Last week I got two calls regarding unusual symptoms starting to appear on ripening rice plants. In both cases, the symptoms were described as medium sized round patches turning reddish or orange. Close inspection of plants showed typical symptoms of K deficiency. 

Leaf tips turned reddish and yellow and started to dry out. Stems looked thin and soft. At first, the symptoms were only in small patches, but as plants matured the symptoms extended to the whole field, with some areas showing more symptoms than others.

The critical flag-leaf K value in mature plants is 1.2% (optimal is between 1.5-2%). Leaf samples from asymptomatic parts of the field were at 1.22%, while samples from areas where symptoms were clear were around 1.18%.

I've been seeing K deficiency for several years now. The take-up of K by plants is similar to N, around 150 lbs/a. Most of the K (80%) goes to the straw, while a smaller fraction (20%) goes to the grain. This means that even in fields where straw is incorporated, 20-30 lbs/a of K are removed each year. Over time, K levels in fields thought to be high in K could be reduced substantially. Currently, Bruce Linquist is examining soil K levels and his research shows that K fertilizers may be needed when soil K is below 120 ppm. 

The annual Rice Field Day will be Wednesday, August 27, 2014, at the Rice Experiment Station (RES), Biggs, California. You and your associates are cordially invited to join us to observe and discuss research in progress at RES. The Rice Field Day is sponsored by the California Cooperative Rice Research Foundation and University of California with support from many agricultural businesses.

7:30 - 8:30 A.M.: REGISTRATION

• Posters and Demonstrations

8:30 - 9:15 A.M.: GENERAL SESSION

• CCRRF Annual Membership Meeting
• D. Marlin Brandon Rice Research Fellowship
• California Rice Industry Award

9:30 – NOON: FIELD TOURS OF RICE RESEARCH

• Variety Improvement
• Disease Resistance
• Insects and Control
• Weeds and Control

NOON: LUNCH

The program will begin at 8:30 a.m. with a General Session that serves as the Annual CCRRF Membership Meeting. Posters and demonstrations will be in place during registration until after lunch. Field tours of research will emphasize progress in rice variety improvement, disease, insect, and weed control. The program will conclude at noon with a complimentary luncheon. The RES is located at 955 Butte City Highway (Hwy. 162), approximately two and one half miles west of Highway 99 north of Biggs, California.

The Weed program at the Rice Experiment Station (RES) in Biggs conducts herbicide resistance testing for the major rice herbicides used in California. This information helps growers improve their weed control programs if resistance is confirmed, and also helps the rice industry as a whole to keep track of resistance issues.

Sprangletop and smallflower umbrella sedge seeds are the first weed seeds to mature. If you suspect herbicide resistance, bring seeds of these weeds to the RES to be tested, together with the Resistant Weed Testing form available here. Follow the guidelines below to ensure that enough seed of the right species is collected. Later in the year we'll update these guidelines to include ricefield bulrush and watergrass.

General guidelines to collect suspected herbicide resistant weed seeds:

• Don't wait until harvest to collect seed. By then, most weeds have shattered their seeds. If you collect after harvest, you'll probably be collecting seeds from weeds growing around the field that may not be the correct species.
• Collect seed when they are mature and dislodge easily from the seedhead. In general, sprangletop matures the earliest, between rice PI and heading. Early watergrass, barnyardgrass, smallflower umbrella sedge, and ricefield bulrush follow, between rice heading and maturity. Late watergrass is the last weed to mature.
• Collect seed from areas of the field where you are certain the herbicide application in question was appropriate. Avoid field borders, tractor tire tracks, skips or areas where you suspect the herbicide was not sprayed correctly.
• Collect seed from several areas of the field. Usually, when herbicide resistance is the problem, weed growth will be distributed uniformly across the field and not just in one “hot spot”.
• Collect seeds, not seedheads. Take the seedhead and gently shake it inside a paper bag. Seeds that shatter are mature and will readily germinate. If seedheads are collected, seeds might not be mature or might have shattered already.

Collect seeds by shaking the seedheads gently into a paper bag

• Collect enough seed. In order to have conclusive results, several replications of the herbicide resistance testing are needed. When not enough seed is provided, replications may not be possible. For small sized seed weed species such as sprangletop, smallflower or sedge, collect seeds from at least 20 mature seedheads. For barnyardgrass, early and late watergrass, collect from at least 30 mature seedheads.
• Monitor seed development. If seeds are not dislodging during collection, they are probably still immature. Return in 2 or 3 days and try again.

Species specific guidelines: 

• Sprangletop seeds shatter easily. Mature seedheads that have shattered their seeds will look dry, while seedheads with immature seeds will look green. Seedheads with mature seeds will have a thicker appearance than empty seedheads.

• There are two sedge species similar to smallflower. Whitemargined flatsedge (Cyperus flavicomus) and ricefield flatsedge (Cyperus iria) grow in field borders and shallow areas in the field. Be careful not to confuse them with smallflower.

If the current temperature trend continues then plant maturity may be accelerated as compared to the last couple of years. The National Weather Service predicts warmer than normal August temperatures in the Sacramento Valley (http://www.cpc.ncep.noaa.gov/products/predictions/30day/). In the 2014 Statewide Variety Trials, panicle initiation was ahead of the expected plant development schedule by about 7 days.

Remember that the rate of drying for M-202, M-205 and M-206 varies between years. For example, in a ‘warm' year (e.g. 2009) the rate of moisture loss from the maturing kernel was double that of a ‘cool' year (e.g. 2010). In this case, the average rate of moisture loss across varieties was 0.8 percentage points per day as compared to 0.4 in the cooler year (Table 1).

Table 1. Daily moisture loss in percentage points from M-202, M-205, and M-206 2009 & 2010.

Keeping close track of grain moisture is particularly important in warm harvest seasons. M-105, M-205, and M-206 can be harvested at quite low moisture contents without compromising milling quality. However, there are limits to this resilience. Rice at a moisture content of less than 17-18% will crack if rehydrated by prolonged dew or rain. The harvest season over the last few years have been unusually dry with very few hours of dew. Consequently it was possible to harvest high quality rice at very low moisture contents. An El Nino weather pattern is developing the Pacific Ocean. This may result in early rains and/or more dew events, thus increasing the chance of kernel fissuring due to rehydration in low moisture rice. Don't assume that the rice can be harvested at the ultra-low moisture levels in all years.

On a related topic, there is some concern that water supplies may run short at the end of the season in some areas. In other words, there may not be enough water to finish the crop. If you are faced with this possibility, remember that drain time experiments demonstrated that rice fields in with heavy clay soils can be safely drained 24 to 28 days after 50% heading without compromising yield and quality. Applying this concept to lack of irrigation would mean that the deep water from blanking protection would be held and allowed to slow subside; some additional water may need to be applied. The target is no standing water but the soil is still fully saturated in the intake check at 24 days after 50% heading (assuming that the water recedes in the intake check first). Compared to typical practices this could shorten the irrigation season by about 7 days. If you have any questions regarding this management option, please give me a call (530.521.6670)