Chicken has become a cut-price protein. We consume more of it than any other meat, and while the price of chicken has dropped, production has boomed and critics say it has a cost to animal welfare. This is behind New Zealand's most popular meat.

The chicken you sit down to feast on tonight is far removed from the chicken your parents or grandparents would eat 50 years ago, when chicken was still a luxury.

While in the past there were breeds specially raised for meat, most were dual-purpose for laying and meat. In fact the chicken a lot of people ate were one- or two-year-old hens that had ended their useful days as layers.

Fifty years ago it took over 12 weeks for a meat chicken to reach the 2 kilogram weight considered ready for the table.

SUPPLIED Environmental conditions are kept strictly controlled in broiler sheds with the aim of producing quick growing, good sized chickens.

So how come they can now reach 2kg in only five weeks?

Specialised breeding companies began to selectively raise chickens with the right attributes. United States company Cobb Vantress, now owned by giant food multinational Tyson Foods, and Aviagen from Scotland created the Cobb and Ross breeds, the most common used in New Zealand.

The eggs from these two companies are imported into New Zealand on a regular basis throughout the year and are hatched by the big four – Tegel, Inghams, Brinks, and Turks – who then send the day-old chicks to their farms.

Peter Meecham/Stuff The inside of chicken sheds with 9 day old chicks at a farm in Auckland, which produces free range chickens.

What else helps them put on weight?

Feed. From day one until five weeks later, chickens are fed on a highly specialised diet over five phases. It begins with the pre-starter, and graduates through the starter, grower, finisher and withdrawal phases.

Typically the ingredients are wheat (50 per cent), soybean meal (25 per cent), barley (10 per cent), with meat and bone meal, tallow oil, limestone in small amounts.

A number of feed additives are added to the mix to boost health or ward off disease, including antibiotics, enzymes, antioxidants and pigments.

123RF Chicken has become a regular part of the diet whereas once it was an occasional luxury.

What else?

Don't forget water, the most important nutrient for animal growth. Fresh water is constantly piped through to nipples from which the birds drink. They have replaced bell drinkers which spill water over the litter, helping cause disease.

Primed for production

Not only are chickens growing heavier faster, they are doing so on less food. Between 1995 and 2015 the feed conversion ratio (unit of feed consumed per unit of body weight gain) has dropped dramatically, largely due to advances in genetics and nutrition.

But New Zealand is also recognised to have superior feed conversion ratios compared to other countries because it is free from three of the major diseases that plague poultry elsewhere – infectious bursal disease, Newcastle disease and avian flu.

Hatch to despatch

Everything in a broiler shed is designed to help chickens grow to the optimum weight. Before they arrive the shed is disinfected and filled with wood shavings (the litter).

For the first four days of a chick's life, it is provided with continuous light to stimulate it to start feeding, which is reduced so that from then on it has at least four hours of uninterrupted darkness every 24 hours.

Chickens have to be kept at the right temperature, easy to do in the controlled environment of a broiler shed. At a few days old the shed has to be heated to 32 degrees Celsius, and this is gradually reduced to 23C by week 3C.

As the chickens grow older and bigger, they become better insulated, and through eating more food, generate more heat. They risk becoming overheated, which is one of the reasons why animal welfare groups would like to see lower stocking rates.