Other Corn Production Issues

Early Season Cold Stress

Frost damage in May or June will generally have little impact on the crop, provided the growing point of the corn plant is still below the soil surface. This is the case until the young plant reaches roughly the sixth-leaf stage (V6). On more advanced plants and/or where damage is more severe, split the stalks to see if the growing point has been damaged. This procedure will require some time to make the correct recommendation. It takes about 3–5 days following a frost to accurately determine the degree of damage to verify the presence of healthy growing points (yellowish-white and firm) or to see new leaf growth.

Frozen leaf tissue bleaches to a straw colour several days after freezing. In some cases, it also develops a “knot,” which may restrict expansion of the undamaged tissue lower in the whorl (Figure 1). Producers have attempted to mow frost injured fields to clip these knots and help the plant recover though research has shown plants can recover as quickly and yield just as much if they are left alone.

If the forecast calls for a risk of frost, consider delaying inter-row cultivation, nitrogen side-dressing or herbicide applications until warmer temperatures return. Soil disturbance at the surface introduces more air into the soil and insulates the corn plants from the heat of the soil mass, thus increasing the risk of frost damage. Similarly, crop residues and weeds act as a barrier for heat transfer from the soil to the corn plant. Dry soils are more prone to frost damage due to their lower capacity to store heat during the day and thus less heat to transfer and protect the corn plant overnight.

Heat Stress

Heat stress is different from drought stress. (Figure 2). Corn can usually tolerate temperatures as high as 38°C before injury occurs, as long as drought conditions are not present as well. Temperature and drought sensitivity varies by hybrid. Drought-tolerant hybrids may result in yield drag and are not good hybrids to use in a normal growing season.

Hail

Corn plants damaged by hail may experience a reduction in leaf surface area, bruising of the stalk and ear, and in serious incidences, stalk breakage (Figure 3). Hail damage may also provide an entry point for diseases such as smut. Yield loss due to hail is dependent on the stage of the crop at the time of the hail event and the level of defoliation. Yield loss is greatest when the corn is defoliated during tasselling. Younger plants may experience a delay in growth and development due to hail, but yield loss is usually minimal. Yield loss is minimal when defoliation of plants occurs near maturity. See Table 1, Estimated percentage corn grain yield loss due to defoliation at various growth stages when making yield loss estimates due to hail damage.

Table 1. Estimated Percentage Corn Yield Loss due to Defoliation at Various Growth Stages

Growth Stage¹	% Leaf Defoliation
	10	15	20	25	30	35	40	45	50	55	60	65	70	75	80	85	90	95	100
	% Yield Loss
7 leaf	0	0	0	0	0	0	1	1	2	3	4	4	5	5	6	7	8	9	9
9 leaf	0	0	0	1	1	2	2	3	4	5	6	6	7	7	9	10	11	12	13
11 leaf	0	0	1	1	2	3	5	6	7	8	9	10	11	12	14	16	18	20	22
13 leaf	0	1	1	2	3	4	6	8	10	11	13	15	17	19	22	25	28	31	34
15 leaf	1	1	2	3	5	7	9	12	15	17	20	23	26	30	34	38	42	46	51
17 leaf	2	3	4	5	7	9	13	17	21	24	28	32	37	43	48	53	59	65	72
18 leaf	2	3	5	7	9	11	15	19	24	28	33	38	44	50	56	62	69	76	84
19–21 leaf	3	4	6	8	11	14	18	22	27	32	38	43	51	57	64	71	79	87	96
Tassel	3	5	7	9	13	17	21	26	31	36	42	48	55	62	68	75	83	91	100
Silked	3	5	7	9	12	16	20	24	29	34	39	45	51	58	65	72	80	88	97
Silks brown	2	4	6	8	11	15	18	22	27	31	36	41	47	54	60	66	74	81	90
Pre-blister	2	3	5	7	10	13	16	20	24	28	32	37	43	49	54	60	66	73	81
Blister	2	3	5	7	10	13	16	19	22	26	30	34	39	45	50	55	60	66	73
Early milk	2	3	4	6	8	11	14	17	20	24	28	32	36	41	45	50	55	60	66
Milk	1	2	3	5	7	9	12	15	18	21	24	28	32	37	41	45	49	54	59
Late milk	1	2	3	4	6	8	10	12	15	18	21	24	28	32	35	38	42	46	50
Soft dough	1	1	2	2	4	6	8	10	12	14	17	20	23	26	29	32	35	38	41
Early dent	0	0	1	1	2	3	5	7	9	11	13	15	18	21	23	25	27	29	32
Late dent	0	0	0	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
Mature	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Adapted from the National Crop Insurance Services Corn Loss Instruction (Rev. 1994). Used with permission.
¹As determined by counting leaves using the leaf-over method (i.e., those with 40%–50% of leaf exposed from whorl and whose tip points below the horizontal).

