9234 cows in 524 herds were divided into two groups in each herd, those holding to first service and those not holding to first service. The differences between the milk-yield means of each group in each herd were tested by an analysis of variance procedure, which showed no significant difference in milk production between the two fertility levels. It was also shown that the sample was homogeneous. The study shows that the amount of their milk yield during the month of service does not adversely affect the fertility of dairy cattle.

1. In a winter-feeding experiment with eight Ayrshire cows the effects of feeding a high- and a low-protein concentrate at two rates (4 and 2 lb./gal.), in addition to dried grass and silage, were studied.

2. With these cows and the feeds used increasing the rate of concentrate feeding from 2 to 4 lb./gal. increased the total dry-matter consumption by 2 lb./day and the average milk yield by 1·2 lb./day.

3. For every 1 lb. increase in concentrate fed there was a decrease of 0·68 lb. grass dry matter eaten.

4. The level of protein in the concentrates had no effect on total dry-matter intake, and only a slight, though positive, effect on milk yield.

5. It is concluded that in this work where the diet was made up with high-quality dried grass and silage, feeding either a high- or a low-protein concentrate mixture at the rate of 2 lb./gal. milk gave the greatest profit margin per cow.

1. The effect of feeding levels of protein and energy appreciably higher than the Woodman standards on the yield and composition of the milk from twelve Ayrshire cows has been studied in two winter-feeding experiments.

2. In the first experiment the S.E. of the concentrate ration was held constant at 63½% and the D.C.P. content adjusted to 10, 15 and 17%. The 10 and 15% D.C.P. concentrates derived their protein from home-grown feedingstuffs, whereas the 17% D.C.P. concentrate was a bought-in commercial mixture. Feeding the three levels of D.C.P. led to no statistically significant differences in milk yield, milk composition or live-weight gain, although the commercial mixture (17% D.C.P.) resulted in milk with a slightly lower fat content.

3. The second experiment reversed the feeding conditions of the first, i.e. D.C.P. was held constant at 16% and three concentrates were fed which contained 59, 67 and 75% S.E. respectively. The two rations of higher S.E. content gave significantly more milk than the low S.E. ration and tended to give slight increases in live weight. There was a slight increase in the S.N.F. content of the milk when the S.E. of the concentrate was raised from 59 to 67% S.E. This was due almost entirely to an increase in the protein content of the milk. There was no further increase in S.N.F. with the 75% S.E. concentrate, and this latter concentrate produced milk with a slightly lower fat content.

4. Comparison of costs and returns showed that the cheap low protein concentrate, ration C, even though it gave the least milk, yielded the highest profit. It was shown, however, that at any reasonable fixed cost of starch equivalent the most profitable level of feeding above the Woodman maintenance standard was 3–3·3 lb. S.E./10 lb. of milk of 3·8–4·0% fat. Alternatively, the total ration should be 110–117% of the Woodman standard.

The rate of acid production by a slow variant of Streptococcus cremoris in milk was raised to that of the fast parent strain when peptone or acid hydrolysed peptone were added. Lactose utilization was similarly affected. However, the stimulation took place without any change in the rate of growth. Other sources of available nitrogen gave similar results.

It is concluded that the change from a fast to a slow culture which occurs on continued transfer is due to the loss of ability to utilize the nitrogen compounds of milk.

A description has been given of a dairy herd, its breed composition, history and management over a period of 8 years. A close study of infection and mastitis was made in the herd, and the bacteriological methods used have been outlined. The data collected have been analysed and the methods of analysis have been described. There were 530 complete lactations for which full records were available and, because of the frequency of milk sampling and the care with which the bacteriological and other tests were made, these data are considered to be suitable for a precise study of the effect on the incidence of infection and mastitis of stage of lactation, season of the year, age and previous infections. These factors will be the subject of a later report.

1. Two experiments were carried out to compare spring-dried grass and autumn-dried grass. In both experiments a 3×3 Latin square changeover design was used with periods of 4 weeks. In the first the control ration was a home-grown concentrate mixture (S.E. 58, D.C.P. 12·5%), in the second a high-quality mixture of imported feeds (S.E. 73·5, D.C.P. 16·6%). The concentrates or dried grass were fed in addition to a basal ration which supplied only the maintenance requirements of the cow.

