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V. Evaluation of Zero-Tillage in Farming Systems
sion. Hence, preservation of this soil structure with zero-tillage resulted
in highest grain yield observed (cf. Fig. 13, treatment 1 ) .
Soil conditions resulting from .a previous row crop generally reduce the
ability of these soils to transform precipitation and soil moisture into grain
yield, even with a good ground cover present. In this situation, loosening
the soil with minimum tillage and interrow cultivation resulted in higher
corn yields than those obtained by zero-tillage (cf. Fig. 13, treatment 6
as compared to 7 ) . Removing any soil cover left by a row crop resulted
in the lowest yield, even with zero-tillage (Fig. 13, treatment 10). All these
effects of mulch cover and tillage intensity did not influence the yield when
sufficient water was made available by irrigation.
tillage operat ions
f e crop
d i s k -.-D
FIG. 13. Yield response of maize (mean grain yield, 1958-1965) to tillage systems
on “crusting” soil in Ohio. (From Van Doren, 1971.)
From these results it can be concluded that the response mechanism
for soil stirring is presumably similar to that for soil cover and that on
crusting, sloped soils in Ohio, the intensity of tillage must be increased
as surface cover and structural stability of these soils decreases. Yet, the
decision whether to employ some secondary tillage is influenced by the
fact that overall secondary tillage can lead to as great as 10-fold increases
in soil loss. Hence, zero-tillage may be advisable despite reduced yields
in some situations.
The data presented in Fig. 13 concern the immediate crop response to
a cropping-tillage treatment; long-term effects of rotation-tillage systems,
however, are of equal importance. As shown in Table 111, yields of corn
grown without tillage were greater for each rotation examined than with
conventional tillage for crusting, sloping silt loams in Ohio. From the foregoing discussion, this could not be expected for zero-tillage in combination
with row crop rotations. Triplett et al. (1970) speculated that perhaps
K. B A E U M E R A N D W. A. P. B A K E R M A N S
after several years without plowing, a stable structure that would not require high levels of mulch for maintaining satisfactory infiltration rates
would develop at the soil surface.
The same authors investigated long-term tillage effects on level, structural stable soils with clay loam to clay surface textures. Due to the high
clay content, cracks which rapidly absorb water during summer rains form
upon drying. Therefore, runoff and soil erosion is reduced to a minimum.
On these soils, the timing of tillage operations is most difficult and tillage
costs are highest; hence, the farmers' need to employ zero-tillage is
Yet, from a standpoint of immediate yield responses, as shown by the
maize data in Table 11, fall plowing appears to be indispensable on these
soils. Again, this conclusion is modified by long term rotation-tillage effects
(cf. Table 111). On poorly drained, noncrusting, heavy soils in Ohio, corn
Long-Term Itotation-Tillage Effects 011 IXffcrent Soil Types:
Corn Yields (kg/ha) Expressed as No-Tillage Minus
Well drained silt
Corn, oats, meadow
Poorly drained silty
clay loam to clay soil
Data from Triplett et d. (1970), average for fifth through seventh
years whenever weed control and stands were equal on all treatments at
a given location. Two locations for each soil.
yields were essentially equal for both tillage systems in the corn-oatsmeadow and corn-soybean rotations. With continuous corn, however, the
yield declined with zero-tillage. The reason for this is not readily apparent
since, in contrast to the Ohio results, Moschler et al. (1972) observed
the highest yield increase (39 % ) by zero-tillage on the least productive
soil examined, a poorly drained clay loam in Virginia cropped with continuous corn. In this 5-year trial, maize was planted onto a cover crop
of rye, which might have been indispensable to the favorable results obtained with zero-tillage.
The above-quoted results of long-term trials indicate that, in an almost
weed-free environment, zero-tillage can be sustained for several years with-
out encountering a yield reduction. This was confirmed in a 6-year comparison between conventional, zero-tillage and alternate corn production
in Virginia by Shear and Moschler (1969). Plowing a loam soil cropped
with a rye mulch and continuous corn every second year resulted in a significantly lower yield than the continuous application of zero-tillage and
an essentially equal yield as compared to the continuous use of conventional tillage methods.
To sum up, maize can be successfully grown without tillage where rotation with sod or cover crops is possible and profitable on structurally unstable soil which is exposed to wind and water erosion or on which increased water infiltration increases the yield and, with some limitations,
on structurally stable clay soil. The marked interaction between tillage,
soil type, and cropping system noted above underlines the need for more
information about system responses under various ecological conditions.
This applies equally to other row crops, for which zero-tillage probably
would enhance the adoption of double cropping systems.
