Growth and yield of indeterminate soybeans. 2. Effect of removal of the mainstem apex

Abstract

The mainstem apex was removed from indeterminate soybeans (Glycine max, varieties Chaffey and Farrer) in the field over 3 years to estimate their ability to recover from damage to terminal buds by foliage feeding and stem boring pests. In years 1 and 2, the growing tip was plucked off the main stem of 50 or 100% of plants (variety Chaffey) early in vegetative growth (V2), at the beginning of flowering (Fl), at full flowering (F100) or at the end of flowering (EF100) as single treatments. In year 3, tips were removed from 20, 33 or 50% ofplants (variety Farrer) at V3, F1 or F100 as single treatments or as repeated treatments starting at those times and repeated twice at weekly intervals thereafter. Tip removal at V2 or V3 delayed flowering (F50) and pod set (R3) by up to 10 days but did not delay maturity (P95), whereas tip removal at EF100 hastened P95 by 5-8 days. Damage during flowering did not affect phenological development. Indeterminate soybeans have the ability to compensate vegetatively for damage to the mainstem apex. Tip removal at V2, F1 or F100 significantly reduced the number of nodes on the main stem, increased branching and did not reduce the total number of nodes per plant at maturity. Tip removal at V2 increased the number of nodes per plant by increasing the number of nodes per branch. Repeated tip removal prior to flowering and single or repeated treatments during flowering reduced final plant height. Tip removal from all plants reduced yield by around 10% irrespective of the time of damage. Fifty per cent damage prior to flowering or after full flowering did not affect yield but, at the beginning of flowering, removal of tips from as few as 20% of plants reduced yield by at least 10%. The most severe yield reduction was 24% following repeated tip removal from 50% of plants starting at V3. Repeated damage to 50% of plants starting at F1 reduced yield by 18%. Seed size was not affected by any treatment. We conclude that pests in indeterminate soybean crops should be controlled if they have removed the growing tip from the main stem of 50% of plants before flowering or 20% of plants at the start of flowering and are continuing to cause damage.