A study was conducted in summer, 1998 at the Texas Agricultural Experiment Station in Amarillo/Bushland with the objective to evaluate the effect of different irrigation regimes on sorghum grown at five planting densities. The long term goal of this research is to identify the optimum irrigation regime that will minimize waste of irrigation water, reduce pumping expenses, and at the same time maximize yield.

The field study was conducted at the USDA-ARS, Conservation & Production Laboratory, Bushland, Texas. Sorghum variety Cargill 647 was planted at five population densities (30,000 plants per acre (ppa), 70,000 ppa, 105,000 ppa, 135,000 ppa, and 198,000 ppa) in twelve-row plots on May 15, 1998. Irrigation was supplied by a center pivot irrigation system, with 6" drops equipped with LEPA nozzles.

The land under the center pivot was farmed in a circle to reduce runoff and improve irrigation distribution uniformity. Different amounts of water were applied based on PET recommendations, 75% of PET and 50% of PET. Multiple infrared thermometers (IRTs) were directly attached to the center pivot to record real time plant stress conditions while the pivot was moving (Figure 1), and plant water status was determined weekly using thermocouple psychrometers.

         On June 26, and August 13, 1998, 2 meters subplots were harvested and plant fresh and dry weight was determined to monitor plant growth. On July 28, the number of heads was determined on a 3-meter subplot. On October 19, 1998, plots were harvested using a combine equipped with GPS yield monitoring device. In addition to the combine harvest, each plot was manually harvested and grain yield calculated.

Environmental conditions were particularly dry during the 1998 summer growing season, making this a difficult year for growing a high yielding sorghum crop.

Table 1 shows the differences in fresh and dry weight among the five plant populations at two harvest dates. On June 26, as plant population increased so did fresh weight. Only plant populations 4 and 5 showed no significant differences between them, indicating that the difference in plant density had no effect on fresh weight production for these two treatments by the June 26 sampling date. These same differences were also reflected in the dry weight. By the second harvest date (August 13, 1998) there were no significant differences in fresh and dry weight among the five plant populations, indicating that plant growth was the same regardless of the planting density.

The effect of irrigation treatment on the fresh and dry weight of the 5 plant populations at two harvest dates is shown in Table 2. No significant differences were found among the treatment irrigated at 50, 75 and 100% PET at the first harvest date. This suggests that if water is a limiting factor and a full water profile is available at planting, growers could water their crop at the beginning of the season using 50 % of the recommended PET and obtain plant growth similar to a crop fully irrigated. However, later in the season, differences in fresh and dry weight were significantly affected by irrigation treatments. By the time of the second harvest, reduced irrigation significantly reduced plant growth.

A significant interaction between plant population and irrigation treatment was found on July 28 in terms of number of heads per unit area (Table 3). Plant population 1 and 2 showed no significant differences when irrigated at 100, 75 or 50% PET. Plant population 3 and 4 showed no significant reduction in number of heads when irrigated at 100 or 75% PET. Plant population 5 significantly reduced the number of heads per unit area when irrigation became limited. These results support those from first harvest (Table 1). Until mid-July, with the environmental conditions of this study, growers could have watered 50% of the recommended PET with a low plant population or 75% PET with a medium plant population without compromising plant growth or number of heads.

Table 1: Fresh and dry weight (in grams) differences among five sorghum plant populations at two harvest dates.

	June 26 harvest		August 13 harvest
	Fresh weight (g)	Dry weight (g)	Fresh weight (g)	Dry weight (g)
Plant Population 1	165.93 D	30.33 D	3084.2 A	939.17 A
Plant Population 2	300.86 C	55.36 C	3070.0 A	913.06 A
Plant Population 3	438.76 B	79.51 B	2976.7 A	909.72 A
Plant Population 4	551.51 A	104.45 A	2953.3 A	865.00 A
Plant Population 5	601.35 A	118.28 A	2887.8 A	851.00 A

Plant population 1, 2, 3, 4 and 5 represent 30,000, 70,000, 105,000, 135,000 and 198,000 plants per acre respectively.
Means followed by the same upper case letter within a column are not significantly different.

Table 2: Fresh and dry weight (in grams) differences among three irrigation regimes at two harvest dates.

	June 26 harvest		August 13 harvest
	Fresh weight (g)	Dry weight (g)	Fresh weight (g)	Dry weight (g)
100% PET	419.84 A	79.02 A	3433.0 A	984.67 A
75% PET	428.03 A	78.98 A	3030.5 B	906.33 B
50% PET	387.16 A	74.75 A	2519.7 C	784.17 C

Means followed by the same upper case letter within a column are not significantly different.