2. In both experiments the milk yield from spring grass was 3–5% higher than that from autumn grass, but the differences were not statistically significant. In Exp. 1 the yield from the home-grown mixture was the same as from spring grass. The yield from the imported mixture in Exp. 2 was significantly higher than from both grass rations. There was a significant increase in milk yield when the cows went on to spring grazing in Exp. 1.

3. Expression of the feed input as a percentage of the Woodman standards showed that the level of feeding was similar in all three rations in the first experiment, and that the increased milk yield from the high-quality ration in the second experiment could be explained by the increased level of feeding which it gave. It was considered that the increased yield on spring grazing might have been caused simply by an increase in the amount of nutrients consumed.

1. The detailed udder health records of 530 lactations have been analysed on a monthly and lactation basis, so that separate studies of the first infection rate, re-infection rate, total new infection rate and total infection and mastitis incidence for both cows and quarters could be made. These data have been used to study the relationship of stage of lactation, age and season of the year with udder infection and mastitis.

2. With advancing lactation there was a decline in the rate of first infection, reinfection, total new infection and in mastitis in infected cows and quarters.

3. In early lactation there was a marked tendency for infected cows and quarters to develop clinical symptoms.

4. During the last 4 months of lactation there was no significant rise either in infection or in mastitis.

5. There was an increase in infection and mastitis with advancing age, but infected older cows were not more liable to develop clinical symptoms than younger cows.

6. There was an indication that the longer a quarter was free from infection the greater were its chances of remaining so.

7. No major seasonal trends were found in the incidence of udder infection and mastitis, although the incidence was slightly higher in the summer months.

Cultures of a strain of Streptococcus cremoris HP after maintenance in two countries under different conditions preserved their main characteristics unchanged for 18 years.

In the investigation of an outbreak of acetone taint in bulk milk it was: (a) shown not to be due to the lactating animal; (b) reproduced by the inoculation into milk of a strain of Bact. cloacæ isolated from the tainted milk; (c) shown that seventy other strains of coliaerogenes organisms (including sixteen Bact. cloacæ) did not possess this ability to produce an acetone taint.

A search of the literature failed to produce a record of this taint being produced by factors other than acetonemia of the animal.

Note. Since writing the article another strain of Bact. cloacæ giving a positive Rothera's test has been isolated at Wye from a sample of milk which developed a typical sickly sweet taint on storage (Billing, E., personal communication, 5. xii. 55).

1. An experiment was carried out with seventy-five dry cows to study the effect of taking samples of udder secretion for bacteriological examination on the incidence of new infection in the early dry period. The incidence of new infection in two quarters of each cow, sampled at weekly intervals after drying off, was compared with that in the remaining two quarters sampled in the fourth and fifth week of the dry period.

2. No difference in the new infection rates was found between the quarters that were sampled early in the dry period and those that were not.

3. Significantly fewer (P <0·05) staphylococcus infections occurred in quarters sampled in the early dry period compared with those sampled at a later stage.

4. About half of all the new infections persisted until the next lactation.

5. It was concluded that if natural sealing of the teat orifice or teat sinus takes place it is of minor importance in preventing infection in the early dry period.

Samples of sterilized milk from fifteen dairies in the Midlands have been examined, and seasonal variations were detected in the incidence of mesophilic and thermophilic sporeformer contamination. Seasonal variation in the rate of spoilage at 30° C. was also apparent.

The lower incidence of contamination in summer was considered to be due to (a) less sporeformer contamination during milk production at the farm in the period when the cattle are not housed, and (b) increased processing times and temperatures in warm weather.

Cleansed milk bottles were examined and found to be an important source of contamination by mesophilic sporeformers, but the incidence of thermophiles of the type commonly found in sterilized milk was low.

Methods of reducing contamination in sterilized milk were discussed.

Batches of cream were artificially infected with M. tuberculosis, and subjected to varying heat treatments under conditions of H.T.S.T. and holder pasteurization. Guinea-pig and phosphatase tests were applied to the treated creams.

The minimum time-temperature conditions required for the inactivation of M. tuberculosis and phosphatase were found to be similar to those pertaining to milk.

