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III. Agronomic Impacts of Crop Rotation

III. Agronomic Impacts of Crop Rotation

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D. L. KARLEN ET AL.



12



that corn, grown in a 2-year rotation with soybean, yields 5 to 20% more than

monoculture corn have been published (Strickling, 1950; Welch, 1976; Kurtz et

al., 1984; Voss and Shrader, 1984; Peterson and Varvel, 1989c; Crookston et al.,

1991). Data from a 15-year study in Iowa (Table 11) show the typical response.

Crookston et al. (1991) reported that annually rotated corn yielded 10% more

than continuous corn, and that first-year corn, following 5 years of soybean,

yielded 15% more than continuous corn. Based on these results, they suggested

Minnesota farmers consider using longer crop rotations. However, yield response to rotations greater than 2 years may (Crookston et al., 1991) or may not

(Lund et al., 1993) occur.

Increased emphasis on crop residue management to reduce soil erosion may

also encourage crop rotations because they can largely eliminate corn yield

decrease observed between no-tillage and conventional tillage production practices (Karlen et al., 1991). This response is particularly evident on poorly

drained soils (Dick et al., 1991). Furthermore, because many cropping systems

have a small profit margin, a 5% yield increase for corn may result in a 50%

profit increase (Crookston, 1984).



Table I1

Crop Rotation Effect on Corn Grain Yield



in Northeast Iowa

Year



Continuous



Rotation



Mg ha-l

1978

1979

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992



8.0

9.5

9.5

9.9

7.5

5.3

5.5

7.6

10.2

8.1

5 .O

6.6

10.6

8.3

9.1



9.4

10.2

9.7

10.4

7.7

6.8

7.3

8.8

10.8

9.0

6.3

8.0



8.0



9.0



11.2

9.4

10.0



LSD(O.05) = 0.3



cv = 4.3%



15-year average

LSD(O.05) = 0.2



CROP ROTATIONS FOR THE 2 1st CENTURY



13



Crop yield increases due to rotation are not limited to corn. Grain sorghum in

rotation with soybean (Brawand and Hossner, 1976; Clegg, 1982; Gakale and

Clegg, 1987; Peterson and Varvel, 1989b; Roder et al., 1988; Langdale et al.,

1990) or corn (Robinson, 1966) showed increased yield compared to continuous

grain sorghum. Soybean yield also increased when grown in a rotation with corn,

grain sorghum, or simply following a fallow period (Crookston, 1984; Dabney er

al., 1988; Peterson and Varvel, 1989a).



B. WATERUSEEFFICIENCY

The need to develop more water-efficient crop management practices may be

one of the strongest incentives for adopting crop rotations. Crops should be

managed in a rotation sequence so that complementary root systems fully exploit

available water and nutrients (Karlen and Sharpley, 1994). Sadler and Turner

( 1994) suggested “opportunistic cropping” as a means for increasing agricultural

sustainability through water conservation or by increasing productivity from

applied water. Opportunistic cropping is not crop rotation in the typical sense,

but this management practice requires farmers to remain sufficiently flexible to

adapt their farming practices to utilize rainfall and/or irrigation water as efficiently as possible. Therefore, opportunities to rotate crops spatially and temporally may become increasingly important.

Roder er al. (1989) evaluated yield and soil water relationships for a sorghum

and soybean cropping system. They found that crop rotation increased soybean

yield, but that nitrogen fertilization did not. The soybean yield advantage from

rotation decreased as the amount of spring rainfall increased.

Increasing temporal and spatial diversity by using different crop rotations may

mimic natural ecosystems more closely than current farming practices. This

change may lead to increased agricultural sustainability (Karlen et al., 1992).

