Agriculture Reference
In-Depth Information
crop requirements increased (Blackmer and Schepers, 1995; Pagani et al., 2009). A normalization
of absolute SPAD readings is necessary because greenness intensity can vary up to 10% among
corn hybrids at a fixed N level (Schepers, et al., 1992). Moreover, interactions between hybrids and
seeding dates have been reported in greenness intensity (Jemison and Litle, 1996). To avoid this
interference, a strip without N deficiency is necessary in each field to provide a reference of the
maximum GI (greenness index), which is used to normalize the absolute crop greenness intensity
through nitrogen sufficiency index (NSI). It has been shown that a threshold value of 0.95 separates
sites with and without response to N applications (Fox et al., 1992).
3.6 NITROGEN HARVEST INDEX
Nitrogen harvest index (NHI) is defined as the amount of N accumulated in the grain divided by
the amount of N accumulated in the grain plus straw. It is an index and hence has no unit. However,
sometimes it is expressed as a percentage. A higher NHI in crop plants or genotypes is desirable
because it has a positive association with grain yield (Figure 3.31). Figure 3.31 shows that a 71%
variability in the grain yield of lowland rice grown on Brazilian Inceptisol was due to NHI. The
NHI values varied from crop species to crop species and among genotypes of the same species.
Figure 3.32 shows that the NHI of five lowland rice genotypes varied from 0.53 to 0.64. Genotype
CNA 8569 was having the lowest NHI and genotype BRS Bigua was having the highest NHI. This
may be related to the larger remobilization of stored N in the grain during the grain filling period.
The author studied the influence of two N rates on the NHI of upland rice genotypes (Table 3.13).
There was a significant difference among genotypes in NHI at a higher N rate (400 mg N ha
−1
).
Overall, NHI was higher with the application of N rate compared to the control treatment.
Novoa and Loomis (1981) reported that the high harvest index of modern wheat varieties implies
that a larger fraction of the stored N is remobilized during grain filling. McNeal et al. (1971) also
observed that wheat varieties differ in their ability to remobilize N, and Huffaker and Rains (1978)
have underlined the importance of remobilization for its influence on the protein content of wheat
grain. Remobilization is a major determinant of NUE by the whole crop. The extreme remobiliza-
tion common to soybean, lentil, and pea that led Sinclair and De Wit (1975) to their self-destruction
hypothesis may simply represent an adaption for high efficiency in the use of scarce supplies of N.
Leopold (1961) reported that remobilization of N is so efficient in oats that the plant can acquire
enough N during vegetative phase for the entire life cycle. Novoa and Loomis (1981) reported that
it may be possible to select cereal and legume cultivars that are highly efficient in remobilization
under conditions of low water supply.
Y = -16.4099 + 40.4288X
R
2
= 0.7089**
20
15
10
5
0.40
0.60
0.80
1.00
Nitrogen harvest index
FIGURE 3.31
Relationship between NHI and grain yield of lowland rice. (From Fageria, N. K. 2009.
The
Use of Nutrients in Crop Plants
. Boca Raton, Florida: CRC Press. With permission.)
