Cotton growers aiming for top yields must pay close attention to plant population.

Texas studies indicate that, for optimum irrigated cotton yields, populations should range from 1.5 to 4.5 plants per foot of row in 40" rows. For maximum yield, aim for 2.5 plants per foot of row. And dryland producers can expect best results at about two-thirds of those levels.

Studies by Randy Boman, a Texas Ag Extension Service agronomist, also show that a too-high or too-low plant population can cause problems.

"Our optimum window of plant populations is 20,000 to 50,000 plants per acre," says Boman. "Anything higher or lower normally results in yield reductions.

"If plant populations are too low, there will be fewer fruiting branches and more vegetative branches," he explains. "Plants will be taller and wider and will produce a higher plant yield. But, there will be lower yield per unit base. There will be lower harvesting efficiency and more sticks in stripper-harvested cotton."

While 2.5 plants per foot usually maximizes yield, it remains above 90% of the maximum at the 1.5 level, but quickly drops off at lower populations.

Higher plant populations bring different problems.

"There is lower yield per plant, lower fiber quality, smaller and more compact plants and a smaller stalk diameter," says Boman. "The root system is more shallow. This increases the height of the first fruiting branch and first boll. The earliness of plants is affected because, as population increases, boll retention can decrease."

Yields don't drop as dramatically with higher populations, but still go from about 93% of maximum with 4.5 plants per foot to just above 80% for 5.5.

Seed size varies. Large-seeded varieties have 3,000 seeds per pound, while small-seeded types have more than 5,000 seeds per pound. Paymaster HS26 and HS200, common varieties, have about 4,400 seeds per pound.

The germination rate normally is 80%, and a seedling survivability rate of 60- 70% can usually be expected due to cool, wet periods just after planting.

"Growers should target stands for the upper portion of the optimum production range to obtain a cushion for stand loss," Boman suggests. "For a target stand of five plants per foot of row, they should plant about six seeds per foot, if 80% germination is expected.

"If the environment causes a one-third loss, they would still have four plants per foot."

At six seeds per foot of row, about 78,000 seeds should be planted per acre. That calculates to about 18 lbs of seed, based on 4,400 seeds per pound.

Growers with 30" or narrower rows should aim for a similar number of plants per foot of row.

"The stand minimum distance of about 3" per plant, or about four plants per foot, remains constant regardless of the row spacing," notes Boman. "By narrowing the distance between rows, the number of plants can be increased without crowding."