One example is in semiarid areas where saline seeps began to develop about 30

years after cultivation began, and especially after about 10 years of an alternateyear, crop-fallow rotation (Ferguson er d.,1972). Formation of saline seeps

gradually became a problem as production agriculture disrupted annual crop

growth associated with native plant communities in semiarid regions. Ferguson

and Bateridge (1982) found that 50 years of crop-fallow farming significantly

reduced soluble salt content of some soils. Although this was beneficial from an

edaphic perspective, they found that up to 90 Mg ha-1 of salt was moved toward

the water table where it resulted in groundwater salinization and became a source

of salts for saline seeps.

Undoubtedly, some water moves below the root zone of native vegetation, but

the quantity is not large. Native vegetation is diverse with varying growth habits

and rooting depths. Therefore, most precipitation infiltrating the sod is transpired



14



D. L. KARLEN ETAL.



(Ferguson et al., 1972; Halvorson and Black, 1974). With cultivation during

periods of above-normal annual precipitation, and with improved soil water

storage and conservation during fallow, increased use of summer fallow enhances percolation of water below the root zone and thus contributes to formation

of saline seeps (Halvorson and Reule, 1976). By using flexible crop rotations

involving small grains, grasses, deep-rooted crops, and a minimum amount of

summer fallow, soil water loss by deep percolation could be prevented and

development of saline seeps could be alleviated (Halverson and Black, 1974).



C. NUTRIENT

USEEFFICIENCY

1. Nitrogen



Increased use of crop rotations may be mandated to improve nutrient use

efficiencies and reduce losses of nitrogen to surface and groundwater resources.

Crop rotation per se is important, but the sequence with which crops are grown

may be more important (Carter et al., 1991; Carter and Berg, 1991). Karlen and

Sharpley ( 1994) reviewed several studies showing how crop sequence could

influence nitrogen movement through the soil profile and ultimately into groundwater resources. Several studies showed that soybean and alfalfa, which do not

require supplemental nitrogen inputs, can effectively use or “scavenge” residual

nitrogen remaining in the soil from previous crops (Johnson et al., 1975; Mathers et al., 1975; Muir et al., 1976; Olson er al., 1970; Stewart et al., 1968).

Alfalfa roots may grow to depths greater than 5.5 m in some soils, and

research has shown that nitrate can be utilized by the crop from any depth where

soil solution is extracted by plant roots. Mathers et al. ( I 975) reported that alfalfa

removed nitrate from the soil profile at a depth of 1.8 m during the first year of

establishment and to a depth of 3.6 m during the second and third years. Olson et

al. (1970) found that crop rotation reduced soil solution nitrate concentrations at

a depth of 1.2 to 1.5 m by 34 to 82% compared to continuous corn. They found

that the decrease in solution nitrate was directly proportional to the number of

years in oats, meadow, or alfalfa production, and attributed this to combined

recovery of nitrate by shallow-rooted oat crops followed by deep-rooted alfalfa

crops.

Soybean can also effectively scavenge residual soil N (Johnson et al., 1975;

Havlin et al., 1990; Karlen e t a l . , 1991), but in Wisconsin, soybeans were not as

effective as alfalfa because of their more shallow rooting depth (Jackson et al.,

1987). This finding was supported by Olson et al. (1970), who also concluded

that recovery of subsoil nitrates by deep-rooted legumes such as alfalfa will

probably be more effective on medium and heavy textured soils than on sands.

One of the persistent nutrient management questions associated with crop



CROP ROTATIONS FOR THE 2 1s t CENTURY



1s



rotation is whether the nitrogen contribution from legume fixation is responsible

for much, if not all, of the beneficial rotation effect. Bullock (1992) reviewed

several studies focusing on the fertilizer replacement value as the method for

assessing nitrogen contributions from legumes grown in rotation with nonlegume

crops such as corn or grain sorghum. He reported that this method overestimated

the nitrogen contribution by legumes and underestimated the rotation effect. For

example, soybean is given a fertilizer replacement value of 25 to 40 kg ha-1 in

many midwestern states. The actual nitrogen contribution by the soybean crop is

often much less or even negative. In the midwestern United States, soybean in a

2-year corn and soybean rotation may acquire only 40% of its nitrogen from

dinitrogen fixation, while the remaining 60% is taken up from the soil (Heichel,

1987). When the grain is harvested and removed, there is an estimated net loss of

84 kg N ha-1 due to the large nitrogen content of soybean grain: The nitrogen

contribution from alfalfa in rotation with maize in the upper midwestern United

States is also less than suggested by fertilizer replacement value methodology.

