the persistence of state orders for the two main cash crops, cotton and wheat. Farmers receive binding directives on the area to be cropped to these commodities and are obliged to surrender their harvest to designated marketers at state-fixed prices. The incomes of farmers and agricultural workers are substantially lower than the national average because the government pays them less than the world prices for their cotton and wheat, using the difference to subsidize capital intensive industrial concerns, such as factories producing automobiles, airplanes, and tractors. Consequently, many farmers focus on production of fruits and vegetables on their small household plots, because the prices of these commodities are determined by supply and demand, not by government decrees. Farmers also resort to smuggling cotton and especially wheat across the border with Kazakhstan and Kyrgyzstan in order to obtain higher prices. The government's discriminatory pricing for the main cash crops, cotton and wheat, is apparently responsible for the exceptionally rapid growth of the cattle herd in recent years, as the prices of milk and meat, like those of fruits and vegetables, are also determined by market forces. The number of cattle increased from 4 million head in 1990 to 7 million head in 2006, and virtually all these animals are maintained by rural families with just 2-3 head per household. Sales of own-produced milk, meat, and vegetables in town markets are an important source for augmenting rural family incomes. The Soviet practice of using "volunteer labor" to help gathering the cotton harvest continues in Uzbekistan where schoolchildren, university students, medical professionals, and state employees are driven en masse out to the fields every year. A recent article posted by a domestic news agency (admittedly with strong anti-government leanings) describes Uzbekistan's cotton as "riches gathered by the hands of hungry children". Natural resources and energy In 2019, the country was the 5th largest world producer of uranium; 12th largest world producer of gold; 7th largest world producer of rhenium; 12th largest world producer of molybdenum; 21st largest world producer of phosphate, and the 19th largest world producer of graphite Minerals and mining also are important to Uzbekistan's economy. Gold, alongside cotton, is a major foreign exchange earner, unofficially estimated at around 20% of total exports. Uzbekistan is the world's seventh-largest gold producer, mining about 80 tons per year, and holds the fourth-largest reserves in the world. Uzbekistan has an abundance of natural gas,