Agriculture Reference
In-Depth Information
PYwtranspiration(T)
=
×
TE
×
HI
(1.3)
where
∫PAR
i
is the integral of photosynthetically active radiation (PAR, MJ) intercepted by green
tissue over the life of the crop, RUE, or RUE, is the efficiency with which PARi
i
is converted into
aboveground biomass (g MJ
−1
). For PYw, T is the amount of water taken up and transpired by the
plant (mm), and TE is the transpiration efficiency for creating dry weight (mg g
−1
of kg ha
−1
mm
−1
).
A parallel to Equation 1.3 for PY
N
, N-limited potential yield, can be written as N absorbed and
NUE. There are many variations of these identities (Mitchell et al., 1998), but they all point toward
efficiency with which a limiting input (radiation, water, N) is captured, then used to create dry
weight and how efficiently that biomass is converted into grain (HI).
Progress in the potential yield increase in the past few decades has largely come through better
crop nutrition, especially N nutrition, producing greater leaf area of longer duration, hence increased
PAR and modest increases in RUE (Muchow and Sinclair, 1994; Bange et al., 1997). Progress in
breeding for increased PY over the past 50 years has been very significant, and is generally attrib-
uted to increase in HI, often via shorter stature in wheat, rice, and tropical corn (Johnson et al.,
1986). An exception is temperate corn adapted to the United States, where HI has remained rela-
tively stable under favorable conditions and PY has increased because TDW has increased (Duvick,
2005a).
1.2.7.2 Projections of Potential Yield of Major Cereals
A significant yield increase in cereals and legumes has been achieved with breeding and cultural
practices in the past few decades. In the future, yield increase will be possible but it should be
through a combination of breeding work and better management practices, including mineral nutri-
tion; N. Loomis and Amthor (1999) stated that although achieving high yield is conceptually simple
(i.e., maximize the extent and duration of radiation interception, use the captured energy in efficient
photosynthesis, partition the new assimilates in ways that provide optimal proportions of leaf, stem,
root, and reproductive structures and maintain those at minimum cost), the processes involved are
complex. Fischer et al. (2009) gave a summarized discussion on projected yield potential of wheat,
rice, and corn. The views of these authors are presented in this section and the importance of N is
associated with this yield potential.
1.2.7.2.1 Wheat
A well-researched estimate of wheat yield potential for the United Kingdom (Sylvester-Bradley
et al., 2005) based on reasonable assumptions, including an RUE of 2.8 g MJ
−1
and HI of 0.6, while
deploying stem dry matter as efficiently as possible to minimize lodging risk, resulted in 19 Mg
grain yield ha
−1
under most favorable environmental conditions (well watered); this could result in a
50% increase in average farm yields to around 13 Mg ha
−1
by 2050 (Fischer et al., 2009). To achieve
this yield level, significant increases in nutrient levels of N, P, and K are required. Among the three
nutrients, especially N will be required in a higher amount due to its loss (>50%) in the soil plant
system. According to Raun and Johnson (1999), about 33% of applied N by chemical fertilizers
is recovered in cereals. Jaggard et al. (2010) also reported that in addition to the use of chemical
fertilizers, especially N, much of the increasing yield of annual crops would be owing to increased
light capture achieved by breeding for delayed senescence. This will be especially important, in
the future, to counteract the effect of a warmer climate that would make grain crops mature earlier.
1.2.7.2.2 Rice
Mitchell et al. (1998) predicted that conventional selection could result in a tropical and subtropical
rice potential yield of 11.3 Mg ha
−1
for IR72 maturity. However, Fageria (2014) reported that poten-
tial yield of irrigated or lowland rice under favorable environmental conditions is about 16 Mg ha
−1
.
Still there is scope of increase potential yield by 15%, which means a yield of about 18.4 Mg ha
−1
