Unusually warm weather in March and early April provided the opportunity to start planting the 2012 corn crop earlier than normal. While there is little doubt that some corn has been planted much earlier than usual, determining whether the crop in total is being planted at a record pace is not as straightforward as it may seem at first glance. The problems include: i) corn planting is generally spread over a relatively long time period, ii) the timing of planting differs substantially by region of the country, and iii) there are several ways to characterize how early or late the crop is planted. One consistent measure is the date at which planting in those states in the heart of the Corn Belt reaches 50 percent completed, as reported in the USDA’s weekly Crop Progress report. We previously examined this measure in the report found here.
Here we review the history of corn planting progress in the states of Illinois, Indiana, and Iowa for the period 1960 through 2011. We calculate the number of days before or after May 1st that planting reached 50 percent complete in each of the three states. Since planting progress is reported on a weekly basis, the date of 50 percent completion is calculated assuming equal daily planting progress during the week that 50 percent was reached or exceeded. For example, if planting progress for a particular state was reported at 42 percent on May 1st and 63 percent on May 8th, we assume that 3 percent of the crop was planted each day during the week and that planting progress reached 50 percent on May 4th. For that year, planting progress reached 50 percent 3 days after May 1st. click image to zoom
The history of the number of days before and after May 1st that planting progress reached 50 percent in Illinois is shown in Figure 1. The first observation is that over time there has been a clear trend of reaching 50 percent completion earlier. In the 1960s the 50 percent date tended to be about 15 days after May 1st while in recent years it has tended to fall near May 1st. This implies that corn is planted in Illinois about two weeks earlier than was the norm five decades ago. More formally, the fitted trend line indicates that 50 percent planting progress in Illinois has been reached nearly 0.4 days earlier each year since 1960. The trend is smaller, but similar, in Iowa (0.34 days) and Indiana (0.26 days).
click image to zoom The overall tendency towards earlier planting over time means that a 50 percent date that is 15 days after May 1st in say, 1975, is comparable to a 50 percent date of May 1st in 2012. So, in order to make “apples-to-apples” comparisons of planting progress over time we de-trended the 50 percent planting date observations, much like yield observations over time are de-trended to reflect changing production technology and management practices. This is accomplished by subtracting the fitted trend line prediction each year from the actual 50 percent observation. The result is a “de-trended” series of 50 percent dates that are comparable over time.