Corn populations and deficit irrigation in Western Nebraska

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click image to zoomGary HergertFigure 1. The field on the left in this early July 2012 photo received 0.5 inch of water earlier and then the remaining 4.5 inches over the next five weeks for a total of 5 inches. Corn in this trial yielded 118 bu/ac at 16,000 population and 125 bu/ac at 24,000 population. The area on the right had received 3.5 inches of the 15 inches it would receive by this time and yielded 200 bu/ac, even in a very dry year. Many producers in the Nebraska Panhandle will be faced with an uncertain water supply this coming year. Surface irrigators are watching snow reports in the Rockies and won’t know their allocations until spring. Many producers in ground water management areas may be nearing or at the end of an allocation period and may have less than their full allocation. With seed corn approaching $4 per 1,000 seeds, producers are questioning what populations they should plant if they face limited water.

The quandary for most producers is that they have seen continually increasing corn yields with adequate water and they have continued to push populations higher as newer hybrids withstand crowding. When you face an uncertain water supply and cannot predict the weather, you need to decide the upside and downside of your decision. If you know you will have less water, how low should your populations be? If you have limited irrigation water but it’s a wetter year, how much yield will you lose with the lower population? If you have a drier year and have a higher population, will it hurt your yield? How much? We attempted to provide research data to answer those questions.

Western Nebraska Deficit Irrigation Research

Deficit irrigation experiments in a no-till winter wheat-corn-dry bean-spring canola rotation have been conducted at Scottsbluff since 2005. The soil is a Tripp, very fine sandy loam with plant available water of 1.5 inches per foot. The primary objective of this experiment was to determine yields from deficit-irrigated corn, winter wheat, dry beans, and canola grown in a no-till cropping system versus full irrigation.

Each phase of the rotation is present each year under a linear move sprinkler irrigation system and the same irrigation level is repeated over time, so low water levels are low year after year. The irrigation levels for the crops are 4, 8 and 12 inches for dry beans, canola and wheat and 5, 10 or 15 inches for corn. The highest irrigation level was designed to be near the long-term average non-ET limiting irrigation. The irrigation levels were not set up as a percentage of ET, rather they were set to correspond to a set allocation a farmer might have. With limited water they would put on normal irrigation, but over a much shorter time period. Repeating the research over years was intended to incorporate real-world temporal variability.

Corn plant populations for the low, medium, and high irrigation levels were originally 16,000/acre, 24,000/acre, and 32,000/acre based on other research, but after the drought years of 2006 through 2008, we decided to look at what we considered “high” and “low” corn populations at each irrigation level. Populations were:

5 inch irrigation —16,000 and 24,000 plants per acre (ppa),
10- and 15-inch irrigation — 24,000 and 32,000 ppa starting in 2009.

click image to zoomFigure 1. Average grain yields from different populations and deficit irrigation rates for 2009 to 2011. (Source: Data in Table 1). The lowest level of deficit irrigation for corn was usually not applied until one week before tassel emergence. We essentially treated it like dryland ecofallow corn but applied the water when it would produce the most yield. Irrigations of 1 to 1.5 inches per week approximating farmer practice were applied from late vegetative stage until water was used. For the higher irrigation levels, irrigation was started earlier in the vegetative period and extended longer into grain fill.

What the Yields Show

Table 1 shows corn yields for the last four years. These are representative good yields for the Panhandle. We are planting 92 to 95 CRM varieties that fit our growing season. Remember, this is not central Nebraska with deep silt loam soils, a two-week longer growing season, and 24 inches of annual rainfall. click image to zoom

The first three years of the experiment turned out much wetter than normal and the higher populations outperformed the lower populations. We did get an abnormally dry year in 2012 and were somewhat surprised by the results.

Let’s look at the results from the wetter years. For the low irrigation level, we should probably have populations near 20,000 to 22,000. For the 10 inches, there was an about a 7 bushel/acre advantage for the additional 8000 seeds. That’s $32 per acre and with $7/bushel corn, it makes sense. With $4.50 corn, you make more money with the slightly lower population. When you have full irrigation, you need to plant higher optimum populations.

Now, what about the “dry” side? Did the higher populations hurt yields, especially at the 5- and 10-inch levels. The answer is “No” at 5 inches, so again a population recommendation of 20,000-22,000 would not hurt yields and provides adequate population if you have a wetter than normal year. At 10 inches, there was no advantage to the 32,000 versus 24,000 populations. If you wanted to hedge your bets, you might consider planting about 26,000. This rate would not reduce yields when you are on the dry side, but would provide sufficient population to produce extra yield if you have above-average precipitation.

Conducting Your Own On-Farm Research

More data from dry years would be nice, but we’re hoping that doesn’t happen. This does provide current information that allows you to consider plant population in the drier regions of the state. Similar data all across Nebraska would be great. There is an opportunity to join the Nebraska on-farm research network and work with UNL Extension educators to do this type of research on your own farm. It could provide good returns in the future. For more information, visit the Farm Research section of CropWatch.

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