Tải bản đầy đủ - 0 (trang)
Chapter 5. Dryland Cropping Intensification: A Fundamental Solution to Efficient Use of Precipitation

Chapter 5. Dryland Cropping Intensification: A Fundamental Solution to Efficient Use of Precipitation

Tải bản đầy đủ - 0trang

198



H. J. FARAHANI ETAL.



improve yields (on a per harvest basis) and reduce total crop failure and labor. A

dryland farming practice known as “summer fallow” soon dominated the North

American Great Plains in regions that receive annual precipitation of less than 500

mm. With this practice, no crop is grown during the fallow, and weeds are controlled by cultivation or chemicals to enhance soil-water storage and nutrient availability for the subsequent crop.

Both winter and spring wheat-fallow systems are practiced in the Great Plains.

In the central and southern Great Plains, hard red winter wheat (triticum aestivum

L.) is the dominant dryland crop primarily because of its high-yielding potential

(Greb et al., 1979) with limited crop substitutions (Johnson, 1977). In the northem plains, hard red spring wheat is dominant. For winter wheat, the fallow period

is approximately 14 months, running from harvest in July to planting in September of the next year. The fallow period for spring wheat is about 21 months, extending from early August harvest to planting in the second spring.

To conserve soil water during fallow, in the early days of dryland farming,

weeds were controlled by multiple tillage (plow, harrow, one-way disk) operations.

As the acreage of clean-fallowed land increased, the hazards of water and wind

erosion multiplied, resulting in the Dust Bowl in the 1930s. Scientists and farmers in the plains then turned to stubble-mulch (a V-shaped sweep or blade pulled

at shallow depth) or subsurface tillage to control erosion (Duley and Russel, 1939).

Stubble-mulch tillage is currently the dominant method of summer fallow in the

plains. It is well documented that fallow increases the probability of having adequate soil water at planting to maximize initial wheat stand establishment and development, and therefore we do not dwell on this. In this chapter we further investigate “the paradox of summer fallow” first noted by Haas et al. ( 1974).



A. THEPARADOX

OF SUMMER

FALLOW

Most farmers in the Great Plains agree that water is the primary limiting factor

controlling dryland production. Yet only a small portion of the precipitation received is stored during fallow, and soil evaporation far exceeds other losses by

weeds, volunteer plants, runoff, deep seepage, and snow blowoff. In a classic

USDA Conservation Research Report, Haas et al. (1974) state, “It seems paradoxical that water should be proclaimed the primary factor limiting crop production in the northern Great Plains, when more than 1 year’s precipitation is lost during the fallow period for spring wheat.” For no-till winter wheat-fallow in the

west-central Great Plains, Farahani et al. (1998) found that on average, only 20%

of the precipitation received during the fallow was stored in the soil profile. For

the region, average (1948-1995) precipitation for the 14-month fallow is 552 mm,

resulting in 442 mm of lost precipitation. That is indeed more than an average

year’s precipitation of 410 mm.



DRYLAND CROPPING INTENSIFICATION



199



Mathews and Army ( 1 960) summarized soil-water and precipitation data for 25

stations representing over 450 wheat-fallow years on well-managed fallow lands

in the Great Plains. The average soil-water storage during the fallow (for both winter and spring wheat-fallow systems) was 100 mm or 16% of the precipitation (617

mm), corresponding to a 84% loss of precipitation. They attributed this loss to

evaporation from the soil, since runoff and deep percolation losses were known to

be very low. Although significant progress in fallow tillage and management has

been made since then, investigators still report unacceptably low fallow water storage efficiencies, even under modem conservation practices of reduced- and no-till

(Unger, 1984; Stewart and Steiner, 1990; Norwood, 1994; Jones and Popham,

1997; McGee ef al., 1997).

