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In experiment 1, 12 cows (treatment L) received natural daylight (mean 8 h), 10 h supplementary light (mean intensity 481 lux) and dark (mean 6 h), and 12 cows (treatment N) received natural daylight (mean 8 h) and dark (mean 16 h) for 8 weeks. Supplementary light increased milk yield and tended to increase food intake and loss of live weight. There were no significant effects on milk composition although milk fat concentration tended to be reduced for cows in treatment L. Daily feeding times were not affected by treatment although more feeding took place during the supplementary light in treatment L. Supplementary light increased the time spent lying down per day and reduced the distance walked per day, although cows in treatment L spent less time lying during the light supplementation period. In both treatments, the activity rate was increased on the day of oestrus, but in treatment L the incidence of oestrus-specific behaviour was significantly reduced compared with treatment N.In experiment 2, 16 cows received natural daylight (mean 9 h) and 10 h supplementary light of mean intensity 0, 101, 191 or 529 lux in a change-over design. There were no significant effects of supplementary light or its intensity on the food intake, milk yield or live-weight change of the cows, but milk fat concentration was reduced for cows receiving supplementary light at 101 or 529 lux and milk protein concentration was reduced for cows receiving light at 529 lux.

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