1. Twelve crossbred does (New Zealand White × California) were offered a diet of high protein and metabolizable energy content (249 g/kg dry matter (DM) and 13·6 MJ/kg DM respectively) throughout a 32 d lactation at one of four feeding levels (240, 280, 320 or 360 g/d). Each feeding level was replicated three times.

2. Milk output was measured by weighing the does before and after their one daily suckling period.

3. The lactation was divided into four consecutive 8-d periods. Each doe was placed in a direct calorimeter for 48 h around the mid-point of each of these periods and measurements of energy exchange were made. Nitrogen balance was also measured throughout the study period.

4. Milk samples were taken from a parallel group of animals and the estimates of milk composition were applied to the main group of does.

5. From the second period of lactation onwards nearly all does mobilized body tissue to support milk energy secretion, although there was no loss of weight. Multiple regression analyses were used to examine the apparent efficiency with which metabolizable energy and body-tissue energy were utilized for milk production. Overall, the relationship was described by the equation:

Period of lactation … 2 3 4

Milk E = 0.735 me intake – 0·938Body ER –296 –280 –276

(se 0-020) (se 0-039)

where Milk E is the milk energy output, ME intake is the metabolizable energy intake and Body ER is the body energy retention, all expressed in kJ/kg body-weight0·75 per d.

All does appeared to be in positive N balance throughout lactation on this high-protein diet.

1. Fractional rates of protein synthesis in tissues of young growing rats were estimated by injection of flooding amounts (1·5 mmol/kg body-weight) of [3H]phenylalanine. Rates of 63·6%/d and 90·4%/d respectively were obtained in the skin and bone (tibia) of fed animals. These rates were comparable to that in liver (86·3%/d) but considerably higher than in muscle (16·9%/d).

2. Absolute amounts of protein synthesized in tissues of fed rats were estimated. Together the skin and bones accounted for 25% of whole-body synthesis, a value similar to the contribution of liver (15%) and muscle (25%).

3. In fed rats the ratio, RNA: protein in skin and bone was lower than in liver, but much higher than in muscle. However, the amounts of protein synthesized per unit RNA in skin and bone were higher than in both liver and muscle.

4. After 2 d of starvation the fractional rates of protein synthesis in skin and bone fell by 26% and 31% respectively. This was greater than the fall in liver (17%) but less than in muscle (66%). In bone the fall in synthesis was accompanied by decreases in both RNA: protein and synthesis per unit RNA, but in skin there was a fall in RNA: protein which was partially countered by an increase in the rate of synthesis per unit RNA.