Fertilizer recommendations for corn following alfalfa in most midwestern states

credit the alfalfa crop with a nitrogen contribution of 100 to 125 kg N ha-'

(Bruulsema and Christie, 1987; Fox and Piekielek, 1988) based on fertilizer

replacement methodology. However, the actual contribution measured with '5N

methodology was only 24 kg N ha-1 (Harris and Hesterman, 1990). Based on

these studies, Bullock (1992) concluded that rotation with legumes does not

provide as much nitrogen as fertilizer replacement methodology estimates and

that much of the yield benefit which has been credited to nitrogen contribution is

actually due to other factors.

Jensen and Haahr (1990) also concluded that with winter cereals, the rotation

effect of pea was probably more important than the residual nitrogen effect. For

winter oilseed rape (Brussicu nupus L.), the residual nitrogen effect from pea was

equivalent to 30 to 60 kg N ha-1 if applied following oats. Removal of the

above-ground pea residues, which contained less than 1% nitrogen, had no effect

on the residual nitrogen value.



2. Phosphorus, Potassium, and Other Nutrients

There is very little direct evidence that crop rotation affects phosphorus relationships (Bullock, 1992). Karlen and Sharpley (1994) concurred, but suggested

that appropriate selection and use of a crop with a higher affinity for phosphorus

may reduce soil phosphorus stratification and increase phosphorus-use efficiency,

particularly if the nonharvested portion of the crop is returned to the soil. They

suggested that selection of crops which can more efficiently utilize residual soil

inorganic and organic phosphorus may be economically viable for farmers and

enhance the sustainability of soil phosphorus fertility.

Vivekanandan and Fixen (1991) reported that corn sampled at the six-leaf



16



D. L. KARLEN ET AL.



growth stage had a higher phosphorus concentration when following soybean

than when following corn. Similarly, Copeland and Crookston (1992) observed

that corn in a 2-year rotation with soybean accumulated significantly more phosphorus than did corn in continuous monoculture. This suggested that corn yield

increases associated with crop rotation may have been due to improved general

plant nutrition. However, in the same study, total phosphorus content of soybean

was not affected by crop rotation except for the very early vegetative stages.

Similarly, there was no consistent increase in leaf phosphorus concentration

when sorghum was grown in rotation with cotton (Brawand and Hossner, 1976).

They concluded that although there was a rotation effect, it could not be attributed to improved phosphorus nutrition.

Copeland and Crookston (1992) reported that K and total micronutrient content increased for corn in a 2-year rotation with soybean as compared to continuous monoculture. They proposed that general improvement in plant nutrition

may have been due to an improvement in corn root function and that causal

agents such as mycorrhizae may have played a role. Following a similar argument, the same research group (Copeland et al., 1993) reported that increased

water use by first-year rotated corn or increased water use efficiency of rotated

soybean, as compared to continuous monoculture, demonstrated that rotation

increased root surface and/or root activity which in turn improved water relations

and increased grain yield. Inclusion of legume cover crops into a crop rotation in

the southeastern United States also resulted in a beneficial redistribution of

potassium to the soil surface from deeper in the soil profile (Hargrove, 1986).

Extractable calcium and magnesium levels do not appear to be affected by crop

rotation. Increased availability of micronutrients including iron, copper, and zinc

because of microbiologically enhanced chelation may also be a beneficial effect

of crop rotation and cover crops (King, 1990).



