Monitoring Early Crop Development

Challenging planting conditions could affect crop emergence and early development. Additionally, a significant portion of the corn and soybean acreage was planted with limited opportunity for pre-emergence herbicide application. These factors will increase the importance of early crop scouting and staging to assess stands establishment and determine proper timing of post-emergence herbicide application.

Stand Establishment

An easy way to calculate corn population is by counting the number of plants in 1/1,000 of an acre and multiplying the count by 1,000 to obtain the number of plants per acre. Table 1 lists the row length equal to 1/1,000 of an acre at various row widths. For standard 30 inch corn rows, count the number of plants in 17 feet, 5 inches of row and multiply that number by 1,000 to calculate the plant population per acre.

Table 1 – Estimating Plants Per Acre Using Row Lengths

What’s 1/1000 of an acre
Row Width In Centimetre (inches)	Length of Row Equal to 1/1,000 Acre
38.0 cm (15″)	10.62 m (34 ft., 10 in.)
50.8 cm (20″)	7.97 m (26 ft., 2 in.)
76.2 cm (30″)	5.33 m (17 ft., 5 in.)
81.3 cm (32″)	4.98 (16 ft., 3 in)

To determine plant population in narrow row crops such as 7 or 15 inch row soybeans, place a sampling frame with a known area on the ground to do the count. This is most easily done with a circular frame, either by making one out of plastic hose or using a Hula-hoop. The circular frame or hoop method is shown in Table 2. Using the hoop, determine the number of plants per acre by counting the number of plants found inside the hoop and multiplying that number by the predetermined factor for the diameter of your hoop, listed in Table 2. Figure 1 is an example of a 24 inch diameter hoop, where the number of plants (eg. soybeans, weeds, etc.) inside the hoop is multiplied by 13,865 to determine the number of plants per acre. Similarly, Table 3 lists plant population per acre for various plant counts using a 24 inch (61 cm) diameter hoop. Table 4 lists the plant count per foot of row for the different cereals planted in 7½ inch rows to obtain the target population (not counting tillers).

Table 2 – Estimating Plant Per Acre Using a Hoop

Diameter of Hoop	Factor by Which to Multiply the Number of Plants Within the Hoop to Equal the Number of Plants Per Acre
91 cm (36″)	6,165
81 cm (33″)	7,334
76 cm (30″)	8,874
69 cm (27″)	10,956
61 cm (24″)	13,865

Figure 1: 24 inch hoop

24 inch hoop: # of plants X 13,865 = plants/acre

24 inch hoop: # of plants X 13,865 = plants/acre

Table 3 – Plant Population Using a 24 inch (61 cm) Diameter Hoop

Number of Plants Inside the Hoop	Plant Population (plants per acre)
3	41,600
5	69,300
7	97,000
9	124,800
11	152,500
13	180,200
15	208,000

Table 4 – Target Population for Cereal Crops Seeded in 7.5 Inch Rows

Crop	Plants per Foot of Row
Barley	14 to 21
Oat	12 to 18
Mixed grain	12 to 21
Spring wheat	18 to 23
Winter wheat	21 to 26

Regardless of the method used to determine plant population levels, at least 10 random counts should be taken in each field to determine an average.

Crop Development & Weed Control

Accurate crop staging is essential to maximize the efficacy of a weed control. For crop safety, most herbicides need to be applied at a particular growth stage. Additionally, early in the growing season crops need to be kept weed-free for a specific growth period to minimize yield loss due to weed competition. The weed-free period, called “critical period of weed control” is the crop growth stages during which the crop must be weed-free to prevent a yield loss of more than 5% from weed competition. If weeds are controlled throughout the critical period, the weeds that emerge later will not affect yield and can be controlled prior to harvest, if necessary, with a harvest aid to burn down the weeds. The weed-free period is specific to each crop and can vary somewhat depending on weather, soil type, weed pressure and growing conditions. For example, the critical weed-free period will be slightly earlier in the growth stages for fields with light-textured soils under moisture stress conditions when weed densities are very high.

For corn, the critical weed-free period starts at the 3 leaf stage and extends to the 8-leaf stage of the corn. For soybeans, the critical period extends from the 1st to the 3rd trifoliate stage of soybean growth. Excellent weed control must be maintained throughout this critical period. In cereals, most broadleaf weed herbicides should be applied when the cereals are in the 2 to 5 leaf stage. In new forage seedings, most broadleaf weed herbicides should be applied when alfalfa, bird’s-foot trefoil or clovers are in the 1-4 leaf stage and seedling forage grasses are at the 2-4 leaf stage. Refer to the label for specific herbicides.