End of Season Heat Unit Accumulation for Northerly Texas Sorghum

Dr. Calvin Trostle, Extension Agronomy, TAMU Dept. of Soil & Crop Sciences, Lubbock, (806) 746-6101, ctrostle@ag.tamu.edu;

This item originally appeared in the “Sorghum Insider,” an online newsletter of the Texas Grain Sorghum Association.

A crop consultant texted August 30th about a grain sorghum field near Plainview in Hale County, Texas.


“Would you spend any money on sorghum if it is still pre-boot near Plainview, TX?  I am seeing 5 to 10 army worms per whorl.  Could be devastating but not sure if the sorghum has time to reach maturity.  Heading is expected in about 10 days.”


There is a long-time rule of thumb from the former grain sorghum breeding faculty at Texas A&M AgriLife Lubbock.  They suggested grain sorghum must be flowering (defined as “half bloom”, when 50% of the plants in the field are at some stage of flowering) by Labor Day in order to make a good harvest.  Typically, that would mean about another 5 weeks or so to reach black layer which is physiological maturity.  But this depends on how warm it is, and how cool our nights get.


First, Labor Day is a poor reference as it varies ± 3 days.  And if this rule applied generally at Lubbock, then Dimmitt, Hereford, Canyon, Dumas, Stratford are further north or northwest and accumulating fewer heat units.


The maturity of the hybrid may have an influence if the above grain sorghum hybrid is early or medium early maturity.  Normally we need to be well into flowering at the latest by about Sept. 7 for Lubbock to be confident of reasonable maturity.  This is what our last recommended planting dates are for different hybrid maturities, e.g., plant a medium maturity grain sorghum hybrid by June 30 at Lubbock, or June 25 at Dimmitt, about June 20 in the northeast Panhandle.  (As for the army worms, as long as they are feeding above the growing point they won’t have as much effect on yield unless they eat into the developing head and heavily defoliate the crop.)


The Extension document “Sorghum Growth and Development” at http://publications.tamu.edu/ lists Table 1 which gives heat unit accumulation for a short maturity and long maturity grain sorghum hybrids.  We know there are some flaws in Table 1 comparing short vs. long-season hybrids.  It would appear a normally developing grain sorghum hybrid needs about 1,000 heat units for a medium-maturity grain sorghum hybrids to develop from flowering to full maturity (black layer).


Grain sorghum degree days are calculated with a base temperature of 50°F and a maximum of 100°F.  This suggests that there is no meaningful growth below 50°F or above 100°F.  (For reference, cotton has a base temp of 60°F.)  What chance does a hybrid flowering on Sept. 7 at Lubbock or Plainview have of making maturity or another location in the northern Rolling Plains or anywhere in the Texas High Plains?


Table 1.  Average monthly hi/low temperatures and first average freeze date, selected Texas locations.


Sept. Average             Oct. Average           Average               Monthly

High/Low                     High/Low                  First              Average Daily

Location                County               Temp (°F)                     Temp (°F)           32°F Freeze          Heat Units                    

September     October

Amarillo         Randall/Potter          84 / 58                          73 / 45                  Oct. 20          21                    9

Lubbock               Lubbock                 85 / 60                          76 / 48                   Nov. 1           22                   12

Vernon                Wilbarger               88 / 63                          77 / 52                   Nov. 9           25                   14



If I use the Lubbock data for the above Plainview example, in simplistic fashion 23 days of September 7 to 30 is slightly less HU than the monthly average, so I use 21 HUs per day.  For the first 10 days of October, I use 12 HU per day.  So, (23 X 21) + (15 X 12) = 663 HU.  That is not enough time.  In fact, in this scenario to reach 1,000 heat units, I would need to flower about August 27.  This example assumes there is low heat unit accumulation after about October 10 at Plainview.  Sorghum maturation will be quite slow, and then a likely freeze before the end of the month.


We know that late grain sorghum in the northern part of Texas and especially the High Plains can run into lack of maturity.  It will depend on the year.  You can’t count on a warm open Fall or avoiding cool night-time temperatures in late September.  Sorghum will be harvestable, but some grains, especially the lower part of the head and likely some tillers—which flower later by up to 7 days—may be immature hence reduced yield and lower test weight.


“Would you spend any money on sorghum if it is still pre-boot (Aug. 30th) near Plainview, TX?”


Probably not.  I looked at the 10-day forecast for Plainview (Sept. 3-12).  Temps are predicted to average 89°F & 66°F for daily high/low.  That is 27 heat units per day which is good this time of year.  But not enough to make a major difference.  If the sorghum fights it out, better able if it is medium-early or early maturity.  If does make suitable harvestable grain, good.  But expectations are moderately low at best.


I won’t spend the money.


Late-Season Heat Units for Cotton & Corn


These calculations are worked out, especially for cotton in Texas.  If you would like more info. on late-season heat units in cotton and corn, contact Trostle for a referral.

Comments are closed.