At this point in the growing season, seed set and yield potential have already been determined in corn development. Thus, the effect that heat stress (typically temperatures over 92°F) has on the crop at this stage relates to fulfilling that potential through the final kernel weight. While the overall effect on late grain-fill is typically less deleterious to overall yield than heat stress during pollination and early kernel development, the consequences can still be significant. One primary consequence of sustained heat during the grain-filling period is the increased growing degree days (GDD) accumulated at night (higher respiration rates at night utilize sugars produced during the day, so plants need to increase photosynthesis to maintain yield). This increase hastens maturity, which reduces the length of the grain-fill period and hence the average kernel weight. Furthermore, water stress typically accompanies heat stress creating a synergistic effect in reducing final yield. Through down-regulation of gene expression, much of the decrease in final kernel weight is through a decrease in the development of the endosperm - the portion of the kernel containing most of the energy-rich carbohydrates (Figure 1), which causes small, deformed kernels.
Figure 1. Primary components of the corn kernel.
USDA-NASS data on current crop progress suggests that most of the state’s corn had reached the dough (R4) and dent stages (R5) by the time the heat wave hit the northern Plains. With the onset of heat, the primary effect may be on increasing the rate of maturity and hence the dry-down of the corn kernel. The average GDDs to maturity for typical hybrids in South Dakota ranges from 2200 – 2800 GDDs or roughly an 86 – 112 day relative maturity. Current GDDs from the northern to southern parts of the state range from 2200 to over 2600 GDDs reflecting the wide variation in maturity relative to current conditions in the field. Regardless, one likely effect in many corn fields is an earlier onset of kernel black layer or physiological maturity where kernel weight will no longer increase. Typical kernel moisture at this stage nears 30% and dries at an approximately linear, decreasing fashion with rates ranging from 0.5-1% per day over the next few weeks.
Ultimately, GDD accumulations across the state reflect varying degrees of corn maturity. Figures 2 and 3 show current accumulated GDDs moving from northwest to southeast across the state as well as expected GDDs in two week periods for the rest of the corn season. On average, GDD accumulation from mid to the end of September is approximately 190 GDDs, which suggests that, particularly for the southeastern portion of the state, harvest maturity may be near depending on corn hybrid and despite any yield losses due to heat stress in late grain-filling, the USDA is still projecting record corn yields for South Dakota at 769 million bushels. Current outlooks indicate likely near to above average temperatures continuing through the end of the month continuing the quick GDD accumulation to season end.
Figure 2. Accumulated growing degree days for stations across South Dakota.
Figure 3. Average growing degree days for September and October. Averages are based on the five year period from 2008-2012.
- Ober ES, Setter TL, Madison JT, Thompson JF, Shapiro PS (1991) Influence of water deficit on maize endosperm development: enzyme activities and RNA transcripts of starch and zein synthesis, abscisic acid, and cell division. Plant Physiol 97: 154–164.
- Yu L.X. & Setter T.L. (2003) Comparative transcriptional proﬁling of placenta and endosperm in developing maize kernels in response to water deﬁcit. Plant Physiology 131, 568–582.
- http://www.nass.usda.gov/Charts_and_Maps/Crop_Progress_&_Condition/2013/SD_2013.pdf (accessed September 13, 2013)
- Nielsen, R.L. 2011. Field Drydown of Mature Corn Grain. Corny News Network, Purdue Univ. http://www.agry.purdue.edu/ext/corn/news/timeless/graindrying.html
- On an area basis the expected yield is 145 bu/ac, which is the second highest on record