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.