Four experiments, each with 24 growing sheep, were carried out to investigate the effects of 8- or 16-h daylengths on growth and carcass characteristics. In the first two experiments, with 6- to 9-month-old castrated males, and 3- to 5-month-old female and castrated male animals, pair feeding of a concentrate feed was adopted. Live-weight gains were significantly greater under 16-h daylength, but carcass weights were not significantly increased because most of the live-weight difference was due to gut-fill. Carcass weights and measurements suggested that long daylength stimulated the growth of non-fat tissues at the expense of fat.
The third experiment compared castrated males fed either ad libitum or restricted to a daily ration proportional to each animal's own weight from 6 to 9 months of age. Food intake was higher in the ad libitum sheep but not significantly so. Weight gains were, however, significantly greater with ad libitum feeding and long daylength, compared with restricted feeding and short daylength, respectively. Gut-fill was greater in those kept in 16 h daylength and the small advantage in carcass weight was not significant. Carcass characteristics were not markedly affected by daylength, while carcass weight and fatness were higher following ad libitum feeding than restricted feeding.
In the fourth experiment 12 female sheep, aged 9 months, were individually rationed according to live weight and slaughtered after 4 weeks exposure to 16- or 8-h daylengths; another 12 were slaughtered after 7 weeks of treatment. Gut-fill was significantly greater in long daylength at both slaughter times, showing that the effect of daylength on gut fill has occurred within 4 weeks of the start of treatment, while live weight has not been observed to be affected at this stage in any experiment in this series.
Overall these experiments show little stimulating effect of long daylength on growth, and a large effect on gut-fill.