In a recent review, Peterson et al. (1996) examined the effects of tillage and

residue management on fallow soil-water storage from Canada to Texas. Water

storage efficiencies using no-till summer fallow in the Great Plains were reported

as 10% in Texas, 22% in eastern Colorado, and 25-30% in western Kansas for the

14-month winter wheat-fallow system; and from 18 to 37% in the northern plains

for the 21-month fallow of spring wheat. From their summary, an average efficiency of 25% was found for water storage during fallow (both winter and spring

wheat) in the Great Plains. Comparing this with the earlier findings of Mathews

and Army ( 1 960), one may conclude that from the dust mulch days in the early

1900s to the present era, fallow efficiency has only improved from 16% storage to

25% storage with no-till fallow. A huge loss, 75% of the fallow precipitation, still

remains a reality, even with our best known soil and water conservation practices.

Summer rainfall prevails in the Great Plains, with nearly 75% of the annual precipitation occurring from April to September. Ironically, precipitation-storage efficiency during fallow is lowest, even negative at times, during summer periods

when precipitation is greatest. Paradoxically,fallow is not only inefficient but most

inefficientduring the periods when precipitation is most substantial (i.e., summer).

There appears to be little possibility of further reducing evaporation by use of surface residue, particularly since residue production in Great Plains dryland agriculture is limited for efficient water storage (Peterson et al., 1996). Existing soil

and water conservation practices, very important to erosion and soil productivity,

are at or near their practical limits. A different approach to water conservation and

efficient use of precipitation is obviously needed.

Enhancing the efficient use of precipitation is the primary key to a sustainable

dryland agriculture (Peterson et al., 1996). It appears that the most direct and practical solution to improving efficient use of precipitation is the inclusion of a summer crop (i.e., corn [Zea mays L.], sorghum [Sorghumbicolor L. Moench], millet

[Panicurn miliaceum L.], or sunflower [Helianrhus annuus L.]) in the year following the wheat crop that would utilize the summer precipitation. Peterson and

Westfall (1 996) stated, “Planting a spring crop that can utilize both the stored water and the summer precipitation is the key; . . . the summer precipitation is used



H. J. FARAHANI ETAL.



by the crop instead of being lost to evaporation during the second summer of fallow.” The 2-year wheat-fallow system is replaced by a 3-year wheat-corn

(-sorghum, millet, or -sunflower) fallow rotation. The former produces one crop

every 2 years; or a 0.5 cropping (and 0.5 summer fallow) intensity per year. In the

3-year system, cropping intensity increases to 0.67 (two crops every 3 years), and

summer fallow intensity decreases to 0.33 (one summer fallow every 3 years). The

term “cropping intensification” is used as an umbrella term, defining dryland systems with more crops and less summer fallow per unit time.

In the Great Plains, dryland-cropping intensification has shown pronounced increases in annualized grain yield and biomass production (Peterson et al., 1993,

1996; Halvorson et al., 1994; Norwood, 1994; Jones and Popham, 1997). Even

soil-surface organic matter has increased in some instances (Wood et al., 1991).

Dhuyvetter et al. (1996) summarized economic studies from across the Great

Plains and concluded that more intensive systems also yielded greater net returns.

What principles govern the efficient use of precipitation in intensified systems?

The underlying concepts that favor cropping intensification as a solution to inefficient fallow are not entirely evident from the literature. The question is, How does

intensification provide the potential for growing more crops (per unit time) in a

given precipitation regime that traditionally produced only one wheat crop every

2 years?



B. OBJECTIVE

Our objective in this article is two-fold: ( I ) to explore the concept of drylandcropping intensification as a fundamental and practical solution to improved use

of precipitation, and (2) to propose a systems approach for analyzing, evaluating,

and comparing intensified dryland-cropping systems. In this quest, we first present

a review (Section 11) of research on precipitation storage and efficiency during different parts of the fallow period in the Great Plains. The review is not intended to

be exhaustive, but it examines significant findings in winter and spring wheat-fallow systems. The emphasis in this chapter is mainly on systems involving winter

wheat, but the concepts discussed are equally relevant to spring wheat. We then

provide (Section 111) a more in-depth examination of the various periods of fallow

using data from a long-term dryland-no-till cropping systems field study.

The number of crop and noncrop periods in an intensified cropping system depends on the degree of intensification. Evaluation and comparison of intensified

systems are made difficult because the duration and frequency of crop and noncrop periods vary, and their time-of-year precipitation characteristics vary among

systems with differing crop choice and sequence. Quantitative measures and indices are needed to evaluate intensified rotations on a system basis. In Section IV,

we propose a systems approach to intensification and present a collection of single-



DRYLAND CROPPING INTENSIFICATION



201



value system indicators that allow comparison of cropping systems on an equal

basis, i.e., irrespective of the cropping intensity. Our goal is to simplify cropping

systems analysis for the purposes of research and application.