Flooding

Flooding stresses the plant by cutting off the supply of oxygen to the root system. Younger corn plants die if submerged in water for more than 5 days, especially in warmer weather conditions. If air temperatures are high, death may occur in only a few days, as plant processes are sped up and the need for a supply of oxygen to the roots is high. In cooler weather, submerged plants may live for up to a week. After the 8-leaf stage of corn, plants can tolerate being submerged in water for more than 8 days but may be more susceptible to disease (e.g., crazy top, Figure 4) and may experience limited root development while under water. Yield loss due to flooding is most substantial for plants submerged immediately before and during tasselling and silking. Plants in the later vegetative growth stages (10–16 leaves) and/or during the grain filling period, suffer little yield loss to flooding.

Drought Stress

The corn crop requires approximately 50 cm of water to produce high yields. This can be supplied over the growing season from a combination of stored water in the soil, rainfall or irrigation.

Lack of water causes the leaves to wilt and turn a greyish colour (Figure 5). Corn is most susceptible to dry conditions during the tasselling-to-silking stage and may experience yield loss if under stress at this time. During the later vegetative stages of growth (V8–V14), the plant may benefit from dry conditions, as it forces the more rapid downward growth of the roots. Drought conditions during silking can reduce pollination and a lack of silk emergence, while drought after silking may cause a reduction in grain fill.

Bird Damage

Birds can damage emerging seedlings. However, the more serious bird damage occurs to grain in August and September (Figure 6). Birds eat the kernels off the cob causing direct yield loss. Kernel damage may result in mould growth. Birds can also damage the ear while searching for ear feeding insects like western bean cutworm. Bird damage can be easily confused with seedling damage caused by black cutworms or ear damage caused by grasshoppers. Noisemakers, propane cannons, exploding shotgun shells, the Phoenix Wailer and recordings of bird distress calls may be successful deterrents if more than one technique is used and their pattern is changed frequently. If crop damage due to birds or wildlife is substantial, contact your local Ministry of Natural Resources and Forestry (MNRF) office for control options.

Late Season Cold Stress

Cold temperatures during the grain-filling period in August and September may cause yield and quality losses. The extent of these losses depends on the developmental stage of the corn and the temperatures recorded.

As temperatures drop to 0°C, frost damage first occurs to the leaves of the corn plants. This damage will eliminate any further photosynthesis, reduce grain filling and will often have a negative effect on stalk strength. However, as long as air temperatures do not fall below -2°C, stalk tissues will remain viable and stalk constituents will be mobilized to fill the ear as much as possible. If temperatures fall below -2°C, both leaves and stalks may be damaged and no further photosynthesis or remobilization can occur. This will terminate grain filling, and kernel black layer will develop. Table 2, Estimated risks to grain corn yield and quality from late-season frost damage outlines the potential risks to yield and quality for grain corn experiencing different levels of frost damage.

Table 2. Estimated Risks to Grain Corn Yield and Quality from Late-Season Frost Damage

Note: This table is meant as a guide; differences among hybrids, overall plant vigour at time of frost and subsequent temperatures will all affect final grain yield and quality.
Crop Growth Stage	Frost Damage	Estimated Grain Yield Loss	Grain Quality Concerns
Mid-dough	Leaves only	25%	Severe
Mid-dough	Complete plant	40%	Severe
Early dent	Leaves only	15%	Moderate
Early dent	Complete plant	25%	Moderate
Half milk line	Leaves only	0%-5%	None
Half milk line	Complete plant	10%	Minor

Generally, the early dent stage is the cut-off point where corn can withstand frost damage to the leaves and still produce a reasonable grain yield. This stage is characterized by having kernels showing small indentations in the crown of the kernel, at least in the lower half of the cob.

The other question regarding cold nights revolves around the corn crop’s ability to continue grain filling after experiencing several cold nights without frost damage. Dr. Thys Tollenaar formerly of the University of Guelph conducted research that measured 50% reductions in photosynthesis and rate of grain filling due to cold nights of 2°C. When these plants were restored to higher temperature conditions, they resumed plant activities at rates similar to those plants that had never experienced the low temperatures. If cornfields can escape any serious frost damage during cold nights, grain filling should resume once normal temperatures return.

In some situations, frost damage will preclude harvesting the crop as grain and will force the producer to consider harvesting it as silage. There are important concerns involving frost damage in silage corn as well. Following a frost, silage corn frozen before reaching the half milk line on the kernel may be too high in moisture to properly ensile. Ideally, in cases of frost, delay corn harvest until the entire plant reaches the desired moisture content for ensiling.