Applying extra water to pivot-irrigated alfalfa in May and June can help bring in higher yields later in the summer.
Adding more water than the crop needs early in the season fills the soil root zone, ensuring that sufficient water will be available when the crop needs it most. It's necessary, says Howard Neibling, because pivots typically can't keep up with alfalfa's water needs during the hottest summer months.
“Most center pivots are not designed to meet the crop demand in the middle month and a half to two months of the season,” says Neibling, a University of Idaho extension water management engineer.
Alfalfa's fastest growth and greatest water needs occur from mid-June to early August. The weather is hot and often dry and windy, so evaporation losses also are high.
During that period, alfalfa may need 0.35" of water per day. But a typical pivot system can apply only about 0.29". The difference must come from stored soil moisture, and if that runs out, crop growth slows or even stops.
“At least in this area of the country, it takes about 5” of water to grow 1 ton of hay,” says Neibling. “So if you end up with a cumulative shortage of 5”, you've lost a ton of hay. And it can get worse than that.”
Even if your system can apply enough water to meet peak demand, your yields are likely to suffer if the soil profile isn't full at the start of that period. That's because each pivot trip around the field supplies only enough water to rewet the top 12-15".
“So even though alfalfa is a very deep-rooted crop, you're only able to exploit the nutrients from the top foot to 15" if it's dry below there,” he says.
“With a hand line or wheel line system, or with surface irrigation, you can usually add enough water per irrigation to refill the soil profile whenever you want,” he adds. “But with a pivot you simply can't.”
To make sure you'll have enough water during the critical late-summer period, dig a hole shortly before the crop breaks dormancy in early spring. Dig 3-4' deep with a soil auger, posthole digger or other tool. Alfalfa roots go deeper than that, but most water uptake happens in the top 4', says Neibling.
Dig the hole where it's representative of soil conditions in most of the field. If the field has a wide range of soil conditions, you may want to dig more than one hole.
If soil around the hole or holes doesn't have ample moisture from top to bottom, consider applying extra water during early season irrigations. In Idaho, mid-May to mid-June is the logical period to do it. But don't fill the soil profile before the crop is growing well.
“You don't want to do it too early or it actually cools the soil and retards crop growth,” says Neibling.
Occasionally, a very early spring irrigation is warranted. Most years, wintertime precipitation is enough to get the crop off to a good start. But the soil may be dry following an open winter. The crop will start slowly and yields will suffer if it doesn't have adequate moisture when it breaks dormancy.
“So when you're digging before or at about the time the crop breaks dormancy, see how much moisture you've got,” Neibling advises. “If the soil is really dry, give the crop a good drink to get it started.
“Then you'll need to fill the rest of the soil profile before the middle of June. And you'll do that by running the system to put on water in excess of crop use.”