N management in wet year: revised based on cool weather.
Earlier this week I sent out a blog discussing N management during a wet year. In the blog, one scenario (Scenario 2) mentioned was “I decided not to apply aqua but just go ahead and flood field when the rains started”. I was traveling around the valley yesterday looking at fields and I saw that rice that had been planted was not coming up – or was coming up very slowly. These rains have been accompanied by very cool weather (almost 10 degrees cooler than normal for this time of year) and the 10 day forecast shows continuing cooler than normal weather. This means plant growth and N uptake is going to be slower than normal. I suggested in my earlier blog post to apply your N around two weeks after planting. Given the cool weather and slow plant growth, I do not think this is the best strategy. You will have too much N sitting on the soil surface for a long period of time as the plants and this N will be highly susceptible to N losses. Given the weather we have had, I think a better strategy is to apply a starter blend containing about 30 lb N/ac 10 to 14 days after planting. Then at four weeks apply the remainder of the N balance as urea. This balance of urea should be applied in two to three applications about applied seven to ten days apart. Fields should not have water flowing through them when urea is applied and the crop still needs to be accessed using the Leaf Color Chart or GreenSeeker at PI to see if additional N is necessary. I say four weeks after planting to apply the urea; however this urea needs to be applied before the plants start to tiller. If weather warms up sufficiently and plant growth speeds up, this urea may need to be applied a bit earlier.
Arthropod and disease management
Just like rice, arthropod development is slowed down when weather is cool. Rice water weevil likes to fly during warm nights. Tadpole shrimp develop fast during warm weather. The cool weather we are experiencing might result in very little rice water weevil flight to recently flooded rice fields. If rice has been seeded and is germinating slowly, keep an eye for tadpole shrimp. Products for control might also take longer to kill them.
The slow germination of seeds can favor the development of seedling disease and seed rot. A higher seeding rate can help compensate for seed rot problems. Check the seeds, and look for fungal growth on the surface of the seed. Sometimes algae grow on this growth, giving it a greenish look. If seed rot becomes a problem, draining the field should help. More information here: http://ipm.ucanr.edu/PMG/r682100111.html
A halo of mycelium grows over the surface of seeds infected with seed rot and seedling disease./h3>
- Author: Bruce Linquist
Rain in May, especially in the amounts we have seen this year are far from ideal for rice growers. It makes establishing seedling, N management and weed control extra challenging. Rain in May is not new. In 1995, 1996, 1998, 2005 average May rainfall in the Sacramento Valley was 2.3 inches and average statewide yields of those years was 73 sacks/ac. Interestingly, in 2011 the Sacramento Valley also received about 2” in May, but yields were 83.5 sacks. Statewide yields over the last 10 years have averaged about 85 sacks/ac.
Here I want to address some nitrogen management concerns. I address three potential scenarios below and some ideas on how to best manage N fertilizer.
Scenario 1. I applied aqua-N before rain to all or part of my field but was not able to get field flooded before the rains.
Aqua-N that was applied before the rain and did not get flooded will be very susceptible to losses. Since the soil became wet (but not flooded), the aqua-N fertilizer will begin to convert to nitrate (NO3) which will be lost to atmosphere (via denitrification) when the field gets flooded. How much is lost? Well it is a bit of a guess but some early research suggests that you lose about 2 lb N/ac per day the field is not flooded. Therefore, if the field received aqua and it was wet for 10 days before flooding, about 20 lb N/ac was lost which will need to be made up later in the season. This is a rough estimate and you should evaluate using a Leaf Color Chart or Green Seeker at panicle initiation (45-50 days after planting). If you see the field yellowing before panicle initiation you should think about adding more N when you see the yellowing.
Scenario 2. I decided not to apply aqua but just go ahead and flood field when the rains started.
In this situation, I believe the best approach is to apply your N around two weeks after planting. The N fertilizer should be applied when water is not flowing within the field. The reason to apply at around two weeks is that before this time, the plant takes up very little nitrogen fertilizer. It really needs it starting around three weeks.
If you plan to keep the field flooded, apply the full amount of N at this stage. As far as N source, urea is a better choice than ammonium sulfate as it will move more easily into the soil. Ammonium will tend to stay at the surface where there is greater chances of losses. Therefore, apply the highest amount of N as urea as possible. You will also need to apply starter fertilizers around this time. Again, assess the crop at panicle initiation to see if more fertilizer N is needed.
Scenario 3. I did not apply aqua or flood my field before the rains
In this situation, you will likely be applying aqua-N into a soil that is wetter than normal. Usually it is applied when the soil is dry and the aqua N is relatively stable for a while (we recommend having field completely flooded within 10 days of application). In a wet soil, the nitrogen will start to convert to nitrate relatively soon (see explanation in scenario 1). Therefore, to minimize N losses, flood the field as soon as possible after applying aqua.