Small grains and forage crops appear to be the second group of crops
well adapted to zero-tillage practices. It has been proved, especially by
experimentation in Europe, that cereals and some fodder crops can be
grown successfully without any tillage on soil which iies in the textural
range from light to medium, if only one essential requirement is fulfilled:
satisfactory weed control, especially of persistent grass weeds. This appears
to be most difficult in rotations in which cereals are grown continuously
and where possible fallow periods are too short for growing competitive
“break” crops, which allow herbicide treatments to kill all noxious weeds
Due to increzsing infestations of untilled plots by gramineous weeds,
several long-term trials with extensive cereal cropping have had to be terminated. Unless new herbicides or herbicide cropping systems prove to
be more effective the application of zero-tillage as a continuous system appears to be restricted to rotations in which a cereal crop alternates with
maize, a Brassica crop or a ley which is killed by a suitable herbicide and
leaves a weed-free environment for the following cereal crop.
Since such rotations are feasible only in connection with animal production, the application of zero-tillage methods can be envisaged mainly with
some systems of “mixed” farming. Here too, it must remain an opportunist’s technique for the present time. Thc same holds for “pure” arable
farming, in which drilling green manure crops or winter cereals may be
developed to the main area for application of zero-tillage methods. Nonetheless, the experience gained with zero-tillage has promoted the development of minimal tillage techniques aimed at circumventing the problem of
weed infestation and maintaining the advantages of zero-tillage effects.
K. BAEUMER AND W. A. P. BAKERMANS
Thus, zero-tillage does not have the wide application first envisaged.
This is especially disappointing for those who have to farm very heavy
soil. Here it would be profitable to omit moldboard plowing and to use
zero-tillage methods. So far, most such attempts have failed, especially
when the heavy soil was waterlogged during spells of rainy weather. As
the need to apply zero-tillage is greatest in such situations, research must
be continued to find the causes and develop remedies.
Only little information is available about the suitability of zero-tillage
methods for farming other marginal soil. H. Kuntze and R. Bartels (personal communication, 1972) reported increased cereal yields on peat soil
with continuous zero-tillage application over a period of 4 years. Bachthaler (1971 ) obtained a higher yield of winter wheat, but slightly lower
yields of spring barley and oats on loamy clay soil with a stone content
To sum up, the possibility of growing cereal and fodder crops with
zero-tillage with reasonable success has been shown repeatedly. Where
wind and water erosion or other conditions restrict arable farming, continuous zero-tillage may offer a solution. Yet, with intensive farming, due to
the higher risks and costs of complete weed control with chemicals only,
continuous zero-tillage cannot be regarded as an alternative to conventional methods at present. If needed, zero-tillage may be the last resort
when proper sowing cannot be achieved by conventional means. Finally,
where soil conditions and requirements of a crop allow the application
of zero-tillage, it will certainly be an economically attractive choice.
Since in dryland farming tillage expenses make up a far greater percentage of total farming expenses than in humid regions, it would be economically attractive to use zero-tillage methods if the yields obtained are
similar to those with stubble mulch farming or the still widely used black
fallow by plowing. Whereas Stibbe and Ariel (1970) observed reduced
yields on Grumusol clay soil in Israel, the same yields were produced by
zero-tillage and conventional dryland tillage methods on light- to mediumtextured soil in the United States.
With continuous zero-tillage, weed control still presents problems. Phillips ( 1972), experimenting with tillage systems in a wheat-sorghum-fallow
rotation in Kansas, concluded that the zero-tillage concept was not sufficiently consistent and that herbicides should be supplemented with some
tillage. Controlling a volunteer crop may be difficult when a system of continuous sorghum is used (Unger and Wiese, 1972). Hence, further research on the applicability of zero-tillage in dryland farming is needed.
Zero-tillage refers to tillage systems in which soil disturbance is reduced
to sowing operations and traffic only and where weed control must be
achieved by chemical means. It has been demonstrated to be of practical
value in maize production on sloping, structurally unstable soil, in double
cropping systems with cereal and forage crops, and in pasture renovation.
More than any other tillage system, zero-tillage maintains crop residues
on the soil surface; hence it protects the ground against wind and water
erosion. Therefore, zero-tillage acreage will probably increase where erossion hazards limit arable farming. In other situations, further adoption of
zero-tillage depends on potential production benefits with this new system
which would exceed those with conventional tillage. Except for the abovementioned cases, those benefits could not be shown to occur in all cases
at the present time.
