Accumulated Heat

One of the first things successful farmers learn with regards to plant growth is that it's a lot more necessary to think about and compensate for what you're environment *doesn't* provide than it is to worry about the stuff that it does. In most years in most of the US I've come to the conclusion that the largest limiting factors for plant growth might be lack of soil fertility, lack of soil depth to bedrock, lack of water availability or lack of timely water. I've come to this conclusion mostly based on the availability of maps and data that explore every one of these items in detail. Locally I can access more data than I care to think about on every one of those topics for free either through our local UT extension office or through our local NOAA weather forecast office.

We worried about all of the aforementioned things in Ontario when I was farming there too, and our local OMAF office also had reams of data available on soil fertility, average precipitation and soil moisture, etc. But none of the things I mentioned even came close to our biggest worry. In spite of being in the southernmost part of Canada most of Ontario has a marginal climate for maturing grain corn and other heat sensitive warm season crops. There is also a significant yield drag on early maturing grain corn varieties....not as big as there used to be but still very real. Thus the safest crop varieties to grow from a maturity perspective are also the ones with the lowest potential yield which is far from ideal from a farm income perspective. In order to mitigate risk, every Ontario farmer (me included) selected corn hybrids and other crops based on the amount of accumulated heat (corn heat units) we could expect on our farm as per the map below (map courtesy Ontario Ministry Agriculture, Food and Rural Affairs).



Every corn hybrid in Ontario was classified by days to maturity (normal measure in the US) AND corn heat units. Thus when I bought a hybrid suitable for my farm SW of Peterborough I felt comfortable selecting hybrids up to roughly 2850 CHU. If I planted it before the fifteenth of May and if we didn't get an unusually cool summer or an unusually early frost it would reach physiological maturity, or close enough to it, most years. We kept on farm weather records and recorded our own corn heat unit accumulations for many, many years. 

Using corn as an example it's widely known and widely disseminated through print media exactly how much accumulated heat it will take to get any given hybrid of corn with a known heat unit rating to waist high, tasseling, silking, dough stage, and black layer, etc. Farm newspapers track this information every week throughout the growing season and compare it to long term averages and  information from the previous year. The same information can be and is applied to alfalfa maturity, grass maturity, small grain maturity and to a lesser extent soybean maturity. 

There are several measures of accumulated heat used in the US but as far as I know there aren't any which are systematically used to select crops across most of the country. Perhaps growing degree days (GDD) base 50F might be the most widely utilized for corn but most grain corn gets sold on days relative maturity which is no more than a rough guide as to when a crop might mature. This makes some sense because in most years most of the US corn belt is warm enough that accumulating enough heat to mature any temperate season crop isn't a big concern.

Strangely enough, accumulated heat does become a concern here in this part of the south, toward the northern end of both the cotton belt and the C4 warm season grass belt. While there is some general information on which cotton or hybridized warm season grass varieties might do best under normal local summer conditons as far as I know there is no systematic yearly tracking of GDD base 60F temperatures as applied to the relative maturities and yields of appropriate local warm season grass varieties. If there is I would sure like to see it. Any UT extension personnel reading this blog is welcome to correct my information.