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Michigan Study Looks at Yields and Quality When Reducing Irrigation Water Inputs

The Tomato Magazine
Feburary 2005

A Michigan study shows that withholding irrigation immediately after planting tomatoes may save significant amounts of water while increasing fruit yield.

The study, "Can You Reduce Irrigation Water Inputs While Maintaining High Yield and Quality?" was reported during the Great Lakes Fruit, Vegetable and Farm Market Expo tomato session, held Dec. 7, 2004, at the DeVos Place Convention Center in Grand Rapids, Mich. The research was conducted by Mathieu Ngouajio and Ronald Goldy, both with the Department of Horticulture, Michigan State University.

Due to the uneven distribution of rainfall from year to year and within individual growing seasons, most vegetables in Michigan are grown using irrigation, the researchers reported. Unfortunately, it is costly to pump and deliver irrigation water to the crop. Also, water use in agriculture is being increasingly regulated in Michigan, thereby putting more pressure on growers to reduce amounts used. It is therefore important to find crop management practices that reduce the amount of irrigation used while maintaining high crop yield.

"For many years, some plasticulture vegetable growers have withheld irrigation for some time after tomato transplanting," Ngouajio and Goldy said, adding that this practice "may reduce irrigation input and nutrient leaching and promote deeper rooting." Deep roots are believed to help improve yield through more efficient water and nutrient utilization.

In their trials, the researchers planted Mountain Spring, a popular tomato variety, in southwest Michigan using recommended plasticulture practices for such fresh market varieties. The tomatoes were transplanted on May 24, 2003, and May 20, 2004, on raised beds covered with black plastic that had been previously fumigated. Plots were drip irrigated and fertigated. Irrigation treatments were initiated either at planting, after transplant establishment, at first flower, at first fruit or at fruit ripening. A sixth treatment received only enough water to apply fertigation.

The total volume of water applied to each treatment was measured using flow meters, the two reported. During the season, plant height was measured after irrigation was started in all treatments. Fruits were harvested seven times during each year. Weight and number, measured by grade, were recorded. Root depth was estimated at the end of the 2004 season by digging a trench on one side of each bed and measuring the depth of the deepest root.

Total marketable yield was calculated as the sum of yield of all grades except the culls. Irrigation water use efficiency was calculated and expressed in ton/ha/inch of water.

There were differences in the amount of natural rainfall in 2003 and 2004, Ngouajio and Goldy noted. A total of 6.5 inches fell in 2003 compared to 16 inches in 2004. 2003 was considered a dry year versus 2004 which was "more normal."

"In 2003, the volume of water applied varied from 44 inches for treatments where irrigation started immediately after planting to 4 inches in the treatment where irrigation was used only during fertigation," the two researchers said. "In 2004, a similar volume of water was used, but slightly reduced due to more rainfall."

The findings? A significant amount of water can be saved by delaying irrigation after planting with no yield penalty, the researchers said. For example, delaying irrigation until the vegetative stage in 2003 saved about 15 inches of water, representing 34 percent less water. In 2004, 20 percent less water was used by delaying irrigation until the vegetative stage.

"Short-term delay of irrigation after planting did not seem to affect tomato height," Ngouajio and Goldy pointed out. "However, when irrigation was withheld until flowering, plant height was reduced. Delaying irrigation increased root depth. Root depth was 120 cm in the fertigated only treatment and 90 cm in the treatment irrigated from transplanting.

Yield in all treatments was smaller in 2004 compared to 2003, the researchers said. They expressed the view that weather conditions (especially soil moisture) that follow immediately after tomato planting "have a significant effect on final yield."

"It has long been asserted that excessive soil moisture during the first couple of days (or weeks) following planting may have adverse effects on tomato transplants and on final yield," they reported.

Planting in 2003 was followed by about 30 days of dry conditions, resulting in high yields, the two said. In 2004, however, planting was followed by frequent and heavy rains, producing very low yields.

The treatment that was irrigated only for fertigation produced the smallest yield in both years. However, the yield reduction due to lack of water was severe only in 2003 (25 percent yield reduction compared to 5 percent in 2004).

"Tomato yields responded significantly to the different timings of irrigation initiation after planting," the researchers reported. "In both years, withholding irrigation following tomato transplanting showed a positive response in the form of yield increases. In 2003, a 10 percent yield increase was observed by delaying irrigation until the end of transplant establishment. In 2004, further delay until the end of the vegetative stage produced 15 percent more yield.

"The difference may be associated with the difference in rainfall between the growing seasons and is another confirmation that high soil moisture early in the season may not be beneficial for tomatoes," Ngouajio and Goldy noted, adding that their studies show that after planting, it was possible to withhold irrigation until 35 days in 2003 (dry year) and 85 days in 2004 (wet year) without yield losses.

Irrigation water use efficiency (IWUE) is one way of looking at the effect of irrigation on yield, the researchers suggested. This parameter eliminates the effects of natural rainfall in order to estimate the contribution of irrigation to total yield. This is to measure the amount of tomatoes obtained from every inch of water applied.

"IWUE was optimum when irrigation was delayed until the end of the vegetative stage," the two said. "Any treatment where irrigation was started earlier than that stage wasted water. Delaying irrigation beyond that stage was not beneficial because the plants were so affected by the lack of water that they could not efficiently use subsequent irrigation water."

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