The problem of how to eradicate persistent weeds with continuous application of zero-tillage has yet to be solved. At present, incomplete weed
control is the main obstacle to further adoption of zero-tillage. If this system is to be widely used in humid areas, substantial improvements must
be made in the development of more effective herbicides and/or croppingherbicide systems. In order to predict the range for zero-tillage application,
much more information is needed on interactions between soil type and
climate on the one hand and cropping and tillage systems on the other.
Erosion losses and, hence, pollution are minimized by zero-tillage. This
fact alone justifies continued study of this system. A clean tilled field is
certainly pleasing to the eye, but such a tillage objective has to be questioned with regard to the essentials of crop production and to maintaining
environmental quality. Thus, the feasibility of zero-tillage can be an incentive for changing farming practices toward better management in relation
to the environment.
As part of applied research, there is a need to investigate the practical
aspects of zero-tillage. Furthermore, this system appears to be an excellent
tool for basic research in the field of agronomy. As demonstrated by the
approach of the research group at the Ohio Agricultural Research and
Development Center, realistic tillage requirements can be established in
a weed-free environment. The results obtained indicate that in some cases
the important factor in tillage is not exclusively weed control, but improvement of the soil structure.
Compared to the conclusions drawn by McCalla and Army in 1961
about the state of knowledge on stubble mulch farming and the need for
further information about the effects of such a system so close to zero-til-
K. BAEUMER AND W. A. P. BAKERMANS
lage, the increase in practical experience and experimental results gained
since is obvious. Yet, as shown by our review, only little has been gained
in understanding plant production systems influenced by tillage methods.
It has to be questioned whether the effort to obtain ever more pertinent
data with extended and refined research will lead to substantial improvement in understanding systems such as zero-tillage. Clearly, a synthesis
is required; it may eventually be attained with a more fundamental rather
than empirical approach. It is hoped that system modeling will be a means
of predicting tillage effects and that, in order to achieve this goal, zero-tillage research will function as an abundant source of information.
The authors are indebted to many colleagues who kindly supplied information
and unpublished experimental results. Thanks are due to Mr. B. Fodiman for
correcting the grammar and style of this manuscript.
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THE GENETIC CONTROL OF FLOWERING
AND GROWTH IN SORGHUM
J. R. Quinby
Pioneer Hi-Bred Company, Plainview, Texas
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11. Genetics of Flowering . . . .. . . . . . . . . . . . . . . . . . . . . . . .. . .. . . . . . . ..
111. The Floral Stimulus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IV. Physiology of Flowering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V. Genetics and Physiology of Cell Elongation . . . . . . . . . . . .. . . . . . . . . .
A. Genetics of Height . . . . . . . . . . . . . .
B. Physiology of Cell Elongation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. Height Genes and Leaf Size . . . . . . . . . . . .. . . ...,.. . . . ... ...
D. Height and Peduncle Length . . .. . . . . . . .. . , . . . . . . . . . . . . .. .
E. Height and Maturity Genes and Panicle Size . . . . . . . . . . . . . . . . .
F. Types of Internode Distribution . . . . . . . . .. . . . . . . . .. . . . . . . . . . .
VI. Influence of Photoperiod, Temperature, and Leaf Area on Flowering . .
A. Photoperiod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. Leaf Area . .. . . . . . . . . . . . . . .
VII. Influence of Maturity Genotype on PI
owth and Adaptation . . . .
A. Influence on Plant Growth . . . . . . . . , . . . . . , . . . . . . . . .. . . . . .
B. Adaptation to Climate . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .
VIII. Morphological Effects of Hybrid Vigor in Sorghum . . . . . . . . . . .
A. Panicle Size, Number of Seeds per Head, and Grain and
. . . . . . . . . .. . . . . . . . . . . . . . . .
B. Leaf Blade Size
G. Size of Root System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
H. Seed Size and Test Weight per Bushel . . . . . . . . . . . . . . . . . . . . . . .
I. Seed Size and Grain Filling Period . . . . . . . . . . .
J. Protein Content of Seed . . . . , . . . . . . . . . . . , . . . . . . . . . . .
K. Summary of Morphological Effects of Hybrid Vigor . . . .
Genetic Control of Hybrid Vigor
A. Effect of Heterozygosity at Maturity Loci
B. Effect of Heterozygosity at 0
C. Discussion of Effects of Heterozygosity . . . . . . . . .
X. Sorghum Genotypes as Experime
A. The MILO Maturity Genotypes
B. Maturity Genotypes Recessive a
r Maturity . . . . . .
. ...... ....... .
C. Pairs of Varieties that Differ at