Overall, U.S. corn yields have been miraculous since 1936, and yields continue to increase.
From 1866 to 1936 (Fig. 1), from open pollination to the era of farmer planting, yields remained almost unchanged. The variation (slope) of yield trends is less consistent over time from year to year.
Figure 1: U.S. maize yield trends from 1866 to 2022
From 1937 to 1955 (Fig. 1), the era of double-breeding, there was an increase of 0.8 bushels/acre per year. The main reason is that Pioneer, Funk, and DeKalb specialists specialize in the production of commercial seeds, which have improved seed quality, improved drought tolerance, lodging resistance, density tolerance, and the use of chemical fertilizers.
Figure 2: Trends in U.S. corn yields 1950-2023
From 1956 to 2022 (Figure 1), the monocross era, there was an increase of 1.89 bushels/acre per year. benefits from variety improvement, nitrogen fertilizers, pesticides, agricultural mechanization, as well as soil and crop management, genetically modified transgenics, etc.; From 2013 to 2023 (Figures 3 and 4), the increase in yield slowed to about 1.05 bushels/acre per year;
Figure 3: U.S. maize yield change and trend model 1980-2023
Figure 4: Changes in U.S. maize yields from 1994 to 2022
Figure 5: U.S. corn, soybean and wheat yields are slowing
In 2013, U.S. soybean, wheat and corn yields all slowed significantly.
I. Thinking
What has caused the growth rate of corn yields in the United States to slow or even come to a standstill since 2013? What does the future hold? Can yields continue to increase at high levels?
2. Enlightenment
The slope (trend line) of the yield increase trend is mainly caused by the key changes in agricultural technologies such as hybrids, pest control, soil management, mechanization, precision agriculture, etc. Year-to-year deviations (changes) of trend lines are mainly caused by year-to-year changes in growing conditions (e.g. weather and pests and diseases). Looking at the trend, you need to be very cautious if the time period is short, because the data of individual special years may have a large impact on the estimate. For example, from 2012 to drought, production was significantly reduced. The general analysis is from the time period after 1956, as it accounts for 93 per cent of the change in corn production then and now. The United States began to promote genetically modified corn in 1996, and by 2008 the penetration rate exceeded 80%. In 2007, Schill predicted that corn yields would reach 300 bushels per acre by 2030 due to the spread of insect-resistant, herbicide-tolerant GMOs. In fact, USDA-NBS yield data show that over the past 25 years (1996-2020), yield trends have been basically 1.9 bushels per acre per year (Figure 1), with no significant change in the yield trend line. Reflecting that the transgenic traits currently in use do not really increase the genetic yield potential, these traits only protect the intrinsic yield potential of hybrids, while potentially reducing farmers' dependence on chemical pesticides. From Figures 3 and 4, it is evident that the slowdown in yield growth from 2013 to 2022 is evident.
3. Reasons
1. Since 2006, the increase in corn demand, especially the increase in ethanol demand, has led to an increase in corn acreage in the United States. The average acreage planted from 2006 to 2023 is 82.4 million acres, an increase of about 17% from the 1990-2005 average of about 70.4 million acres. The increase mainly came from low-yield areas such as Kansas, North Dakota, South Dakota, Nebraska and Minnesota, affecting the average corn yield in the United States. The increase in area after 2006 is estimated to have reduced the average United States maize production by 2.2 bushels and also contributed to a slight decline in the United States maize production trend estimates.
2. Scott Irwin (University of Illinois) pointed out that the actual yield is greatly affected by climate, and the Thompson-style regression model takes into account the time trend, the proportion of corn planting, rainfall and temperature in the main planting season. The model explains the change in average U.S. corn yield by 98%. After the actual situation and the use of model analysis to exclude the influence of weather, there was no statistically significant change in the trend of yield after 2013, and the actual yield after 2013 was low, and the yield did not increase according to the trend, largely because the weather was not conducive to growth, rather than the impact of maize production technology.
Other reasons: changes in USDA measurement methods; Growers calculated input-output returns, slowed down the input of chemical fertilizers and pesticides, etc
What does the future hold? Can yields continue to increase at high levels?
After 1950, the corn yield in the United States continued to grow at a rate of 1.9 bushels per acre per year, due to many reasons, mainly due to the low yield base, chemical fertilizers, genetic improvement breeding, high-quality seeds, mechanization, herbicides, insecticides, fungicides and other pesticides, genetically modified technology, planting management technology and other comprehensive reasons. Some articles previously shared by Zhizhong Network show that the yield potential per plant of varieties in the past few decades has hardly changed in the ideal environment, and the main changes are that the new varieties are more dense, more suitable for abiotic stresses in the new climate and planting environment, and the extension of the grain filling period. Mr. Li's chart also shows that the increase in corn yields in the United States is largely due to the increase in planting density. In recent years, the focus of Mr. Tang Jihua's team has also been working hard to test and promote 6000-7000 plants per mu.
At present, the planting density of corn in the United States is a little more than 6,000 plants, with an average of about 5,700 plants per mu. The planting density of the winners of the high-yield competition in the United States is mostly distributed in the range of 5700-7300 plants/mu. How much more can the main planting density increase in the future? At present, the mainstream row spacing of corn planting in the United States is 30 inches (76 cm), while the limit of corn plant spacing is 5 inches (about 12.7 cm). According to the row spacing of 76cm, the plant spacing is 12.7cm, and the density is about 6900 plants/mu. Personally, it is speculated that if the U.S. corn wants to continue to increase the density by more than 7,000 plants/mu, it can only reduce the row spacing, such as wide and narrow rows, 30 inches wide rows, and 20 inches of narrow row spacing. Due to the change of planting mode, the need for sowing and harvesting machinery has led to the recent slowdown in the increase in density. In addition, germplasm resources and breeding improvement have been completed under the traditional row spacing of 76 cm for a long time, and there may not be varieties suitable for narrow row spacing and higher density.
In the past 70 years, the harvest index of corn has basically been 0.52, and it is likely that the yield will continue to increase in the future. Increasing planting density can make more efficient and rational use of resources such as light, heat, water, and fertilizer. It is speculated that before the planting density is lower than 8150 plants/mu, the yield of corn in the United States will continue to increase at a high growth rate, and 8000-12000 plants need to achieve dwarf corn, and the plant height is likely to be between 170-210cm.
The chart is from the PPT of Li Shaokun, a teacher of the Chinese Academy of Agricultural Sciences
Fourth, the enlightenment of our breeding
Dense tolerance breeding: Before our mainstream plant spacing is 60-65cm and the density reaches 7000-7500 plants/mu (the limit plant spacing is 15cm), the main breeding direction is dense tolerance breeding.
Suitable for future planting mode and environment: It is necessary to fully consider the changes in demand and planting mode after 8-10 years, and adopt the planting mode after 8 years in the breeding and testing stage.
Based on the analysis and thinking of limited information, different regions should fully consider the actual local situation and speed to formulate a reasonable breeding direction, such as low salinity tolerance, and the good land in the slope and mountainous areas is definitely different from the good land in the plains.
Finally, U.S. corn yields have slowed down in the last 10 years. The reason is to speculate whether there is a single heterogeneity pattern of mainstream varieties, the homogeneity of germplasm genetic background, and whether it may be related to the large use of haplotype breeds. Teachers who understand the situation are welcome to leave a message on WeChat to exchange and share.
Some references:
1. Bussel/acre = 0.062719012 tons/ha = 4.18126749 kg/mu
2. Opportunities for corn yield improvement in the future
3.Historical Corn Grain Yields in the U.S.February 2023