Don’t just water. Irrigate!

Jarrod Hardke, University of Arkansas

DR. JARROD HARDKE
ARKANSAS
Asst. Professor/Rice Extension
Agronomist
University of Arkansas Cooperative
Extension Service
jhardke@uada.edu

In recent years, the focus of irrigation management in rice has taken on a whole new twist.

The increase in furrow-irrigated rice, or row rice, has now surpassed 15% of the rice acres in Arkansas. While this practice has the potential to reduce irrigation demand, it is not always the case depending on the situation.

Irrigation is often not needed as frequently in furrow-irrigated rice as many may believe.

Consider rice is a semi-aquatic plant that tolerates a flood but doesn’t require one. Rice may be more prone to drought stress than other rotational crops, but it doesn’t require a flooded soil condition to achieve maximum yields.

In general, without the aid of irrigation tools, such as soil moisture sensors and surge valves, irrigation may be needed on a silt loam soil every three to five days or on a clay soil every five to seven days. It should be noted that time between irrigations can be greater, particularly with the help of heavy rainfall.

The greatest risk from a water management standpoint in furrow-irrigated rice is allowing the soil to become too dry during reproductive growth — that is, from panicle initiation through grain fill. Soil moisture sensors can be particularly helpful during this period to ensure we don’t suffer yield loss due to “watering on a calendar” and allowing the soil to dry too much between irrigations.

multiple inlet irrigation

Multiple-inlet rice irrigation, using poly tubing to irrigate all paddies simultaneously, is currently used on more than 30% of rice acres in Arkansas — photo by Vicky Boyd

Multiple-inlet rice irrigation, using poly tubing to irrigate all paddies simultaneously, is always worth mentioning. We currently use this practice on more than 30% of rice acre in Arkansas. When you take zero-grade fields out of that equation, the use is higher. However, there are still many acres that can benefit from this practice.

Using MIRI allows us to flood paddies up evenly, minimizing water loss from trying to cascade water down the field through the gates. In addition, we can set gates higher to allow us to capture rainfall. The biggest burdens are the cost of the pipe and the time to install the pipe and get the gates set, but these can be offset by water savings and management efficiency.

Try MIRI on a field, but be sure to use a rice irrigation app to determine the number of holes needed per paddy or you may minimize the benefits of the practice. Not every field will be as easy to institute the practice, so start with a simpler field and expand from there.

Whether it’s irrigation or anything else rice related, contact your county Extension agent or a specialist if we can help. Best of luck in 2021.

‘Best thing you can put on your field is your shadow’

Luis Espino

DR. LUIS ESPINO
UNIVERSITY OF CALIFORNIA
Rice Systems Adviser
laespino@ucanr.edu

When I was a graduate student in Texas, my major professor, M.O. Way (happy retirement Mo!) always used to tell rice growers in his presentations: “The best thing you can put on your field is your shadow!” He is right.

Monitoring is key for good pest management and to achieve a good stand. Recent experiments have shown that for M-206, the stand that maximizes yield is 25 plants per square foot.

When the stand was reduced to 12.5 plants per square foot, yields were reduced 10%. These numbers can change with variety and year, but they give you a guideline of what to shoot for when assessing stand.

For weed management, herbicide programs are chosen ahead of planting; however, monitoring is needed to make sure you are applying them at the right growth stage of the weed and rice. The UC IPM website has good pictures of rice weeds as seedlings, so you can properly identify them early in the season.

Monitoring for weeds will allow you to adjust your weed control program to address weed population shifts or resistance issues the following year.

When it comes to arthropods, tadpole shrimp and rice seed midge are the pests you want to be on the lookout for soon after planting. Factors that affect these two pests are the length of time to flood and temperature.

When fields take a long time to flood, shrimp have more time to develop to a size that can injure rice; similarly, midge adults will have more time to lay eggs in the water, resulting in a large number of larvae.

seed midge

Rice seed midge damage to germinating rice seed — photo by Jack Kelly Clark, UC Statewide IPM Program

Development of these two pests will be faster when it is warm. Inspect the water and the seeds — the water for presence of tadpole shrimp and the seeds for signs of midge or shrimp feeding. Tadpole shrimp will feed on the germinating coleoptile or roots, cutting them. Midge can do this and consume the inside of the seed. In some cases, midge can feed on the first rice leaves, but this type of feeding usually does not cause a problem. Once the seedlings have a well-developed spike, the risk of damage by these pests is small.

With regard to diseases, seed rot and seedling disease can reduce stands. These two diseases will affect the germinating seeds soon after seeding. Generally, they are a problem during cool springs. The slow seed germination due to cool temperature gives the pathogens more time to produce infections.

To determine if seeds are infected, observe them under water and look for whitish or greenish outgrowths of mycelium coming out of the seed. When the water is muddy, this can prove challenging — pushing a clear glass jar to the bottom of the water can help.

Seedling weed ID is key to early season weed control

ronnie levy

DR. RONNIE LEVY
LOUISIANA
Extension Rice Specialist
RLevy@agcenter.lsu.edu

Identification of early season seedling weeds is often difficult due to the small size and similarity of various seedlings. The “Schematic Diagram for Seedling Weed Identification in Rice” publication is available on the LSU AgCenter’s website (https://bit.ly/38YtWVr).

Weeds are some of the most troublesome pests in rice production in the United States and throughout the world. They compete with rice for water, nutrients, space and light. Direct losses from weed competition can exceed 50%.

Indirect losses, such as increased costs of harvesting and drying, reduce quality and increase dockage at the mill. These losses are not easy to measure but can greatly reduce profits.

Therefore, weed control measures should include all activities used in different production practices and systems.

Although weeds vary in their ability to compete with rice, most fields contain a complex of weeds that will reduce yield and quality if an appropriate weed management strategy is not implemented.

Rice weed control is best accomplished by using a combination of cultural, mechanical and chemical management practices. Relying on a single control practice seldom provides adequate weed management.

amazon sprangletop

This two- to three-leaf Amazon sprangletop can easily be mistaken for other grass species — photo courtesy LSU AgCenter

While using water to control weeds can be a major asset, thorough knowledge of weeds present in each field is critical in developing appropriate management strategies. The most important factor in weed control is selecting the proper herbicides in your weed management program.

The program should begin the previous year or two prior to planting.

A history of the weed pressure will help identify which ones will require the most herbicide dollars. Keeping visual and written records of weeds, herbicide treatments and their control for each field is important. Don’t trust your memory.

Six basic application timings should be considered when choosing your herbicide program: (1) burndown prior to planting, (2) preplant incorporated, (3) preemergence prior to planting, (4) preemergence after planting, (5) delayed preemergence (drill-seeded only) and (6) postemergence.

As you can see, most timings target weeds prior to rice emergence or small weed seedlings. Yield losses occur from early competition. Don’t wait!

General weed control programs are often decided before the season begins and are determined based on the past weed species present in a field and past success or failures in weed control.

However, you should always scout fields for weed species present and modify herbicide applications depending on the ones in the field.