These close conditions require the use of antibiotics to stop the spread of disease and housing systems often prevent animals from exhibiting many of their natural behaviours. Farm animals are often subjected to artificial feeding and lighting regimes, selective breeding, and intensive confinement, often in cages and at extreme stocking densities, to produce the greatest possible output of meat, milk and eggs in the shortest amount of time at the lowest cost. According to Voiceless, thecintensification of farming processes has resulted in large multinational companies dominating the global meat and dairy trade. As of 2006, 50% of global pork production and over 70% of global chicken production came from industrial systems. Two companies supply approximately 70% of Australia's meat chickens, with the balance of the market supplied by about four medium-sized companies. Voiceless states that this intensification of agricultural practices has led to the widespread use of cruel and inhumane meat production practices, such as the debeaking of chickens, tail docking and teeth clipping of pigs, and mulesing of sheep, often undertaken without pain relief. Voiceless's work on factory farming is focused on raising awareness of the practice in order to change the laws which allow it. The commercial kangaroo industry is a multimillion-dollar meat and skin industry, responsible for the killing of almost 90 million kangaroos and wallabies in the last 30 years. Voiceless claims that kangaroos are hunted in the largest commercial slaughter of land-based wildlife on the planet, primarily because they are perceived to be overpopulated in Australia and considered a pest. According to a 2011 report by THINKK, the think tank for kangaroos, the notion of kangaroos as costly pests to Australian farmers has been significantly overstated. While shooters are required by the relevant Codes of Practice to aim to shoot a kangaroo in the brain and therefore cause instantaneous death, it is Voiceless's view that non-fatal body shots are unavoidable and cause extreme injuries. Voiceless also claims that the death of joeys is ‘collateral damage’ to the killing of female kangaroos, with young joeys killed or left to die when the mother is shot. The kangaroo industry code allows joeys to be killed by a single blow to the head (usually with a steel pipe or against the tow bar of the shooter's truck) or through decapitation. On average, approximately 800,000 dependent joeys are killed as collateral damage of the kangaroo industry.