2008 Soybean Yields

Soybean yields ranged from disappointing to absolutely incredible in 2008. Some producers experienced the best crop they have ever grown, while others harvested below average yields. It's too early to know what the final numbers will be for 2008, but in areas north of London yields have generally been very good. Yields of over 60 bu/ac are not uncommon in some counties. That being said, areas in the extreme south-west have again been hit by below average yields. What made the difference? The quick answer is "rainfall".

The number one yield-limiting factor for soybeans is generally lack of moisture during key growing stages. Although this spring and summer lacked in heat and sunshine, more than adequate rainfall was experienced in much of the province. Coupled with the excellent fall weather and low aphid numbers, the result was excellent yields for those that received rain in July and August. A late developing cluster of pods at the top of the plant helped push yields over the top. (Refer to Figure 1.) Those areas that had disappointing yields were generally dry in late-July and August, or suffered from diseases like white mould or SCN pressure.

Figure 1 - Late Developing Cluster of Pods at Top of Plant

What About Tillage?

No-till production sometimes gets blamed for disappointing yields. The only way to know what yield response there is from tillage systems in any given year is to have side-by-side comparisons. Some producers have experimented with spring time one-pass tillage (pre-tillage) for soybeans. In 7 Ontario 2008 replicated field scale trials, pre-tillage with the Salford RTS coulter unit averaged a yield gain of 2.3 bu/ac. This is in line with previous results, which averaged a 1.8 bu/ac response over a three year period. Although tillage does add a small amount to yield, the response is generally 1 - 2 bu/ac. This year does not seem to be an exception.

Moisture Stress

Dry conditions can reduce yield at any stage of development, but there are two stages that are the most susceptible. Sufficient water during germination and during the reproductive growth stages is critical. Dry soil conditions during germination will reduce plant stands. Moisture stress during the reproductive stages will lead to fewer, smaller, and poorer quality seeds. Severe moisture stress is evident by leaf wilting, and under extreme conditions may lead to plant death. However, moderate moisture stress is not seen on the plant during the growing season. A lack of moisture during the vegetative growth stages (May and June) is usually not damaging if enough moisture was present for emergence.

The soybean crop is the most susceptible during the reproductive stages of growth. Once the plant starts to flower, it needs significant water right through to seed-fill. This means that both July and especially August are critical to yield. This year was a good example of how enough moisture during those two months can push yields above 60 bu/ac.

Soybean yield is a function of the number of plants/acre, pods/plant, seeds/pod, and the size of the seed. Each of these yield components is fixed in the above order. Moisture stress confined to a specific growth stage reduces the yield component that is established during that phase. On the other hand, water abundance during a specific phase enhances a specific component. Early stress, during the reproductive stages, reduces seed number. However, if moisture returns that reduction is offset by enhanced individual seed size or mass. Later stress, during the reproductive phases, greatly depresses seed mass, the last yield component to be fixed during reproduction. Refer to Table 1, Yield, Number & Individual Mass Response To Moisture Stress.

Means followed by different letters are significantly different, LSD (P = 0.05).  Reaper and Purcell, 1999.

Soybean varieties grown in Ontario have an indeterminate growth habit. This results in an extended flowering period, lasting from the end of June into early August. Yield that may be lost due to flower abortion early in the reproductive stages can be compensated for by flowers and pods that will be produced later in the season. If plants go through the entire flowering period with insufficient moisture, significant yield losses will occur. Even if water is supplied after flowering, yield losses cannot be regained.

Strategies for Success

Since irrigation is not economical, there are no management strategies to alleviate moisture stress once it occurs. The following practices have been shown to reduce the impact of moisture stress:

1. Planting deep enough for adequate germination moisture.
2. Early planting for greater root depth and density.
3. Establishing a full canopy early in the season through narrow rows.
4. Excellent weed control to reduce competition.
5. Good crop rotation and fertility program.
6. Careful management of disease and insects.
7. Choosing a number of varieties with different maturities to spread the risk of when the dry period will impact a specific variety.

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