Which One Is the Cause of Unfilled Grain of Rice?

Key Takeaways:

Insect infestation, especially rice bugs, is a major cause of unfilled grains in rice.
Excess nitrogen fertilization can lead to excessive vegetative growth and reduced grain fill.
Diseases like panicle blast and armyworm feeding damage developing grains and prevent proper filling.
Cold temperatures during pollen formation and high temps during flowering disrupt fertilization and grain fill.
Other factors like spider mites, sugarcane aphids, and sorghum midge can also cause empty grains in cereal crops.

Introduction

Rice is one of the most widely consumed cereal grains globally, serving as a staple food for more than half of the world’s population. Producing high-quality, well-filled grains is essential for good yields and milling quality. However, rice farmers often face the problem of unfilled or partially filled grains that result in reduced yields. What causes grains to remain unfilled and how can this issue be addressed?

This comprehensive article will evaluate the various factors that can lead to unfilled rice grains. It will provide an in-depth analysis of the major causes, including insect infestation, nutrient imbalances, diseases, and adverse environmental conditions. The aim is to help readers understand the underlying reasons behind unfilled grains and gain actionable solutions to prevent and mitigate this problem. With the information provided here, rice growers will be better equipped to make management decisions that support optimal grain development and filling.

What Is an Unfilled Grain of Rice?

Before examining the causes, it is important to understand what constitutes an unfilled grain of rice. Rice grains contain an endosperm that stores starch and protein to nourish the germinating seed. A filled, well-developed grain has a white, translucent endosperm that fills the entire inner space within the hull.

In contrast, unfilled or partially filled grains have empty spaces in the endosperm, giving the grain a chalky, opaque appearance. The endosperm lacks adequate starch accumulation to completely fill the hull. This results in shriveled, lightweight grains with reduced milling quality and market value.

What Are the Consequences of Unfilled Grains?

The presence of unfilled grains in the harvested rice crop can have several negative consequences:

Reduced grain weight and yields per acre
Lower head rice recovery and milling quality
Increased broken grains after milling, affecting marketability
Lower nutritional value due to inadequate accumulation of starch and protein
Poor germination of seeds for the next planting

Therefore, identifying and addressing the underlying causes of unfilled grains is critical for maintaining high productivity, grain quality, and farm profitability.

What Causes Unfilled Grain in Rice?

Many factors can interfere with the proper development and filling of rice grains at various growth stages. Here, we explore the major causes behind unfilled grains.

Does insect infestation cause unfilled grain of rice?

Yes, insect pests are one of the leading causes of unfilled grains in rice. Insects feed on developing rice kernels, destroying the endosperm and preventing the grains from filling properly.

The most damaging pests include:

Rice stink bugs – They insert their piercing sucking mouthparts into immature rice grains from the pre-flowering stage up to the soft dough stage. This damages the developing endosperm and causes chalky, unfilled grains.
Sugarcane aphids – Both nymphs and adults suck sap from the soft, milky grains causing them to shrivel.
Spider mites – Feeding by mites during the grain filling stage reduces starch content.
Sorghum midge – Larvae feed inside the flowering spikelets of sorghum, preventing grain development.

According to research by Texas A&M AgriLife Extension, rice stink bug damage alone can cause up to 20% unfilled grains resulting in 5-10% yield loss. Effective insect pest management is therefore critical for minimizing grain damage.

Can excess nitrogen cause unfilled or empty grains in rice?

Yes, excess application of nitrogen fertilizers can promote vegetative growth at the expense of reproductive growth, resulting in increased unfilled grains.

Excess nitrogen causes prolonged and excessive tillering, diverting energy from grain filling to vegetative growth. This delays maturity and increases infertility.
High nitrogen also increases susceptibility to diseases like sheath blight and lodging, which negatively affect grain fill.
Research from Louisiana State University found that rice fertilized with high nitrogen beyond recommended rates had 12-15% more unfilled grains compared to optimally fertilized rice.
Excess nitrogen also predisposes cereal crops like sorghum to charcoal rot disease which interferes with grain development.

Therefore, following recommended nitrogen rates based on soil tests and avoiding excess application is important to prevent unfilled grains due to over-fertilization.

How does panicle blast disease cause empty grains in rice?

Panicle blast, caused by the fungus Magnaporthe oryzae, can result in large percentages of unfilled grains.

The fungus infects the rice plant during heading and flowering. It causes rotting of the panicles along with discoloration and death of individual spikelets.
In severe cases, panicle blast can destroy up to 60-90% of the grains. The infected florets fail to produce filled kernels.
Cool and humid weather favors this disease. Floret infections at early heading cause the highest unfilled grains.
Fungicide application at both booting and heading stages is necessary to protect developing panicles from blast infections and reduce grain loss.