D. DISFASE

AND PESTINTERACTIONS

Crop rotation is a fundamental tool of integrated pest management. Francis et

al. (1986a) coined the term “biological structuring” to describe the use of crop



rotations, management alternatives, biological phenomena, environmental conditions, and interactions of these factors to manage crop pests such as disease,

weeds, and insects. Crop rotation affects pest pressure in various ways, but in

general the literature supports Francis and Clegg (1990) who stated that “the

greater the differences between crops in a rotation sequence, the better cultural

control of pests can be expected.” While crop rotation does reduce pest pressure

it should be noted that even when pest pressure is minimal, the rotational effect

still exists. This suggests that pest control is a contributor to the benefit of crop

rotation, but is not responsible for the rotation effect itself. However, it should be



CROP ROTATIONS FOR THE 2 1st CENTURY



17



recognized that we are unaware of all pests which detrimentally affect crops and

thus it can be hypothesized that much of the rotational effect is due to alleviation

of unknown pests.

Crop rotation is an effective tool against certain pests, and efficacy may contribute to the rotation effect, but rotation does not control all pests (Bullock,

1992). Pests which are controlled by crop rotation have the following characteristics (Flint and Roberts, 1988). First, the pest inoculum source must be from the

field itself. Crop rotation does not control highly mobile pests since they have the

ability to invade from adjacent fields or other areas. Pests which can be controlled by rotation include soil and root-dwelling nematodes, soilborne pathogens

(if they do not produce airborne spores), and vegetatively propagated weeds such

as nutsedge (Cyperus). Second, the host range of the pest needs to be fairly

narrow or at least must not include plants which are reasonably common in a

given area. Third, the pest must be incapable of surviving long periods without a

living host. In other words, the pest populations must decrease substantially

within a year or two of removing a living host plant.

1. Weeds



Weeds can reduce crop yields provided their densities reach a biological

threshold. Most research indicates that biological thresholds are greater than zero

(Aldrich, 1987), but there are scattered arguments in the literature that biological

thresholds are zero (Cousens, 1985).

A combination of crop rotation, smother crops, and mechanical cultivation

were used to control weeds prior to the introduction of the synthetic herbicide

2,4-D [(2,4-dichlorophenoxy)acetic acid]. Crop rotation alone was not sufficient

to control weeds; all three methods had to be used as an integrated program with

a primary goal of preventing weed reproduction (Regnier and Janke, 1990).

Crop rotation helps control weeds because they thrive and increase in crops

which have similar growth requirements to their own. For example, grasses

thrive in continuous corn, while broadleaf weeds thrive in continuous soybean.

In Nebraska, rotating corn and grain sorghum with a broadleaf crop is an effective method of controlling shattercane (Sorghum bicolor L.) because it allows for

the use of herbicides which are phytoactive on cereals (Francis and Clegg, 1990).

Similarly, Dale and Chandler (1979) reported that a corn and cotton rotation

enabled growers to control johnsongrass (Sorghum halepense L.) much better

than a continuous corn rotation because grass-specific herbicides could be used

during the cotton phase of the rotation.

Crop rotation introduces conditions and practices that are not favorable for a

specific weed species and thus growth and reproduction of that species are

hampered. For example, Forcella and Lindstrom (1988) found 25 weed seeds

m-2 in a continuous corn field, but only 4 weed seeds m-2 where corn was



18



D. L.KARLEN ET AL.



grown in rotation with soybean. Not all crops are equal in their competitiveness

with weeds. Van Heemst (1985) ranked 25 crops for their ability to compete with

weeds based on a mean reduction in yield. Wheat was considered the most

competitive and given a rank of first. Soybean ranked fourth and corn seventh.

Regnier and Janke (1990) suggested that factors such as rate and extent of canopy

development, plant spacing, and life cycle all contributed to a crop’s competitiveness. It has also been noted that cultivars within a species also compete

differently with weeds. Bullock (1992) cited several references suggesting this

difference may be attributable to production of allopathic compounds, especially

if small grains such as rye, wheat, oats, or barley are included in the rotation.

