Row Spacing, Plant Population, and Yield Relationships

by Jeffrey C. Silvertooth,
Extension Agronomist - Cotton

The manipulation of row spacing dimensions, plant populations, and the overall spacial arrangement of crop plants in a field has been the subject of considerable discussion among farmers and agronomists for many years. The crop canopy has often been manipulated by row spacing and population adjustments in an attempt to improve yields, production efficiencies, and profits. Similarly, plant breeders have altered plant architecture in an effort to improve light interception by crop plants. The development of conventional, narrow, and ultra narrow (UNR) crop systems and breeding for plants with columnar or bush-type architecture are examples of these types of manipulations to improve yield.

The process of crop production requires the conversion of energy in sunlight into dry matter by fixation of CO2 from the atmosphere through the process of photosynthesis. Leaves are the primary site of photosynthesis in crop plants. Therefore, one might assume that the greater the number of leaves in a field, the better interception of sunlight and higher the yield.

A common way of describing the surface area of leaves in a field is by the leaf area index (LAI). The LAI is the ratio of the leaf surface area (upper side only) of the crop to the ground area. The LAI for a crop will normally increase as the crop canopy develops. LAI values of greater than or equal to 4 (i.e. 4 times more area of leaves than ground) have been measured for irrigated cotton in Arizona. Cotton plants will also be effected to some extent by the fact that the leaves track the movement of the sun (heliotropic movement), which increases interception of light by the canopy. However, with an increasing LAI there will also be an increasing amount of shading of lower leaves in the canopy. As a result, agronomists recognize that an optimization of LAI is important in realizing the most efficient interception of sunlight and optimum photosynthesis.

Another primary objective in a crop production system is to direct a high proportion of the dry matter into harvestable parts of the crop plant. The relationship between the harvestable portions of the plant (e.g. lint or seedcotton yield in cotton) and the total amount of dry matter produced by the crop is often referred to as the harvest index (HI). An important goal in a crop production system is to generate a high HI. For cotton production systems this translates to producing as many bolls per unit area (e.g. per acre) as possible to realize high yields. Therefore, maximum cotton production is a direct function of efficiently utilizing resources such as sunlight, water, and nutrients. The balance between vegetative and reproductive growth (i.e. harvest index for cotton) is critical in relation to any effort to improve yields.

As noted above, an essential aspect of any crop production system is the development of a crop canopy that optimizes the interception of light, photosynthesis, and the allocation of dry matter to harvestable plant parts (yield). A crop canopy is commonly managed by manipulating row spacing, plant population, and plant type. Yield per unit area generally increases with plant density. Although, as plant density is increased yield per unit area will approach an upper limit, plateau, and then decline. Yield per plant tends to decrease with increasing plant density because competition for resources (light, water, and nutrients) between adjacent plants intensifies. Optimal plant populations (densities) for both conventional and narrow row cotton production systems often range between 30,000 to 60,000 plants per acre (ppa). Acceptable cotton plant populations have been reported between 20,000 to 75,000 ppa for irrigated cotton production systems in the desert Southwest. The optimal plant population for a given field will depend to some extent on soil type, variety, weather, pest populations, etc.

In modern cotton production systems conventional row spacings commonly range from 36 to 40 inches. Narrow row systems are those less than 36 inches, often having 30 inch row spacings. In recent years, UNR cotton systems have been developed and evaluated in many parts of the US cottonbelt. The UNR systems that have been developed commonly have 6 to 10 inch row spacings and plant populations of 100,000 to 120,000 ppa. In general, these UNR systems have shown the most promise for drastically reducing the length of the growing season and increasing production on marginal lands. Therefore, UNR systems have been touted as having the potential of improving profitability by reducing inputs, risk, and increasing the ratio of yield to inputs. Success with UNR systems require control of plant population and uniformity, plant height with high population, and the use stripper harvesters (a complete change in harvesting equipment for picker harvesting areas). Additional concerns include difficulties in ginning and maintaining good grades. To date, very little information is available relative to the feasibility of developing or utilizing UNR cotton production in Arizona. Despite this fact, a number of Arizona growers are interested in developing and testing UNR systems in several areas around the state in 1999.

The development of UNR systems represents an interesting way to manipulate crop canopy architecture, but it is difficult to predict how well this will work under irrigated conditions in Arizona. A conventional cotton production system in Arizona (e.g. an Upland cotton variety, 40 inch row spacing, and approximately 40,000 ppa) will commonly produce plants with an average of 21 fruiting branches by the time the crop experiences cut-out (end of primary fruiting cycle). Assuming the plants have at least 2 fruiting sites on each fruiting branch, the plants would have an average of at least 42 potential fruiting sites. With 50% retention of those sites at harvest and 3 plants/foot, this would provide approximately 63 bolls/foot of row. Using a general yield estimate of 20 bolls/foot equating to 1 bale lint/acre, this field would have a 3 bale yield potential. For a UNR system to be more profitable a greater number of bolls would have to be produced per unit area, or an equal number produced with lower inputs. Seemingly, the goal of a UNR system would be to optimize the earliest (lower) fruiting sites on the plants. Thus, the emphasis in crop management will likely be oriented toward the early stages of the growing season and fruiting cycle.

Variations in crop canopy architecture as effected by row spacing, plant population, and plant type have been topics of interest for many generations and will continue to be subjects of further study and development. Results with the UNR systems that are being developed and tested should be followed with great interest. We need to evaluate them thoroughly and objectively and document the potentials they appear to offer.

Full Disclaimers

Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director Cooperative Extension, College of Agriculture and Life Sciences, The University of Arizona.

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