Stage 2: V6 to VT

Vegetative growth phase

This is the period of cob size determination and rapid vegetative growth. To maximise the yield potential of crops, growers should aim to have them strong and vigorous as they enter this growth phase.

This is because the number of kernels around each cob is determined around V6. This can be achieved by ensuring adequate nitrogen (N) and moisture is available before the crop reaches V6.

During the early stages of this growth phase (V7 to V10) the number of kernels along each cob is being determined. So strong vigorous growth throughout this period will also maximise yield potential.

One key to maximising yield potential is to ensure N is not limited at any point during this rapid growth and ear size determination period. In other words, ensure that the bulk of your N is effectively applied prior to V6.

Applying side dressed N into dry soil is not as effective as if it is applied into moist soil. Likewise, knifing-in will reduce losses, particularly in dry conditions. Getting side dressing N on before V6 also reduces crop damage caused by tractor tyres or leaf scorching.

Cobs are initiated on several leaf axis’ on the main stalks of each plant. The number of cobs that develop is also determined during this period. If conditions are good and the plants feedback mechanisms indicate it can support two (or more) cobs, then it will direct any surplus resources to the second or third cobs in that order. The primary (top) cob will always have priority over other cobs, unless the top cob fails to develop, is unfertilised, is damaged or is unable to receive resources due to insect attack (e.g. corn stalk borer, which fortunately, we don’t have here in NZ at present).

Liebig’s Law of the Minimum states that growth is determined by the availability of the resource that is most limiting at that time. Any deficiencies that occur during this growth stage will limit the yield potential of the crop.

Over the whole crop growth cycle, final yield is reduced by the sum of the daily effects of the most limiting factor on crop growth and development. This sum of effects creates the difference between the actual and potential yield, often referred to as the yield gap.

During the establishment phase the most common limiting factors are temperature, root oxygenation and available soil water. From V4 onwards it is more likely to be water, sunlight, and/or nutrient availability. If it’s water, there is no benefit to putting on extra fertiliser unless you can irrigate or it is likely that significant rainfall will occur.

Astute growers work hard to understand what is limiting their system. The key

objective is to balance the resources and inputs so that none are applied more than needed, so that the limiting resource is one that is set by the environment, usually water (e.g. rainfall), sunlight, or the ability of the crop to take up water and nutrients(think root architecture and soil structure).

When scouting crops at this stage keep a keen eye out for any signs of stress, leaf disease and problem weeds. Consider the scale of any symptoms; is the issue widespread (macro scale), more localised or patchy (meso-scale) or confined to individual plants (micro-scale).

Collect tissue samples from any plants showing deficiency symptoms for nutrient analysis. Collect and submit samples from healthy plants nearby at the same time. Soil samples from the same locations will help with interpretation. Record the exact location of any potentially serious threats (e.g. noxious weeds). Study the crop; try and understand what it needs to overcome the present limiting factor. Overcoming that factor may have to be done indirectly. For example, implementing strategies to enhance soil structure and health to increase root growth, development and health to enhance water and nutrient uptake.

Be mindful to assess the likelihood of an economic return to fix a problem before implementing a plan. Also consider the longer term consequences if nothing is done. In the case of potentially serious threats (e.g. noxious weeds) consider future revenue losses against the costs of control and/or eradication. Note any differences between fields and/or hybrids for future reference.

LEAF PROBLEMS FROM V6 TO VT

LEAVES DISCOLOURED OR DEAD
Heat/drought	Leaves exhibit a greyish sheen, curl/roll up and die under drought conditions and will scorch when excessively hot.
Nutrient deficiency	See Leaf abnormalities for details of specific symptoms.

WHORL LEAVES DEAD
Disease	Bacterial stalk rot can occur in hot, wet conditions.
Herbicide injury	Caused by the late application of some herbicides directed over the whorl of the plant.
Excessive heat	Uncommon in NZ

LEAVES SPOTTED OR STRIPED
Disease	Refer to Diseases for symptoms and details.
Nutrient deficiency	Macro/meso-scale symptomology. Particularly N, K, Mg, S, Mn, Fe and B.
Hybrid genetics	Macro/meso and micro-scale symptomology. Some hybrids are more prone to striping and/or physiological spotting than others.
Sunscald	Banded scorching on the upper leaves, tips and margins.
Low soil pH	Beaded streaking of leaves which turn reddish-purple and may die.

LEAVES SHREDDED OR CHEWED
Wind damage	Macro-scale loss of leaf area, often with mid-rib broken. Some hybrids are more susceptible to wind damage than others.
Insects	Including armyworm and other insect larvae (i.e. caterpillars).
Animals	Check for signs of tracks and droppings.
Hail damage	Even light hail is usually accompanied with strong winds which can combine to cause significant damage to leaves.

LEAVES ROLLED (PINEAPPLED)
Drought/heat stress	Usually on a macro/meso-scale. Plant leaves will appear rolled and upward pointing (like a pineapple) often with a greyish colour/tinge. Symptoms may appear first on lighter (sandy) ridges/seams and/or fields with a north-westerly aspect.
Poor soil conditions/root development	Often seen in areas where poor soil conditions restrict root growth/health and/or available water capacity (e.g. sub soil acidity, high salt index or compaction).
Root injury	Occasionally results from side dressing implements too close to plant rows.