The importance of crop rotation was diminished with the advent of synthetic

herbicides. However, there is ample evidence confirming that crop rotation irnproves weed control even with synthetic herbicides (Bullock, 1992). For example, after 7 to 8 years of standard chemical and mechanical weed control from

1500 to 3000 weed seed m-2 were found with continuous corn, while in a cornsoybean rotation, the soil had from 200 to 700 weed seed m-2 (Forcella and

Lindstrom, 1988). Withholding herbicides for 1 year reduced continuous corn

yield by 10 to 27% but did not reduce corn yield in the 2-year corn and soybean

rotation.

Interest in using crop rotation to control weeds is gaining popularity, especially

among those persons focusing on sustainable agriculture. For example, compared to growing continuous corn, growing corn in a 2-year soybean and corn

rotation or a 3-year soybean, wheat, and corn rotation reduced giant foxtail

(Seturiafuberi Herrm.) seed at the 0- to 2 . 5 , 2.5- to lo-, and 10- to 20-cm

depths (Schreiber, 1992). Similarly, Ball ( 1992) reported that cropping sequences

were the most dominant factor influencing species composition in weed seed

banks.

Temporal diversity achieved through crop rotation and spatial diversity

achieved through intercropping can markedly reduce weed population density

and biomass production (Liebman and Dyck, 1993). Among 26 comparisons

between monoculture and rotation cropping systems, they found that emerged

weed densities with rotations were lower in 21 studies, higher in 1 study, and

equivalent in 5 studies. They concluded that the success of rotation systems for

weed suppression appears to be based on the use of crop sequences that create

varying patterns of resource competition, allelopathic interference, soil disturbance, and mechanical damage to provide an unstable or inhospitable environment that prevents the proliferation of a particular weed species. Liebman and

Dyck (1993) concluded that the relative importance and most effective combinations of various weed control tactics have not been adequately evaluated and

recommended, therefore three research thrusts should be addressed. These included (1) determining effects of crop rotation and intercropping on weed population dynamics including weed seed longevity, weed seedling emergence, weed



CROP ROTATIONS FOR THE 2 1st CENTURY



19



seed production and dormancy, agents of weed mortality, resource competition

between cultivated crops and weeds, and allelopathic effects; (2) determining

how to combine specific components of rotation and intercropping strategies that

may be important for weed control; and (3) designing and testing new integrated

approaches for weed control at the scale of complex farming systems.



2. Insects

Insect pests which have specific or at least narrow host ranges and which are

incapable of extended migration are particularly susceptible to crop rotation

(Ware, 1980). An example of this is the control of northern corn rootworm

(Diabrotica sp.) in the central United States. In a monoculture corn production

system, rootworm reaches an economic threshold about 30% of the time, but in a

2-year corn and soybean rotation, the economic threshold is reached less than 1%

of the time. However, increased use of a 2-year corn and soybean rotation

throughout much of the northern corn belt has resulted in selection for corn

rootworms with a 2-year rather than a 1-year diapause. Therefore, reports exist

of economic damage for corn grown in a short, 2-year rotation (Ostlie, 1987).

For some insect species, crop rotation is not an effective control practice. For

example, Johnson et al. (1984) reported that black cutworms (Agrotis ipsilon) are

more of a problem when corn is rotated with either soybean or wheat than when it

is grown continuously. Apparently, black cutworm moths are less attracted to

corn residue than to either soybean or wheat residues for oviposition (Busching

and Turpin, 1976).



3. Diseases

Crookston (1 984) suggested that decreased crop yields associated with monoculture cropping systems were caused by increases in some unknown soil pathogen. Although attractive and frequently used to justify crop rotation as a method

for preventing fungal diseases (Curl, 1963), Crookston’s suggestion is not universally accepted (Roder et a f . , 1988). It is not necessarily clear to what extent

disease prevention contributes to the rotation effect. Bullock (1992) found that

monoculture wheat has problems with fungal diseases, in particular take-all

(Gaeumannomyces graminis var. tritici), but the severity of fungal diseases in

continuous wheat often decreases within 3 to 5 years. The reduction in severity is

known as “take-all decline,” and as a natural control of the disease it is effective.

