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The varieties and rates of planting were
as follows:
Variety
Caribbean composite
Tuxpeiio composite
Guerrero 221 (Pepitilla)
Oaxaca 179 (Zapalote Chico)
Row width (em)
Rates of Planting (plants/ha)
54,00067,000100,000167,000
Single plant hills 20 cm apart were used
at all row widths.
Data for one complete year are now available.
The variety of Zapalote Chico completed
three cycles of growth and flowered in
its fourth planting at the end of one year. The
other three varieties were in the grain-filling
stage of their third cycle at the end of one year.
In general, yields seem to increase with increased
plant populations, unless the percentage,of
ear-bearing plants decreased with increase
populations, as occurred in the first
cycle for Pepitilla, the Caribbean Composite
and the Tuxpeno Composite.
Zapalote Chico, at 167,000 plants/ha, produced
14 tons of grain in the first three cycles.
This variety completed an additional one-half
of a growth cycle in the first year of the experiment,
which suggests that its annual production
will probably approach the maximum
annual grain production reported in the tropics
(see CIMMYT Annual Report 1967-68, p.
43). The Zapalote Chico used in this experiment
is an unimproved collection. It is very
early and short in stature.
These results underline the need for additional
study of different plant types in association
with the appropriate cultural practices
necessary for expression of their full yield
potential:
Selection For Root Strength
x
X
X
92
x
X
X
X
75
X
X
X
X
50
X
30
Root lodging is a major problem in all
corn-growing areas of the world. When not
~aused by disease and/or insect attack, lodg­
Ing can generally be traced to an inadequate
root system. One approach to development
of populations with a high degree of resistance
to root lodging is the selection of plants showing
the highest pulling resistance.
The first CIMMYT study in this direction
consisted of a mass selection program for root
strength, initiated in Roque, Guanajuato, Mexico
in 1969. The "Late Interracial Composite"
was used at population levels of 25,000 and
50,000 plants/ha. Root strength was measured
by pulling the root from the soil using a tripod,
a Chinese finger, and a tensiometer to indicate
the force required.
Preliminary analysis of the data collected
seems to indicate that there is some useful
genetic variation for this character. Seed from
about five percent of the plants showing highest
pulling resistance will be used to form a balanced
composite for the next cycle of selection.
Photoperiod Studies
The experiments reported here summarize
a preliminary attempt to pinpoint some important
growth responses, and environmental
factors producing these responses, when experimental
conditions are changed over latitude
and altitude.
Two varieties of each of seven Mexican
race~ of maize were planted in Tepalcingo,
Tlaltlzapan, and Roque, Mexico, at a number
of dates throughout 1968 and 1969. Six of
these races are adapted to altitudes between
1,000-1,800 meters. The seventh race, Zapalote
Chico, is adapted to lower elevations.
Two improved varieties were included each
year as checks.
TABLE 19. Sites, dates of planting, observed
daylength at planting dates, and mean maximum
and minimum temperatures during the 30 days
following planting.
No. of
5 i t e Plantings
Mean temperatures
Planting Observed ' • for 30 days after
Date Daylength planting ( °C)'
Max.
Min.
Roque, Gto. April 21 12h 36' 29.2 10.3
Roque, Gto. 2 May 21 12h 57' 30.0 12.6
Roque, Gto. 3 June 21 13h 12' 26.7 13.4
Tepalcingo,
Mor. 4 Oct. llh 31' 32.5 15.0
Tepalcingo,
Mor. 1968 5 Nov. 1 11 h 04' 30.4 12.2
Tlaltizapan,
Mor. 1969
Tepalcingo,
Mor. 1968 6 Dec. 10h 43' 29.2 10.5
Tlaltizapan,
Mor. 1969
Tlaltizapan,
Mor. 7 Dec. 21 10h 41' 30.1 8.8
1 Observed daylength is period between the times when the sun's
disc was half below the horizon in the morning and evening
when observed from the site of planting.
• Observed daylengths and mean temperatures are for 1 year
only on plantings numbered 4 and 7. For other planting dates
the values are means of 2 years data.
33