Table 4 shows the yield differences among plant populations at each irrigation regime. Plant population 1 and 2 did not significantly reduce their yield when irrigated at 75 % PET compared to their yield at 100 % PET. Plant population 3, 4 and 5, on the other hand, had a significant reduction in grain yield as soon as water became limited.  This result is explainable in terms of plant competition for water. High plant populations generally result in less available water per plant compared to low plant populations. However, within each irrigation regime, only plant population 5 had significantly lower yield than all the others did. Considering that within each irrigation regime number of heads decreased as plant population decreased (Table 3), plants grown at lower densities compensated for fewer heads with heavier seed.

Table 3: Number of heads in 3 meters subplot among five sorghum planting densities.

	100 % PET	75 % PET	50 % PET
Plant Population 1	14.6 D a	12.8 D a	10.5 B a
Plant Population 2	32.6 C a	33.1 C a	27.6 A a
Plant Population 3	50.3 B a	45.3 CB a	25.1 A b
Plant Population 4	60.0 B a	48.5 B a	20.1 A b
Plant Population 5	72.5 A a	63.0 A b	24.0 A c

Plant population 1, 2, 3, 4 and 5 represent 30,000, 70,000, 105,000, 135,000 and 198,000 plants per acre respectively.
Means followed by the same upper case letter within a column are not significantly different.
Means followed by the same lower case letter within a row are not significantly different.

Table 4: Yield differences (lb/acre) among five sorghum planting densities.

	100 % PET	75 % PET	50 % PET
Plant Population 1	7473.8 A a	6859.5 A ab	5405.5 A b
Plant Population 2	7805.6 A a	6669.8 A ab	5101.6 A b
Plant Population 3	9227.5 A a	6748.9 A b	3761.3 A c
Plant Population 4	8201.1 A a	6918.4 A b	4288.2 A c
Plant Population 5	7130.1 B a	5585.1 B b	2775.3 B c

Plant population 1, 2, 3, 4 and 5 represent 30,000, 70,000, 105,000, 135,000 and 198,000 plants per acre respectively.
Means followed by the same upper case letter within a column are not significantly different.
Means followed by the same lower case letter within a row are not significantly different.

This result is of particular importance because it suggests that growers could achieve relatively high yields by watering their crop at 75% of the recommended PET rate if they introduce a low plant density. Such an irrigation regime would greatly save water and increase growers’ profits by reducing the cost of pumping. Furthermore, regardless the irrigation regime adopted, the results of this study showed that lower plant populations have the potential to achieve yield as good as those from medium planting densities and should be implemented when water availability is limited.

Table 5 shows the mean leaf water potential data taken during the growing season. Leaf water potential is an indicator of plant water stress. More negative values indicate a higher water stress. These results followed the same trend as the yield data. Regardless of the irrigation regime, plant population 5 was always under more severe stress than all other plant populations.

Table 5: Leaf water potential differences (Bars) among five sorghum plant populations.

	100 % PET	75 % PET	50 % PET
Plant Population 1	-1.7 A	-7.8 A	-13.4  A
Plant Population 2	-1.8 A	-8.3 A	-14.8  A
Plant Population 3	-1.7 A	-7.7 A	-13.9 A
Plant Population 4	-2.0 A	-7.9 A	-13.5  A
Plant Population 5	-3.1 B	-11.2   B	-19.7  B

Plant population 1, 2, 3, 4 and 5 represent 30,000, 70,000, 105,000, 135,000 and 198,000 plants per acre respectively. More negative values represent higher stress.
Means followed by the same upper case letter within a column are not significantly different.

Infrared thermometers were effective in differentiating water stress treatments (Figure 1). The red, orange and green areas represent areas where the crop was irrigated at 50 %, 75 % and 100 % PET respectively. Further data interpretation is needed to verify if IRTs can distinguish among the different plant populations.

Even with the difficulties of a very dry summer, this study produced encouraging results, indicating optimum plant populations and the feasibility of introducing remote sensing instrumentation for detecting plant stress and managing irrigation accordingly. Just from the results of this year's study alone, it was easy to see the potential savings in water and associated pumping costs that a grower could achieve by implementing water-saving measures in conjunction with a plant population suited to the available water.

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