Can feeding by armyworms lead to unfilled grains in cereal crops?

Yes, armyworm feeding on developing grains within cereal heads can often cause lack of proper grain filling.

Armyworm caterpillars, especially fall armyworms, feed inside rice panicles and sorghum heads devouring the contents of unripe kernels.
This prevents the grains from maturing and accumulating starch resulting in partial or complete lack of endosperm filling.
Heavy armyworm infestations at the heading stage of cereal crops can destroy up to 75% of grains. Late-whorl infestations also reduce grain fill.
Insecticide application during the whorl and flowering stages helps control armyworm caterpillars before extensive grain damage occurs.

How do low temperatures during pollen formation affect grain filling?

Exposure to cold temperatures during the microspore and anthesis stages interferes with pollen viability and fertilization, increasing unfilled grains.

Rice pollen is most susceptible to cold injury during microspore formation and anthesis when temperatures drop below 60°F for prolonged periods.
Cold-induced male sterility inhibits pollen germination and growth of pollen tubes. This leads to unfertilized ovules and prevents grain enlargement.
Even short durations of cold temperatures can substantially reduce grain fill. Research shows just 3 days below 68°F caused 64% unfilled grains.
Sorghum is also affected with temperatures below 65°F during pollen-shedding causing sterility. Avoiding planting too early in cold conditions can reduce unfilled grains.

Can high temperatures during flowering cause empty grains in rice?

Yes, extremely high temperatures above 104°F during flowering interfere with fertilization and grain fill in rice.

Temperatures exceeding 104°F during anthesis causes pollen sterility and low pollen germination. This reduces fertilization and increases blank grains.
Grain filling duration is also shortened by 7-10 days under very high temperatures, reducing accumulation of starch in endosperm.
Research shows that temperatures of 109°F for 4 hours caused complete pollen sterility and 100% unfilled grains.
Adjusting planting time to avoid very high temps during flowering helps minimize heat-induced empty grains.

How to Prevent and Manage Unfilled Grain in Rice

The various causes behind unfilled rice grains point to several recommended management practices to prevent and reduce this problem:

Practice integrated pest management

Scout fields regularly and use economic thresholds to determine need for insecticide use against rice stink bugs, aphids, mites.
Make sure to control armyworms during vegetative stages to prevent later grain damage.
Apply fungicides at both booting and heading to protect against panicle blast.

Follow proper nitrogen fertilization

Avoid excess nitrogen beyond recommended rates based on soil tests.
Split applications with more nitrogen at early stages and less during reproductive stages.
Reduce nitrogen rates with high organic matter soils prone to lodging.

Choose suitable varieties

Select rice and sorghum varieties adapted for your climate with resistance against pre-flowering cold.
Avoid very early or late planting to prevent cold/heat stress during flowering.
Diversify planting dates to spread risk in case of adverse weather at critical stages.

Conclusion

In conclusion, unfilled grain is a complex problem in rice arising due to multiple factors. Insect pests, diseases, high nitrogen, and temperature extremes during pollen formation and flowering are the major causes that hamper proper kernel development. By understanding these reasons and implementing integrated solutions focused on insect pest control, balanced fertility, adapted varieties, and optimum planting, rice growers can greatly reduce losses from unfilled grain and achieve higher productivity. The information presented here equips readers with improved knowledge to make sound decisions for preventing and managing this widespread grain filling problem.

Frequently Asked Questions

What percentage of unfilled grains leads to significant rice yield losses?

Just 10-15% unfilled grains can result in 5-10% yield loss. The higher the percentage of unfilled grains, the greater the reduction in grain weight and rice yields per acre. Over 30% unfilled grains can cause losses exceeding 20% of potential yield.

How does drought stress affect grain filling in cereals like rice and sorghum?

Drought during the grain filling stage reduces accumulation of starch in the endosperm. It accelerates maturity resulting in poorly filled, shrivelled grains with lower weight. Maintaining adequate soil moisture post-flowering enhances grain enlargement.

Can cloudy weather increase unfilled grain in rice?

Yes, cool and cloudy conditions reduce the photosynthetic rate during grain filling. This limits the supply of assimilates like sucrose to the developing grain, resulting in incomplete filling of the endosperm. Sunny weather post-flowering optimizes grain fill.

How do micronutrient deficiencies cause unfilled grain?

Deficiencies of boron and zinc, in particular, cause improper pollen formation and reduced grain setting. This results in empty, unfertilized florets. Timely correction of micronutrient deficiencies is important for proper grain development.

What is the role of crop rotation in minimizing unfilled grains?

Rotating rice with soybean improves soil fertility and reduces disease inoculum that could affect grain fill. It also helps break pest cycles of insects like rice stink bugs and armyworms to reduce grain damage

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