Chemistry Reference
In-Depth Information
contributed to past increases in yield.
63,64
Among the important contributors
to these have been:
N
improved germplasm, able to grow vigorously (e.g. hybrids), resist
pathogens, and respond to fertilisers without lodging (in particular the
use of dwarfing and semi-dwarfing genes in rice and wheat);
d
n
1
r
2
n
g
|
1
N
the application of fertilisers and, particularly, the availability of affordable
nitrogen fertiliser;
N
the development of chemicals to control weeds, pests and diseases; and
N
improved irrigation systems, especially in rice-producing countries and for
some previously rain-fed crops.
These technological innovations, together with institutional and market
reforms, have modified G, E and M to greatly increase yields. As a result of
these innovations, over large areas the yields of many crops have increased
year-on-year (Figure 3). Global yield increases for a number of crops have
typically been linear (Figure 3a), with values of 53 kg ha
21
yr
21
for rice, 41 kg
ha
21
yr
21
for wheat, and 63 kg ha
21
yr
21
for maize over the period 1961 to
2004. Increases in yield have also been linear with time in many individual
countries (e.g. wheat yields in several European countries)
65
although, in a few
instances, technological innovations have produced more rapid, step-wise
increases in yield (e.g. Australia; Figure 3b).
66
Although crop yields have
increased globally and throughout North and South America, Europe,
Australia and much of Asia, a notable exception has been that of Africa
where, for example, per capita food production decreased by about 5-10%
between 1980 and 1995.
67
The reasons for the poor performance in Africa
relative to other countries are many and include social unrest and war, poor
institutions and governance, climatic variability making reliable irrigation
difficult, and weathered soils that are deficient in nutrients. It has been argued
by many that a major factor behind many of the observed decreases in yield in
Africa countries has been the decline of soil fertility accompanied by the lack
of fertiliser application. Much has been written about the need to ''re-
capitalise'' the soils of Africa, especially with regard to P status, but progress
has been limited.
67,68
Because the area of cropped land is likely to increase proportionately less
than the future demand for food, reducing the gap between current yields and
potential yields is a major goal for the future.
69
Potential yield is a theoretical
upper limit to yield imposed by solar radiation (affecting growth), temperature
(affecting development and growth) and water supply (affecting mainly growth
but also development). A review of data from crops of maize, rice and wheat
grown in a range of countries showed that the gap between potential and the
actual yields ranged from about 20 to 80%.
70
In many irrigated cereal systems,
yield appeared to plateau at or about 80% of potential yield while in rain-fed
systems, average yields were commonly 50% or less of potential.
70
While part
of the yield gap is inevitable because of crop losses during harvest, storage and
transport, and the way that land areas are reported,
69
d
n
6
h
3
there are still large
