Prior to the Green Revolution, most increases in crop production were driven by increases in cultivated acreage. The figure below shows changes in the world harvested grain acreage over time (until 1996) since the Green Revolution began.
(Note that we'll focus on grain production as an indicator of how we are doing in food production because grains supply more than 50 % of humans' intake of calories and protein directly, because grain is often used to feed livestock, and collectively, these two (grain and livestock) provide about 80% of the world's food supply. Further, about half of the global crop acreage is planted to grains. Thus, we use grain production as a surrogate for production trends in other agricultural commodities a as well. Corn, wheat and rice constitute about 85% of the total global grain production.)
Figure 1 (Modified from "Vital Signs 1997," World Watch Institute)
Since the advent of the Green Revolution, there have been increases in cultivated acreage, to the tune of about a 24% increase in world grain acreage between 1950-1981.
(Technically the 1981 figure is unrealistically high. In response to high grain prices, farmers in the US and USSR plowed highly erodible land that shouldn't have been plowed, much of which has been taken out of production since then.)
All of the apparent increase isn't actually new grain land. The green revolution crop varieties have increased the opportunity for double cropping because they mature earlier and are less sensitive to daylength. This allows, for example, planting of rice in summer and wheat in winter in China and India where before they were only able to plant one or the other of these crops.
But then harvested acreage began a general decrease (click "Land" if you want to peek ahead at some reasons for this decrease; some are "good" news, environmentally, and some are not). (Harvested grain area did expand slightly in 1996, as land set aside under commodity programs was returned to production in the US (the government has historically paid farmers to idel land in times of surplus, as surpluses decrease prices), and as the European Union also returned some of its set aside lands into production. High grain prices also led, in various regions of the world, to planting land to grain that was normally planted to oilseed.) But, by 1998, harvested grain acreage stood ~7 % below its 1981 peak, and was similar to acreages from the early 1970's. (This is particularly daunting when you remember that we'd added over two billion people to the global population between the early '70's and 1998!)
By 2004, the highest year since 1997, total acreage was about 681 million hectares, still about 6% lower than in 1981.
We saw that between 1950 and 1981, there was about a 24% increase in cultivated acreage. However, over that same interval, world grain yields per ha more than DOUBLED as indicated in Figure 2, below.
Figure 2 (Modified from World Watch Institute documents)
Between 1950 and 1990, grainland productivity (yields per ha) rose more than 2% per year, at least matching and generally exceeding the rate of population growth.
However, between 1990 and 2000 (a period not pictured on the figure above, but included in the handout given in class), the rise was only about 1.2 % per year, while the minimum per capita global population growth rate ("r"), expressed as a percentage was 1.3% during that interval. See any problem with that disparity?
Nevertheless, yields for 2004 broke records, exceeding 3 tons per ha for the first time. While not certain, it is likely that near perfect weather in major grain growing areas of the world in that season contributed to this "bumper crop."
The general picture of struggling grainland productivity has been true particularly for wheat and rice; productivity for both slowed way up in the 1980's ; wheat yields/ha in the US and rice yields/ha in Japan both increased steeply until the early to mid-1980's, and have nearly plateaued since then. Corn yields per ha are the highest of any grain, and don't seem to be struggling yet -- they are still rising.
(We will discuss some reasons for the slowing rates of increase and even plateauing in yields per ha in following sections. You could click on water , land or fertilizers to look at brief treatments to come, or on land degradation for a more extensive treatment of some related issues.)
On a per capita basis, world grain production increased about 40% between 1950-1984 as the figure below illustrates. Note, however, that the increase in per capita production was not sustained. World per capita grain production peaked in 1985 at 343 kg/person. As of 2004, (beyond the range of data graphed below) production was 322 kg/person, about 6% below the 1985 peak.
Figure 3. (Modified from "Vital Signs 1997," World Watch Institute)
One might think that the increase in per capita grain production over 1950-1984 was driven by increases in cultivated acreage. However, recall that the increase in land under cultivation during that time was much smaller than 40%, as we saw above (Figure 1; the increase was about 24%).
