e v e r , b e f o r e u s i n g such a t e c h n i q u e for c o r n p r o d u c t i o n , it is r e c o m m e n d e d t h a t t e s t s are m a d e for e m e r g e n c e p r o b l e m s on a s m a l l a r e a for a few years. I t is also r e c o m m e n d e d t h a t zero-till s y s t e m s be a v o i d e d w h e n a source of w e e d c o n t a m i n a t i o n is n e a r b y , a n d to e n s u r e t h a t a p r o p e r herbicide c o m b i n a t i o n is selected. I f t h e weeds c a n be c o n t r o l l e d satisfactorily, t h e n a zero till s y s t e m w i t h a p p l i e d m a n u r e c a n be a n e c o n o m i c a l t e c h n i q u e to use. W e e d p r o b l e m s t e n d to i n c r e a s e w i t h t h e n u m b e r of y e a r s of zero till, a n d p l o w i n g t h e soil e v e r y few y e a r s or u s i n g a c r o p r o t a t i o n w o u l d be helpful in r e d u c i n g t h e use of herbicides. I t m u s t be k e p t in m i n d also t h a t t h e p r e s e n c e of a m a n u r e m u l c h i n c r e a s e s t h e risk of f r o s t d a m a g e to seedlings.

324 AC KNOWLEDGEMENTS This study was undertaken with the support of the Natural Sciences and Engineering Research Council of Canada (Grant A7491) and of the Quebec Ministry of Agriculture (Grant McA-85-C-1177).

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