Hawaii’s seed crop industry is in a bigger decline than previously thought but still ranks as the state’s top agricultural product by value, according to a report Tuesday.

The National Agricultural Statistics Service estimates the value of the industry shrank 14 percent to $159 million last year from $184 million the year before.

The drop represents the second consecutive year of decline for Hawaii seed crops after three years of flattening that followed robust growth.

Industry observers over the last few years have said that the business of growing seeds in Hawaii for research and development had matured and that maybe companies were becoming more efficient in what they do.

Seed companies in Hawaii experiment with plant traits — mostly for corn — and select certain plants to produce seeds that are sent to the mainland for mass reproduction and sale to farmers.

Because the companies don’t sell the seeds they produce in Hawaii, the Statistics Service uses the cost of production to represent industry value.

Other crop values are represented by sales revenue, which usually is more than production cost.

In its new report, the National Agricultural Statistics Service, which is part of the U.S. Department of Agriculture, said the decline in seed crops in 2013 was larger than previously estimated.

Last year the agency preliminarily estimated that Hawaii’s seed crop value in 2013 was $213 million. The new report revised the 2013 figure down to $184 million, which represents a 24 percent decline from 2012.

Furthermore, the report said the industry cut back farming to 5,900 acres in 2013 from a peak of 6,900 acres in 2012. An earlier report said the seed industry increased farming in 2013 to 7,040 acres. Last year the area farmed in seed crops was estimated to be 4,840 acres.

Last year the industry shipped 8 million pounds of seed outside Hawaii, down from a revised 8.2 million in 2013 and 10.7 million in 2012.

Growing seeds for research in Hawaii dates back decades but really took off in the early 2000s with industry spending soaring by roughly 20 to 40 percent per year. In 2006 seeds became Hawaii’s biggest crop by value.

After seed crops, sugar cane typically has been the state’s No. 2 crop by value, generating around $70 million or $80 million in sales annually. Other top agricultural operations include coffee and ranching at around $50 million a year along with macadamia nuts at around $35 million a year. Pineapple also remains big, but industry value is no longer reported.

Seed companies primarily value Hawaii as a base for research because corn can be planted and raised to maturity three or four times in a year compared with only once on the mainland, thereby allowing faster advancement of research.

Seed research and production is viewed by many Hawaii agriculture leaders as being a major agricultural employer and keeping farmland in farming.

Hawaii seed growers also have been targets of environmentalists and natural-food proponents in recent years because a lot of the work to produce seeds with desired traits is done using genetic modification. Such concerns have led to litigation and state and county initiatives to restrict or ban work with genetically modified organisms.

Five companies — BASF, Dow AgroSciences, Monsanto, Pioneer Hi-Bred International and Syngenta — are involved in Hawaii seed research, and operate 10 farms on Oahu, Maui, Molo­kai and Kauai.

Corn is the main seed produced, though the companies have also grown soybeans, wheat, sunflowers, rice, rapeseed and sorghum in Hawaii.