With 5 Figures)The practical application of the results of closely controlled experiments on the effects of different levels of concentrate intake may be affected by the lack of control of roughage intakes of individual cows which occurs in most dairy herds. Individual cows show considerable variation in their ability to ingest and utilize roughage (1,2,3), which, together with the effects of competition, can give rise to considerable differences in individual plane of nutrition when roughages are rationed on a 'herd' basis. Responses to additional concentrates might therefore be expected to be maximal in those cows which are on the lower planes of nutrition due to low roughage intakes. The experiment to be described was therefore designed in an attempt to assess the importance of the effects of variations in individual rates of eating roughage upon responses to a standard increase in the concentrate ration under a system of feeding and management commonly used in dairy herds in the South of England. MATERIAL AND METHODSTwenty-nine Ayrshire cattle were used. They were divided into two groups (fourteen cows and fifteen heifers) and were housed in separate but adjacent covered yards fitted with mangers for individual feeding. The animals were given a daily 3 hr. exercise period on a bare paddock and were milked in a milking parlour. No concentrates were fed in the milking parlour. The daily feeding routine was as follows: 4.15 a.m., half of the daily allowance of concentrates; 5.0 a.m., milking; 6.30-7.30 a.m., half the daily allowance of hay; 9.45-11.0 a.m., marrowstem kale; 11.0 a.m.-2.0 p.m., exercise; 2.45 p.m., the other half of the concentrates; 4.0 p.m., milking; 5.0-6.0 p.m. the other half of the hay.The animals had access to water at all times except when tied in the yokes for feeding. Table 1. Mean age, stage of lactation and milk yield at the start of the experiment Mean S.D.

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1. The effects of feeding 6 or 9 oz. of milk substitute in 2 or 3 pints of water per feed were studied in Ayrshire calves weaned at 5 weeks, and of feeding 6 or 9 oz. in 3 pints with calves weaned at 3 weeks.2. In both experiments higher milk substitute intakes significantly improved growth rate without detriment to the consumption of dry food.3. Restriction of water to 2 pints per feed tended to reduce live-weight gain and concentrate consumption and increased free water intake, although not sufficiently to compensate for the restriction.4. In calves weaned at 3 weeks the higher amount of milk substitute produced an effect which was still present at 88 days and this indicated that underfeeding in very early life may be permanently detrimental.5. Regressions of live-weight gain upon concentrate and free water consumption and the progressive increase in live-weight gain and concentrate intake before weaning indicate the importance of this intake in providing energy for the calf before weaning and the beneficial effects of individual vigour.6. The marked effects of the various systems of use of similar foods upon the distribution of liveweight gain during the first month and upon cost are described and discussed.

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