B. TRENDS IN ACREAGE AND YIELDS

Copyright 1998, Patricia S. Muir

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 almost more than half of humans' intake of calories and protein directly, because grain is often used to feed livestock (which consumed ~40% of global corn production alone as of 2010 -- nevermind livestock's consumption of global soy, which was about 80% as of 2010 [Vital Signs 2011]) 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 as well. Corn, wheat and rice constitute about 85% of the total global grain production and, amazingly, rice alone provides about 20% of humanities caloric intake [Science 18 July 2008].)

Note also that I'll give updated versions of the following figures in lecture and on Blackboard -- I must be cautious about use of current graphs owing to copyright issues.

BI300010.gif

Figure 1 (Modified from "Vital Signs 1997," World Watch Institute; 1 hectare (abbreviated "ha") is equal to about 2.47 acres)

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 and 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 to decrease slightly, and has basically wobbled around at a fairly constant level for over 15 years (click "Land" if you want to peek ahead at some reasons for this; 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 idle 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. As of 2010, harvested grain acreage was ~ 700 million acres, roughly where it had been since the early 1980's. This is particularly amazing when you remember that we'd added over two billion people to the global population since then!

 

 

TRENDS IN YIELDS AND PRODUCTION

TRENDS IN GRAIN PRODUCTION PER HECTARE

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. (As for the previous figure, I'll make an updated figure available to you in lecture and via Blackboard.)

BI300011.gif

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 plus increased fertilizer inputs in many regions (particularly in China), and even possibly the increased concentration of atmospheric CO2 have contributed to these "bumper crops" [Science 29 July 2011; WorldWatch Institute Nov 125, 2011]. (Note that experiments have demonstrated this CO2 fertilizaton effect under controlled conditions for some grain crops -- but not for corn, which uses a photosynthetic pathway that is not responsive to increased atmospheric CO2 [it is a "C-4 plant". We will discuss the realism of such experiments when we discuss global climate change later in the term.] Similarly high yields per ha have occurred in some other recent years as well.

Wheat yields in the US and rice yields in Japan both increased steeply until the early to mid-1980's, and have not increased as fast since then, while corn yields so far seem to be continuing to increase.

(We will discuss some reasons for the recent increases -- and decreases -- in 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.)

TRENDS IN PER CAPITA GRAIN PRODUCTION

On a per capita basis, world grain production increased about 40% between 1950 and 1984 as the figure below illustrates. After some years of decline, per capita production is now almost back to levels that prevailed during the mid-1980's. It appears that increases in production, are barely able to keep up with population growth. Per capita production is, as you'd expect, variable from place to place -- for example, as of 2007, per capita grain production in the US was ~ 1,230 kg/person/year, while in China that number was 325 and in Zimbabwe, only 90 kg/person/year was produced (WorldWatch March/April 08).)

BI300012.gif

Figure 3. (Modified from "Vital Signs 1997," World Watch Institute) (Notes on availability of updates as above; as of 2007, per capita production was about 350 kg/ha; only slightly higher than its previous peak in the mid 1980's)

One might think that the increase in per capita grain production since 1950 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 has been less than 20%).

TRENDS IN TOTAL GRAIN PRODUCTION

The product of grain acreage and grain production per capita gives total grain production, of course.

During 1950 - 2008, total global grain production (not per capita, but total) more than doubled (see Figure 4 below, and the updated version of this figure shown in lecture and available on Blackboard). Can we account for the doubling in production based on increases in cultivated acreage? See Figure 1 above (Figure "A" in the document available on Blackboard), 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.

WHY WERE INCREASES IN TOTAL PRODUCTION SO GREAT?

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.

Climatic factors may have particularly favorable in some years as well (Science 14 Feb. 03). In particular, summers in some regions were cooler, which favored production by some crops -- if this is so, prospects for agricultural production under changed climate circumstances may be particularly bleak...)

WHAT CHARACTERIZES "GREEN REVOLUTION" CROP VARIETIES?

(1.) 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 allocate 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 varieties 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 rows 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.

(2.) A second factor contributing to high yields from these "green revolution varieties" 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. (This is partly responsible for some of the apparent increases in grain acreage -- the same land can be planted more than once per season.)

(3.) These varieties are also tremendously responsive to fertilizer and 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.

(4.) The new varieties have also been bred to be resistant to most of the major diseases that were known to plague them at the time of their development.

The green Revolution undoubtedly helped to improve nutrition throughout the world. Life expectancy in lesser developed 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.

RECENT TRENDS IN PRODUCTION

Figure 4, below, illustrates changes in total global grain production over time.

BI300013.gif

Figure 4. (Modified from "Vital Signs 1997," World Watch Institute; an updated version is on Course Documents in BlackBoard; as of 2007, production was up to 2,300 million tons (2.3 billion tons)

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.)

Natural factors contributed 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 over much of that time, but just that the rate of increase decreased!)

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! World grain output reached ~ 2.3 billion tons in 2007, which was a 4% increase over the previous year. Yields rose by ~ 95 million tons in response to near perfect weather in many major growing areas and an increase in world fertilizer use. Population growth has, nevertheless, tended to be steady with or to outstrip increases in production, as we saw above when we looked at trends in per capita production.

The next section takes a brief look at some resource limitations that contribute to these trends. (Click ">>" to move there now.)

Page maintained by Patricia Muir; last updated Oct. 29, 2012.

contents.gifprev1.gifnext1.gif