2007 - Texas A&M AgriLIFE Research Center at Beaumont - Texas ...

tional three-way becomes a five-way split of nitrogen.)
PD application
The PD application is made when 30 percent of the
main stems have 2 mm or longer panicles. During this
growth stage, this application is efficiently used (taken up
within 3 days) by plants because the roots cover the flooded
soil surface. If the rice plants appear nitrogendeficient,
apply nitrogen before the PD stage.
The chlorophyll meter is very useful for determining
the need for PD nitrogen. If fields are very uniform in stand
emergence (emergence within 2 days), applications earlier
than PD might be warranted.
Chlorophyll meter use
Because the green color of rice plants as detected by
the human eye varies with cloudiness and the time of day,
it is sometimes difficult to tell if nitrogen topdressing will
be economical. Minolta’s model 502 chlorophyll meter
provides a quick and unbiased estimate of the need for additional
nitrogen during PD and 2 weeks before PD.
For example, research data (Fig. 1) show that, for Lemont
plants with chlorophyll readings of 40 or more, topdressing
will not increase yields enough to justify the cost.
The procedure for using a Minolta model 502 chlorophyll
meter to determine the average chlorophyll reading
in a rice field is to walk into representative areas of the rice
field and insert the edge of a most recently matured leaf, at
a point three-fourths of the way up the leaf, into the measuring
head of the meter. When the measuring head is clamped
on the leaf, the meter will provide an instant three-digit
chlorophyll value.
The meter will store and average up to 30 readings. Fields
having chlorophyll readings above the critical levels given
above are unlikely to benefit from nitrogen topdressings.
Fields having lower chlorophyll values will benefit from
topdressing nitrogen (Fig. 1).
Although plant density can influence chlorophyll readings
in rice fields, plant density usually must be less than 10 to 12
plants per square foot before affecting the chlorophyll value.
Another factor influencing the chlorophyll readings of
rice leaves is that the leaf midrib frequently does not divide
the leaf down the center. The narrow side of the leaf tends
to read one or two chlorophyll values higher than the wide
side. Therefore, to reduce variation in chlorophyll readings
within a field, take readings only from leaves having centered
midribs, or take an equal number of readings on each side of
the midrib.
Other factors that influence chlorophyll readings include
rice cultivar, the position of the leaf on plant and the location
on the leaf where the reading is taken. Keep in mind also that
chlorophyll readings may be influenced by cool weather as
well as deficiencies of phosphorus, zinc and iron.
Table 9. Critical chlorophyll levels above which commonly provide
no yield benefits to additional nitrogen fertilizer.
Variety
Chlorophyll reading
Francis and Jefferson 41–42
Dixiebelle, Presidio, Priscilla
and Sabine
Banks, Cheniere, CL131 and 161,
Cocodrie, Cybonnet and Saber
39–40
38–39
Cypress, Spring and Trenasse 37–38
Yield increase due to topdress N
lb/acre
Figure 1. Relationship between yield increase and chlorophyll readings.
Chlorophyll readings
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