1. A study was made of the milking of the two hind quarters of four cows by teat cup and by teat cannula at three levels of vacuum and by teat cannula at atmospheric pressure.2. The eight quarters milked at significantly different maximum rates by teat cup but at nearly the same rate through a teat cannula, suggesting that the teat orifice is a very important factor controlling rate of milking.3. The maximum rate of milking by both teat cup and teat cannula increased with increasing level of vacuum from 11 to 20 in. Hg. The rate of increase in the teat-cup milking was greater than in the cannula milking, suggesting that the teat orifice was stretched open at the higher levels of vacuum.4. The average amount of strippings increased at the higher level of vacuum in teat-cup milking, due apparently to teat-cup crawl.

With 2 Figures)The discoveries of the last decade have considerably improved our understanding of the neural and hormonal mechanisms controlling milk ejection. This, in turn, has stimulated research into the many practical problems concerned with hand and machine milking. The discovery that milk ejection is a transitory mechanism encouraged the study of the rate of machine milking of dairy cows since it was inferred that less milk is obtained when milking is slow. This research work has shown that there are very wide differences in the machine milking rate of cows in the same herd, even when they are under identical management (l, 2, 3). However, the milking rate of a particular cow is relatively stable over long periods (2, 3, 4) and cannot be altered even by severe changes in management (5, 6,7). The explanation of this appears to be that milking rate is mainly controlled by the structure of the teat orifice (8).The results given in this paper carry this study a stage further. They show the effect of inherent milking rate on milk yield, milk composition and the rate of decline of milk yield. In addition, some preliminary observations on the inheritance of milking rate are given. SOURCE OF DATAThe measurements of milking rate, milk yield and milk composition used in this analysis were all made in the herd of the National Institute for Research in Dairying. For details of the method of collection of the milking-rate data reference should be made to another publication (3). The lactation yields of the cows were all compiled from the daily weighing of the milk produced by each cow at each milking. Samples of milk for milk-composition analysis were collected on one day each month. Elsewhere, Bailey (9) has described in some detail the management, feeding and breeding of the herd. RESULTSFor convenience the following terms have been used in presenting the results:'Peak flow' (16/min)-maximum milking rate attained in any minute during milking. 'Standard lactation yield' (lb.)-amount of milk produced in 305 days or until dry, whichever is the shorter period. 'Lactation fat percentage'-mean weighted fat percentage in the total lactation period. 'Lactation solids-not-fat percentage'-mean solids-not-fat percentage in the total lactation period. 'Lactation length' (days)-days in milk from 4 days after calving until dry. 'Persistency' (days)-the number of days after calving before the weekly yield falls below one-half of the maximum weekly production.

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1. Two levels of steaming-up and two levels of concentrates feeding during the first 84 days of lactation have been compared in a 2 x 2 factorial experiment over three winter seasons, using fifty-two Shorthorn and thirty-six Friesian heifers. The levels of steaming up were 2 cwt. and ½ cwt. of concentrates fed over the last 21 and 14 days of pregnancy, respectively. The levels of concentrates during lactation were 5 and 3 lb. per 10 lb. milk per day. Roughages were fed for maintenance. After the 84th day of lactation all animals were given the same treatment for the remainder of the lactation. Milk yield, milk composition and live weight were measured throughout the lactation.2. The treatments were annotated as HH, HL, LH and LL, the first letter indicating level of steaming-up and the second the level of concentrates feeding in the first 12 weeks of lactation.3. Mean milk yields were as follows:4. The LH treatment reqiuird 3 cwt. more concentrates than HL to produce the same amount of milk.5. The response to additioal concentrates on the LH treatment after calving was l lb. additional milk per l lb. additional starch equivalent.6. Butterfat percentages were higher (3·69%) in both groups on low-lactation feeding than in the two groups on high-lactation feeding (3·43%). The HL group (i.e. the group that received high steaming-up and low-lactation feeding) combined high milk yield with a higher butterfat percentage, and at current milk and feedingstuff prices and under the conditions of the trials, this group showed a greater cash return than the groups on the other three treatments.

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