18. The majority of turkeys grown for the table are kept in windowless houses, with modern systems of environmental control. These systems provide for precise control of heating, ventilation and lighting. Ventilation systems are designed to provide air containing sufficient oxygen for the normal growth and development of the turkeys and to remove excess ammonia, carbon dioxide, dust, moisture and heat. Houses may carry up to 25,000 birds and on larger farms the turkeys are usually distributed amongst several houses. Birds reared on one of these farms are normally all the same age. An "all in/all out" stocking policy helps to prevent diseases being passed from one group of birds to another. On a conventional (enclosed) farm one person may be responsible for about 30,000 birds, depending on the degree of automation of the environmental control, feeding and drinking systems. Turkeys are grown to a wide range of weights and sizes. The normal slaughter age for hen turkeys is 9-11 weeks, for stags is 19-21 weeks and for an as-hatched flock is 12-14 weeks. Stocking density can be adjusted during rearing by removing some birds for early slaughter. During the latter part of the rearing period the stocking density, which is normally set to maximise production, often exceeds the level recommended in the Government's Turkey Welfare Code by a substantial margin. Injurious pecking behaviour can be a serious problem in conventional (enclosed) housing but it is usually controlled by reducing the light intensity within the turkey house rather than by beak-trimming.