PLANT AND STALK PROBLEMS FROM V6 TO VT

PLANTS WITH VARIABLE HEIGHT
Variable growing conditions	Variability in plant height at the macro/meso-scale caused by widely different growing conditions (e.g. ponding, soil texture, water/nutrient availability, salt index, compaction, or shading/competition from trees).
Uneven emergence	Uneven planting depth, seed/soil contact and/or soil moisture.
Previous injury/stress	Caused by any “checking” during the previous growth stage (VE to V6) including pest, disease, chemical and/or mechanical injury.

PLANTS WITH MULTIPLE TILLERS
Growing point injury	Injury caused during previous growth stage (VE-V6) by pests or mechanical injury (e.g. Argentine stem weevil (ASW) or crushing).
Favourable growing conditions	Expressed at the macro/meso-scales.
Low plant population density	Widespread tillering suggests the plant population could have been increased whereas more localised tillering is more indicative of variation in local conditions (e.g. high fertility/water availability or pest pressure).
Hybrid differences	Some hybrids are more prone to tillering than others.

PLANTS STUNTED WITH NO OR VERY SHORT INTERNODES; LEAVES MOTTLED, STREAKED OR DISCOLOURED
Virus or cirus-like disease	Maize dwarf mosaic virus or maize leaf fleck virus.

PLANTS GOOSE-NECKED
Recovery from earlier root damage/lodging	Likely causes include, pests (birds and insects), compaction, poor fertiliser placement, mechanical injury, herbicide injury (e.g. phenoxy types) and wet/windy conditions.

PLANTS TWISTED OR GROWING HORIZONTALLY
Mechanical injury	More prevalent at the meso/micro-scale. Usually other symptoms such as crushing/foliar damage are also apparent.
Herbicide injury	Particularly around headlands or overlap areas. Commonly seen following late applications of Dicamba or other Auxin-type herbicides.

PLANTS LEANING OR BROKEN OFF
Mechanical injury	Typically at the meso/micro-scale. May be systematic, check for patterns across paddock.
Herbicide injury	Particularly around headlands or overlap areas. May be systematic. Commonly seen following late applications of Dicamba or other Auxin-type herbicides.
Wind	Severe wind can cause root lodging when conditions are wet and/or soil lose; green snap can occur in otherwise dry conditions or firm seedbeds.
Soil conditions	Macro/meso-scale variations in soil conditions that affect root growth and health.
Poor nodal root development	Macro/meso-scale variations in soil conditions that affect root growth and health. Shallow planting. Dry loose, rubbly soils.
Hybrid differences	Some hybrids are more prone to root lodging and/or green snap than others.
Animal tramping/feeding	Look for tracks and droppings.

PLANTS WILTED OR DEAD
Disease	Bacterial stalk rot.
Drought	Common at the macro/meso-scale particularly on sandy ridges/seams.
Herbicide damage	Macro/meso-scale damage. Check for misapplication of herbicides or spray drift from neighbouring crops.
Frost	Symptoms are likely to be worse in low-lying areas.

STALKS SPINDLY
Nutrient deficiencies	Likely to be caused by a Mn deficiency, but several other nutrient deficiencies can result in poor stalk development.
Excessively wet or dry soils	Affects root growth and health, resulting in reduced water and nutrient uptake.
Root damage	Affects effective root zone and root health, resulting in reduced water and nutrient uptake.
Excessive population density	Symptoms will be worse in more challenged areas of fields where resource availability (water, nutrients or light) is lower.

CURLED/STUBBY BRACE ROOTS
Herbicide injury	Macro/meso-scale symptoms. Usually late applications of phenoxy-type herbicides are responsible. Most prevalent around headlands and overlap areas.
Soil conditions	Macro/meso-scale symptoms. Excessively wet, dry or hot soil conditions.
Windy conditions	Macro/meso-scale symptoms.

TASSELING PROBLEMS (VT)

GREY OR BLACK GALLS ON PLANT REPRODUCTIVE PARTS
Disease	Common smut.

TASSELS MALFORMED WITH A SOOTY, BLACK, STRINGY APPEARANCE
Disease	Head smut.

EMERGING TASSELS CHEWED
Insects	Corn earworm or cosmopolitan armyworm.

TASSELS MALFORMED INTO GREEN ROPEY CLUSTERS OFTEN COMBINED WITH EXCESSIVE TILLERING & MALFORMED LEAVES
Disease	Crazy top.

Crop Scouting: Stage 1 - Establishment phase

Crop scouting: Stage 3 - Reproductive phase

TOP

Crop Scouting

Stage 2: V6 to VT

Vegetative growth phase

Heat/drought

Nutrient deficiency

Disease

Herbicide injury

Excessive heat

Disease

Nutrient deficiency

Hybrid genetics

Sunscald

Low soil pH

Wind damage

Insects

Animals

Hail damage

Drought/heat stress

Poor soil conditions/root development

Root injury

Variable growing conditions

Uneven emergence

Previous injury/stress

Growing point injury

Favourable growing conditions

Low plant population density

Hybrid differences

Virus or cirus-like disease

Recovery from earlier root damage/lodging

Mechanical injury

Herbicide injury

Mechanical injury

Herbicide injury

Wind

Soil conditions

Poor nodal root development

Hybrid differences

Animal tramping/feeding

Disease

Drought

Herbicide damage

Frost

Nutrient deficiencies

Excessively wet or dry soils

Root damage

Excessive population density

Herbicide injury

Soil conditions

Windy conditions

Disease

Disease

Insects

Disease

Crop Scouting: Stage 1 - Establishment phase

Crop scouting: Stage 3 - Reproductive phase

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