They are all acronyms for formulas that are used in connection with the growth and maturity of agricultural and horticultural crops, and follow the same basic principal of measuring daily maximum and minimum (max and min) temperatures.
The relationship between temperatures, and growth and physiological maturity of crops, has been known for a long time, as has the preferred (and often essential) max and min requirements of many plants. Agriculturists have made use of these measurements for more than half a century to my certain knowledge, and probably longer. The first one I ever used was the T-sum. In the northern hemisphere it is a measurement taken from 1st January each year and is the mean of daily temperatures, in ºC not ºF, added together. The timing is to coincide with the lengthening of the days, but begins when there is no pasture growth due to low temperatures. When the total, i.e. the T (or temperature) sum reaches 200 then grass will be able to make use of Nitrogen fertiliser. All you need is a max/min thermometer, which you should have anyway, and you can make use of the same temperature measurements in your home garden or on the farm.
CHU, the acronym for either Corn or Crop Heat Units, also using the daily mean, is sometimes used for maize, known as corn in USA. GDD (Growing Degree Days) is slightly more complicated, ignoring temperatures above and below certain readings to give a base max and min temperature and appears to be the most common measurement used in the USA. Please note that the USA does use ºF. On the other hand, some Canadian CHU systems are even more complicated in that they use different base temperatures for day and night readings, so more calculations are required. Additionally Canada tends to use calendar and minimum temperature data to create a starting and ending calculation date.
GTI (General Thermal Index) can be used, but is also complicated for maize because it uses a different formula for the period before and after silking. The silk is the female part of the flower that appears out of the top of the unfertilised cob. The silk is fertilised by pollen blowing from the tassel which is the male part and appears out of the very top of maize plants. Bees work the tassels and I have occasionally seen them on the silk, so some insect pollination may occur, but wind pollination is the norm.
CRM (Corn Relative Maturity) sometimes referred to as the Minnesota Relative Maturity Rating System is a means of showing the relative maturity of different maize varieties against a set of standard varieties, using dates rather than temperature, and includes a formula for giving approximate conversions from or to CHU and GDD. There is a considerable amount of research information and guidance notes from agricultural departments on the Internet for anyone who wishes to pursue the various measuring systems in technical detail. Merely input the acronyms in a search engine.
My philosophy is always that something easy and simple is best for an individual grower, so I use my own version of CHU to build records of different crops and varieties within them. I take measurements of the previous 24hrs. max/min air temperatures, plus soil temperature and any rainfall at 9.30 am every day. I record these on a spreadsheet that calculates the mean for the day too. There is also a table with the monthly values, but that is for annual historical information and does not form part of the CHU calculations. I total the daily mean results from sowing to harvest.
My limited use of this simplified system suggests that there is a good year-to-year correlation for CHU requirements for spring sown crops, which is precisely the reason it is used so extensively for maize. But, the maize research has consistently shown that there is a different heat unit requirement for the same varieties in different localities – and this is extremely important. Every property is different and CHU requirements can be expected to vary between properties in close proximity to each other, so do not rely on anyone else’s figures except for use as a general and rough guide. Factors such as daylength, soil fertility and crop stresses like water shortage or water logging will have an effect on maturity too.
Due to its economic importance, researchers began to look for other ways than estimated days to harvest from sowing. Maize is the crop that has been most researched, and varieties now always come with reasonable information relating to season length required to reach maturity. It may not be in the heat units format a particular grower uses, but it is at least the breeding company’s knowledge of relevant maturity dates of its own varieties and comparisons can usually be made against other varieties from other breeders.
Whilst I have not used it for overwintered crops I have the data for several years of such crops - oats and beans on a field scale, and beans, onions and various brassicas in the garden, and can do the calculations. There is not the same necessity for me to know CHUs for these winter crops because they are naturally watered in the vegetative stages and harvested before there is a need to irrigate, but the extremely dry conditions of last winter and the whole of this year (apart from 12 days in late April/early May) proved to be an exception and irrigation was used during winter for some garden crops. This has led to me to consider pursuing this extra “management tool” to winter crops and the permanent ones. I will use it for olives and may extend it to other tree fruits (using flowering to ripe fruit) and it takes literally only a few seconds to do the calculation on a spreadsheet. If you lack spreadsheet skills then it would be no great hardship to do things the old-fashioned way with a pencil and a piece of paper. CHUs do not apply to such things as the return of the storks, but that, like the arrival of the first cuckoo for Europeans, and other birds elsewhere, appears to depend on other factors, which is understandable since birds migrate from elsewhere. It might apply to the blooming of spring flowers; although I do not record the flowering dates, just being pleased to see them when they do bloom.
The main purpose of knowing CHU requirements for different varieties is ease of management decisions as to what to sow. In farming and gardening there are frequently problems in sowing a crop exactly when one wishes to do so, and it is useful to know how long it will take to maturity if there is more than a few days’ delay in sowing. I know there are often guidelines given as to the number of days from sowing, or transplanting, to harvest – similar to the CRM, but I have always found this to be a rather vague way of deciding on a variety because I have no idea where these days were calculated, nor the growing conditions.
This year I had to cease irrigating on 27th July due to lack of water in the river. This cessation has happened twice before, but never this early, and most years we have been able to irrigate right through. The only rain we had allowed crops to be sown into good moisture - and rain can continue to the end of May in some years, but I hedged my bets with maize, and grew four varieties with different maturity dates. The two earliest are making decent crops, the third probably will, and the latest variety might, depending upon how far down its roots need to go for moisture and how much remains down there. Some plants of this variety are already showing moisture stress and I expect these to fail to produce any grain.
As anyone who has grown early and maincrop potatoes will know, there is a yield penalty with early maturing varieties of most crops; late season varieties yielding much more in a normal cropping year, and maize is no exception. An advantage of early maturity though is that I would have continued irrigating the later varieties of maize if the water had been available and this costs time and not a small amount of money. An early variety saves this time and money, albeit with a smaller crop in a good year.
Everyone will be in a different situation, some need to harvest certain crops before it becomes too hot and dry; others before winter sets in; others are restricted by their irrigation capacity or water allocation, and some could be following on with a more valuable crop and need the land cleared. Armed with your personal CHU requirements for what you want to grow you are in a better position to decide which varieties suit your sowing date, seasonal expectations and overall management plan.