During 1950 - the early 1980's, total global grain production (not per capita, but total) more than doubled (see Figure 4 below, and the updated version of this figure on your handout from class). Can we account for the doubling in production based on increases in cultivated acreage? See figure 1 above (Figure "A" on your handout from class), and you'll see that the answer is no! Increases in land under cultivation were not responsible for more than an estimated 1/5 of the increase in global total production -- that is, about 80% of the increase in global total production was due to factors other than increase in land under production. Increases in production per ha were much more important in increasing total production than were increases in cultivated acreage.
Two main sets of factors were involved:
(1) The use of green revolution hybrid crop varieties and
(2) Changed agricultural practices. More detail will follow on these changes, but for now, I'll mention that the global increase in crop yields per ha across 1961 - 1999 were accompanied by a 97% increase in irrigated acreage and 638 %, 203 %, and 854 % increases in use of nitrogen fertilizer, phosphorus fertilizer, and production of pesticides, respectively.
One of the most important things that makes them higher yielding is that they are dwarf compared to conventional varieties. This may sound odd, but it works because they have been bred to allocated more of their photosynthate (the carbon that they fix via photosynthesis) to grain, and relatively less of it to their vegetative parts, such as stems and leaves. The ratio of crop yield (e.g., grain weight) to total aboveground weight of the plant is called the "harvest index." Green Revolution varieites generally have a higher harvest index than do conventional varieties.
For example, old wheat varieties allocated about 20% of their photosynthate to grain, while the green revolution varieties allocate 50-55% of their fixed carbon to grain!. (The physiological maximum is estimated to be 60%, to leave enough for roots, enough leaves for sufficient photosynthesis, etc., so there isn't much room for more improvement this way!)
While breeders have been able to alter the allocation of photosynthate, they haven't been able to change significantly the amount of photosynthate produced per unit of leaf area -- that is, they haven't been able to improve on the efficiency of photosynthesis itself.
High yielding corn in US is not as dwarfed as are high yielding rice and wheat, but is high yielding (on a per area basis) because it is bred to hold its leaves more vertically than older varieties, which minimizes self-shading and allows a greater planting density. Use of herbicides instead of cultivation to control weeds also allows row to be planted more closely. The combination of these two factors allows more plants to be packed into a given area -- increased plant density – which results in increased yield on a per area (rather than on a per plant) basis.
A second factor contributing to high yields from these "green revolution varieites" is that they are relatively insensitive to daylength. (Remember, most plants flower in response to changing daylengths?) This insensitivity means that they can be planted widely across latitudes, and also can be planted more than once per year under appropriate climatic conditions.
These varieties are also tremendously responsive to fertilizer and to water. This responsiveness results in the second factor contributing to enhanced yields, specifically use of heavy inputs of fertilizers and irrigation (which the hybrids require to achieve their yields), along with pesticides in many cases.
The green Revolution undoubtedly helped to improve nutrition throughout the world. Life expectancy in lesser deveoped countries increased by 10 years in two decades (from less than 43 years in the early 1950's to over 53 years in the early 1970's), with a major portion of the increase attributable to improved nutrition.
Figure 4, below, illustrates changes in total global grain production over time.
Figure 4. (Modified from "Vital Signs 1997," World Watch Institute; an updated version will be on your handout in class.)
Between 1950 and 1990, total global grain production increased at an average rate of about 2.6% per year -- comfortably above the rate of population growth for most of that time.
But, you can see that the increases began to falter, and between 1990 and 1996, the harvest increased only at 0.7% per year, no longer keeping ahead of population growth. (This slow down led to record grain prices.)
Some natural factors contribute to some of the precipitous drops in world grain production. For example, in 1987 there was a monsoon failure in India; also the 1988 drought and the 1993 floods decreased US corn production (there was a 31% decrease in US corn harvest between 1992 and 1993).
(Note the data are not saying that grain production actually decreased, but just that the rate of increase is decreasing!)
Total production did, however, increase again in recent years, with production breaking the 2 billion ton (2,000 million tons) barrier as of 2004 for the first time -- good news! Population growth has, nevertheless, outstripped increases in production, as we saw above when we looked at trends in per capita production.
We also have seen that cultivated acreage has decreased or plateaued in recent years. (Figure 1, above), and the generally weakening rates of increase in yields per ha (Figure 2, above) have also contributed to the slowing in total grain production.
The next section takes a brief look at some resource limitations that contribute to these trends. (Click ">>" to move there now.)
