dition, source-sink alteration experiments have indicated that C- and N-storage processes in cereal ker-
nels seem to operate autonomously [199–201].
Further complicating attempts to understand the relation between yield and protein concentration is
the tendency for individual grain weight to vary with grain number [173]. A negative relationship between
grain number and grain weight is often observed [202], which may be due in part to the relative supplies
of C and N from the vegetative plant [203]. There is some indication that the composition of assimilates
(C and N) channeled to the kernel by the mother plant controls the amounts of starch and protein accu-
mulated in maize grain [204,205]. In other work, however, it is shown that the genotype of the kernel pri-
marily dictates the range of grain composition, with external factors modulating the phenotype within this
range either to a large or small degree [206]. Alternatively, zygotic factors and the source supply may in-
teract to control grain composition [207]. Compensation phenomena also complicate our understanding
of how C and N relationships control grain composition because plants can make up for a lack of current
assimilate (both C and N) with enhanced remobilization from the vegetation [172].
On a whole-plant basis, the supply of N often appears more limiting for grain development than does
the supply of carbohydrate. For maize, the capacity of the plant to supply N to the ear was more limiting
than the capacity to provide photosynthate, inasmuch as the net remobilization of vegetative N occurred
earlier and was much more extensive than the remobilization of vegetative carbohydrate [119,140,208].
Although shading plants during grain fill decreased yield and enhanced the remobilization of both dry
matter and N, the availability of newly reduced N was still more limiting to grain fill than current photo-
synthate [209]. Similarly, although supplemental illumination to the lower two-thirds of the canopy in-
creased carbohydrate status and yield, these effects could not be separated from an enhancement in the
total accumulation, and the tissue concentration, of N [210]. Collectively, these data suggest that the
availability of N to and within the plant is more variable than the availability of photosynthate and at least
as limiting to grain development.
IV. CROP RESPONSE TO APPLIED NITROGEN
A. Growth and Yield Response
Increases in crop productivity due to fertilizer N additions may be realized as dry matter yield, protein
yield, or an improvement in quality factors. For cereal crops, grain yield and protein quality exhibit a typ-
ical pattern in response to N supply that can be divided into three main components [211]:
- Grain yield and protein content (total amount present) and concentration (protein per unit
weight) increase in unison with increasing N supply. - Grain yield reaches a plateau, but protein continues to increase with additional increments of N.
- Grain protein content peaks, grain yield begins to decline, and protein concentration continues
to rise with further increases in N supply.
Responses to applied N are affected by many environmental [212,213], cultural [214,215], and soil
factors [14], with the result that the response curves can vary considerably at different locations. For ex-
ample, in a fertile soil with a high residual N supply, applications of N may have no effect or may even
decrease crop yields. Alternatively, if some factor other than N, such as soil moisture or another nutrient,
is limiting, then applications of N fertilizer will not increase growth and yield even if the supply of soil N
is low. The optimal economic N rate also depends on the soil type and the ratio of fertilizer N costs to the
value of the crop. In general, the fertilizer N rate required for maximum yield and the economic optimal
N rate are lower for soils with higher organic matter such as silt loams than for sandy soils [14].
Despite the variation associated with crop response to fertilizer N, an example of a general pattern
for maize is presented in Table 1. In the absence of other limiting factors, addition of fertilizer N will in-
crease maize yields in a curvilinear fashion. As with other growth inputs, response to N fertilizer de-
creases as more and more fertilizer is added. As a result, plants are always the most efficient at utilizing
fertilizer N when it is available at low levels (see Sec. IV.D for a discussion of N use efficiency). In this
example, 202 kg of N per hectare increased yield by 4.8 Mg, compared with no N application, with nearly
half (2.1 Mg) of this increase coming from the first 67-kg increment of N (Table 1). Although the second
and third increments also increased yield, the size of these increases diminished successively (1.7 and 0.9
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