II. SUMMER FALLOW: A SECOND LOOK

The focal point of previous fallow research has been enhanced soil-water storage through improved tillage equipment, reduced number of tillage operations, and

increased surface residue cover. Less tillage coupled with more surface residue

coverage has provided the most practical means of minimizing erosion, enhancing infiltration, and retarding runoff and evaporation. Most previous research,

however, concentrated on evaluating fallow as a whole. Literature on precipitation

storage and efficiency during different parts of the fallow period is limited. A summary of significant research is presented in Tables I (spring wheat) and I1 (winter

wheat).



A. SPRINGWHEAT-FALLOW

SYSTEM

Haas and Willis (1962) summarized data collected over 40 years for the alternate spring wheat-fallow system and reported that 54% of the total 21 -month fallow storage of 1 l 1 mm was stored from August harvest to spring (Table I), and

84% was stored by July 1 (not shown in Table I). Of the 300-mm precipitation received from spring to fall, only 17% (5 1 mm) was stored in the soil profile. On the

average, no precipitation was stored during the second winter of fallow. These inefficient periods of fallow reduced the 40-year mean efficiency for the entire 21month fallow to only 19%. These results were reconfirmed in a study conducted

at Sidney, Montana, by Black and Power (1965). As summarized in Table I, fallow storage efficiency was the highest from harvest to spring (60%) and lowest for

the summer of fallow from spring to fall (5%). On average, of the 109-mm total

fallow storage, 76% was stored the first winter, 9% during the summer of fallow,

and 15% the second winter. Both sets of investigators regarded runoff as insignificant on their sites, suggesting that the evaporation was the major cause of low efficiency. For stubble-mulch and no-till fallow in spring wheat, Tanaka and Aase

(1987) reported that over 60% of water storage occurred from harvest to spring

when the land was in stubble, a lesser amount from the following spring until fall,

and still less during the second winter.

For the northern plains data (Table I), mean precipitation was greatest during

the summer of fallow (2 14 mm), of which 84% (1 80 mm) was lost. Note that the

mean precipitation storage of 74 mm from harvest to spring plus the average 2 14



Table I

Soil-Water Storage (SWS)and Precipitation Storage Efficiency (PSE) during Specific Periods of the 21-Month Fallow in a Spring Wheat-Fallow System

Specific periods of the 21-month fallow"

Harvest to spring



Spring to fall



SWS

(mm)



PSE

(70)



of

total

SWS



SWS



PSE



gr, of

total



(mm)



(%)



60



33



54



51



83



60



76



72

80



51



56



65

62



o/c



Reference



Haas and Willis, 1962

Plow (Mandan, ND)

Black and Power, 1965

Minimum- and no-tiU (Sidney, h4T)

Tanaka and Aase, 1987

Stubble-mulch (Sidney, MT)

No-till



Yearsofdata



8 of

total

SWS



PSE



SWS



SWS

(mm)



17



46



0



0



0



111



19



10



5



9



16



19



15



109



27



34

41



19

23



31

31



4

9



9

19



4

7



110

130



29

35



(%)



SWS

(mm)



PSE



(a)



1915-1954

1956-1964

1981-1 985



Mean precipitation (mm)

Soil water storage (mm)

Precipitation lost (mm)

"SWS (for a given fallow period)



Entk21-mth

fallow



Fall to seeding



15 1



74

77

= profile



214

34

180



64

7

56



429

I15

3 14



soil water at the end minus the profile soil water at the beginning of the fallow period. PSE (for a given fallow period)



= (SWSdivided by precipitation during that fallow period) X 100. Percentage of total SWS = SWS during a given period of fallow divided by total stored water during the entire fallow) X 100.



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

Chapter 5. Dryland Cropping Intensification: A Fundamental Solution to Efficient Use of Precipitation

Tải bản đầy đủ ngay(0 tr)

×