The mechanisms responsible for take-all decline are not completely understood,

but changes occur in the microflora and microfauna in soils where take-all fungus

is established. Part of the take-all decline is due to a build up of competitive and

predatory microorganisms which control the take-all fungus (Crookston, 1984).

Curl (1963) suggested that in some cases the control mechanism may be a pest-



20



D. L. KARLEN ET AL.



predator type of relationship, while in others, the organisms are simply competing for limited resources. Crookston et al. (1991) postulated that a buildup of

beneficial organisms which help to control detrimental organisms might explain

why second-year yields for a continuous corn cropping system often show a

greater decline compared to yields of rotated corn than that observed during the

later years. Meese et al. (1991) reported that withholding corn for 1 year is

sufficient to obtain the maximum rotation effect, but Crookston et al. (1991)

reported that withholding corn for more than 1 year would increase the rotation

effect slightly. Both reported that soybean requires more than 1 year of absence to

negate deleterious soybean yield effects. The exact nature of the agent responsible for the deleterious effect of continuous soybean is not clear, but Whiting and

Crookston (1993), working in the northern U.S. corn belt, have reported that

plant diseases are not playing a major role. Thus, it is reasonable to conclude that

the yield increase observed for soybean in a soybean and corn rotation is not

necessarily due to a reduction in seventy or incidence of plant diseases.

Time does not decrease the seventy of all diseases (Bullock, 1992). Studies by

Stromberg (1986) showed that gray leaf spot in corn (Cercosport zeae-maydis) in

the southeastern United States becomes a severe problem if corn is grown continuously using no-tillage production practices. However, a rotation in which corn

is absent for at least 1 year prevents the disease from becoming an economic

problem.



4. Nematodes

The use of crop rotations to control Meloidogyne and Heteroderu glycines

species of plant parasitic nematodes on tobacco and soybean crops in North

Carolina was established during the 1950s and 1960s (Barker, 1991). This approach for control is currently increasing in importance once again because many

chemical nematicides are no longer available (Flint and Roberts, 1988). Negative

impacts of nematodes on crop production are decreased by crop rotation because

changing plant species generally reduces population levels of most plant parasitic

nematodes (Dabney e f ul., 1988; Ferris, 1967; Edwards et al., 1988).

A reduction of nematode pressure may account for most of the rotation benefit

for soybean in the southeastern United States since cyst nematodes (Heteroderu

glycines Inchinohye) in soybean can generally be controlled by crop rotation

(Dabney et al., 1988). Bailey et al. (1978) reported similar conclusions with

regard to root knot nematodes. Sasser and Uzzell (1991) reported that soybean

yields were improved most by increasing the number of years during which a

nonhost crop was grown.

In other nematode studies, a 1-year rotation with barley (Carter and Nieto,

1975), clean fallow (King and Hope, 1934), or planting a resistant processing

tomato cultivar (Flint and Roberts, 1988) were effective in controlling the cotton

root knot nematode (Meloidogyne incognita).



CROP ROTATIONS FOR THE 2 1st CENTURY



21



The economic impact of 3-year cotton and soybean rotations in soils with

varying population densities of Hoplolairnus Columbus was estimated by Noe et

al. (1991). They calculated maximum yield losses to be 20% for cotton and 42%

for soybean. Maximum nematode population densities at harvest were estimated

to be 182 per 100 cm-3 of soil for cotton and 149 per 100 ~ m for

- soybean.

~

They projected net incomes to range from a loss of $17.74 ha-1 for a soybean,

cotton, and soybean rotation to a profit of $46.80 ha-' for a cotton, soybean, and

cotton sequence. A range of economic assumptions and management conditions

are considered in this study.

