What is the yield impact of August weather conditions as we approach the end of the soybean growing season? That question is not easy to answer until we understand the sequential progression of each soybean R stage from early July through August.
The final soybean reproductive stage is known as its seed-filling phase. It starts at R5 (begin seed) and nominally ends at R6 (full seed), when each soybean seed has enlarged to fill the width of its pod cavity. However, from the end of R6 to stage R7 (physiological maturity) soybean seeds can still enlarge a bit more lengthwise to fill the space between seeds in adjacent pod cavities. If there is no stress prior to R7, the enlarging seed can expand (bulge out) the pod cavity. At stage R7, the mother plant stops depositing carbon and nitrogen metabolites into its seeds. R7 in soybean is equivalent to the “black layer” stage in corn. To determine if your soybean crop has reached R7, open any pod along the suture line. If the interior pod wall membrane clings tightly to the seed (coat), the mother plant is still feeding that seed. However, if that membrane stays with the pod wall and does not cling to the seed coat, the seed has effectively been “weaned” from its mother plant.
There are two basic soybean yield components:
- seeds per acre, which is a function of plants per acre X seeds per plant, and
- seed size which can be estimated in several ways, but seeds per pound is the most common measure.
Assuming a reasonable plant population density, the seeds per acre component is mainly established during pod-setting (stages R3 to R4) which, in Nebraska soybean fields planted by mid-May, occurs in July. The seeds per pound component is mainly established during seed-filling (stages R5 to R6, and up to R7, as noted above), which occurs in August when R7 occurs the first week or so of September.
At this time of the season (mid-August), the primary abiotic factor affecting final seed size is invariably a lack of sufficient soil water to supply the transpiratory needs of the crop for the rest of the season on up to R7. Plant leaves open their stomates during the day to capture CO2 for photosynthesis, but water is transpired from the leaf through those open stomates. Keep in mind that the growth of new root tips (which have water-absorbing root hairs just behind the root tip) has likely slowed down as the mother plant directs more carbon and nitrogen to its seeds, at the expense of other shoot and root organs. Late season water stress is notorious for accelerating what we physiologists call normal plant senescence (i.e., plant maturity). In effect, water stress can substantively hasten the expected maturity date of a given cultivar. This stress-induced earlier maturity is readily observed in most years by comparing the crop maturity in a center pivot field area with the crop maturity in its non-irrigated corners. Other weather factors (i.e., sunny vs. cloudy days, coupled with high vs. low temperatures, humidity, and wind speed) can amplify rather than diminish the cumulative amount of crop water stress and thus its maturity-hastening symptom.
Most producers are probably not fully cognizant of the degree of yield impact that occurs when seed size is reduced (for whatever reason). The table shows five potential seed-per-pound numbers in 250 seeds/pound increments ranging from 3500 seeds/pound (small seed) to 2500 seeds/pound (large seed). The left column shows a range (140 to 300) of seed numbers per square foot. The intersection of a seeds/pound value in a column with a seeds/acre value in a row indicates the seed yield that would result from that combination. (This table assumes 13% seed moisture content.)
If you examine just the red rows, you can see that if the seeds/square foot number is increased by units of 20, the yield impact with a small seed size (3500 seeds/lb) is about 4 bushels per acre, while with a large seed size (2500 seeds/lb), it is 6 bushels per acre. As noted previously, seed number per acre is mainly established during the R3 to R4 period, which in Nebraska occurs in July. So, that yield component was essentially “fixed” back then. Let’s assume you now have 270 seeds per square foot, and that these seeds are well into their August seed-filling stage in one of your fields. You came up with that 270 number by estimating that you have 125,000 plants/acre in that field and that each plant is averaging about 94 seeds per plant ( [125,000 x 94] / 43560 = ~270). Note that changing from 270 to 290 seeds /square foot would require 7 more seeds per plant (94 + 7 = 101) in your field of 125,000 plants. However, you now are asking yourself: What if water stress accelerates the maturity of the variety I have in that field, leading to a smaller seed size for the 270 seed number estimate?
If you look at the table row with 270 seeds per square foot, you will see that if your variety had the potential of producing 2500 seeds/pound, your yield estimate with that seed/acre row and seed/pound column combination would be 78 bushels per acre. Now let’s say (for contemplation purposes) that your seed size was decreasing because your rainfed field hadn’t received any rain since the beginning of August and none was in sight. With each 250 seeds/pound decrease in seed size, you can see your estimated yield of 78 bushels per acre decrease by 7 to 71, then 6 to 65, then 5 to 60, and then 4 to 56 bushels per acre.
Clearly, the last yield component to be fixed by the crop in the season — seed size — will be substantively diminished if soil water is not sufficient for crop transpiration each day during August and early September. This point should not be lost on those who irrigate their soybean fields — make sure that your last seasonal soybean irrigation is scheduled with the proper timing and water amount to ensure no loss in seed size by late-season water stress.