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.
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