Crop rotation research was de-emphasized in the early 1960s as priorities

shifted to the development of resistant cultivars and the evaluation of nematicides

(Schmitt, 1991). Resistant soybean lines (Brim and Ross, 1965, 1966)performed

well in cyst-infested fields and gave the maximum yield potential for the environment in which they were grown. As these and other nematode resistant cultivars

became widely grown, often in monoculture, nematode races shifted and, consequently, the use of resistant cultivars as a management tool is now limited.

Schmitt (1991) reported that following 2 or more years of a nonhost crop,

nematode populations were at low or undetectable levels and that soybean yields

were not affected. Those results suggested that a more prudent use of resistant

cultivars grown in rotation with nonhost crops would increase their longevity in

fields infested with cyst nematode races 1, 3, or 4.



E, ALLELOPATHY

Allelopathy occurs when one plant species releases chemical compounds,

either directly or indirectly through microbial decomposition of residues, that

affect another plant species. Liebman and Dyck (1993) stated that including

allelopathic plants in a crop rotation or as part of an intercropping system may

provide a nonherbicide mechanism for weed control. They found few studies that

focused on use of allelopathy in rotations, but management of allelopathic cover

crops for weed control has been extensively investigated (Bullock, 1992). Results of those studies are directly applicable to crop rotations.

Liebman and Dyck (1993) found that exudation of allelochemicals from living

roots of barley and oats have been suggested, but most studies of allelopathy

have been conducted with cover crops or dead crop residues associated with notillage production practices. Studies such as those by Yakle and Cruse (1983,

1984) are typical of those efforts to understand the effects of this complex

process, especially for monoculture corn production. With respect to weed suppression, Putman er al. (1983) found that compared to unplanted control treatments, residues of several fall-planted cereal and grass cover crops significantly

reduced growth and dry matter production by several weed species during the

following summer. Rye, wheat, and barley, which survived the winter and were



22



D. L. KARLEN ET AL.



subsequently killed by nonselective herbicides, had greater a~l~lopathic

effects

and suppressed weeds much more than oats, grain sorghum, or sorghum-sudan

grass (Sorghum urundinaceum [Desv.] Stapf var. sudunense [Stapf] Hitchc.),

which were killed by winter conditions. Shading and cooling of the soil may

have contributed to this control, but several other studies have shown suppressive

effects that cannot be attributed to the physical presence of mulch. Identification

of the mechanisms governing the differential effect of cover crop residues on

weed and crop species provides a major challenge for persons studying and using

crop rotations.



IV, SOIL QUALITY EFFECTS

The need to reduce negative on- and off-site impacts of agricultural practices

will probably provide one of the s~ongestincentives for reintr~ucingcrop

rotations into farm management plans. Kay (1990) reached a similar conclusion

in stating that a major goal for agricultural research will be to identify and

promote cropping systems which sustain soil productivity and minimize deterioration of the environment. To assess the effects of soil and crop management

practices such as crop rotation on both factors, several projects focus on the

concept of soil quality as an assessment tool (Karlen el a / . , 1992; Doran and

Parkin, 1994; Karlen and Doran, 1993; Karlen and Stott, 1994). Using different

crop rotations may improve soil quality by more closely mimicking natural

ecosystems than current farming systems (Karlen et al., 1992). This woutd occur

because temporal and spatial diversity across the landscape would be increased.

Furthermore, management strategies that maintain or add soil carbon have good

potential for improving the quality of our soil resources.

Critical factors being included in most soil quality assessments include measurements of soil structure, aggregation, bulk density, water infiltration, water

retention, soil erosivity, and organic matter (Karlen and Stott, 1994).All of these

factors are influenced by crop selection and rotation. Therefore, it is logical to

examine the effects of crop rotations on the various soil quality indicators as we

assess the need to re-emphasize rotations in 2 1st century farming systems.



A. SOILSTRUCTURE

Kay (1990) stated that the characteristics of plant species being grown, the

sequence of different species, and the frequency of harvest were ail aspects of

cropping systems that affected soil structure by influencing the formation of

biopores by plant roots and soil fauna. The network of biopores subsequently



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