The practical difficulties of controlling the effciency of cream pasteurization are discussed.

Cultures of sporeforming bacteria isolated from commercially sterilized milk were examined and seasonal variations in the incidence of B. subtilis/ B. licheniformis strains were apparent which may account for the increased rate of spoilage at 30° C. during the winter.

B. subtilis was the most common mesophilic sporeformer isolated, and under favourable conditions it produces rapid digestion of milk.

Thermophilic sporeformers are frequently present, and usually produce a firm acid clot in milk held at temperatures favourable for their growth. The classification of the thermophiles isolated is uncertain and a review of the classification of the aerobic thermophilic sporeformers in general is required.

Cl. thermosaccharolyticum, a gas-producing thermophile, was isolated from sterilized milk.

1. An experimental study was made of the effect of drying-off by ‘stop’ or intermittent milking on the incidence of new infection in the early dry period of dairy cows. Ninety-one and ninety-seven dry periods were examined following the treatments, and the results were analysed on a cow and quarter basis.

2. The incidence of new infection was similar for groups of cows dried off by ‘stop’ or intermittent milking.

3. The quarters of cows that were not infected on drying-off showed a significantly higher incidence (P <0·05) of new infection following ‘stop’ than following intermittent milking.

4. Of the cows that were not infected at the time of drying off, those which had a previous history of infection showed a significantly higher (P <0·01) incidence of new dry-period infection than cows that had not been infected before.

5. There was a much higher incidence of new infection in cows that were already infected on drying-off compared with cows free from infection at that time.

6. About half of all the new dry-period infections persisted until the next lactation, and about one-third of these resulted in mastitis in the affected quarter.

7. The method of drying-off had no effect on infections that were already established in quarters going dry.

8. The types of pathogen causing infection were similar for both methods of drying-off.

9. The method of ending lactation did not affect milk yield in the lactation following the dry period studied.

The use of a column to give countercurrent contact of steam and cream to strip taints from the cream is explained. An analysis based on the engineering concept of a transfer unit is presented, and generalized curves are given which can be used for any counterflow column using fresh steam. The generalized curves give the percentage reduction in taint for values of the stripping factor (mV/L) when the number of transfer units is known.

It is further shown that the number of transfer units for any particular design of column depends on the value of the stripping factor. The total resistance to the transfer of taint (HTU)L depends on a liquid-side resistance (HTU)l and a steam-side resistance (HTU)g such that (HTU)L=(HTU)l+L/mV(HTU)g.

Results are given for an experimental column in which the liquid flows as (a) a spray, (b) a falling film or wetted wall. A strong deodorizing effect occurs at the bottom of the column and the resistances to transfer are

Resistances for a full-size plant 3 ft. diameter processing at 10,000 lb./hr. are assessed from the above figures as:

Restriction of the process of taint transfer is controlled largely by the liquid-side resistance. Because the minimum obtainable taint level depends on the liquid-side resistance (no matter how much steam is used) a low liquid-side resistance or a tall column must be used for satisfactory deodorization. A minimum of 4 transfer units is suggested, as this will allow a taint to be reduced to nearly 1·8% of the original value.

A design method is given based on hypothetical taint conditions, the generalized curves, and the characteristics of any proposed counterflow column.

The extent of removal of the reference substances diacetyl and acetoin from water and cream in the Vacreator under varied conditions of operation has been investigated.

1. It was not affected by change in diameter or length of the uptake-pipe, or, with water, by changing the position of the equilibrium-valve to the top of the uptake-pipe or by introduction of the steam at the bottom of the pasteurizer bend instead of at the normal steam inlet.

2. It was affected only slightly by inclusion of baffles in the uptake-pipe, to provide a more intimate contact of steam and cream.

3. With diacetyl in water it was not affected by introduction of the major proportion of the steam via the infuser connexion, and with diacetyl in cream it was not affected until the proportion of steam introduced via the infuser was greater than 50% of the total amount of steam used. With acetoin in both water and cream there was a progressive slight reduction in extent of reference substance removal with increase in proportion of steam introduced via the infuser.

4. With diacetyl and acetoin in water it was not affected by increase in rate of flow of steam and cream at constant intensity of treatment.

5. With diacetyl it was not affected by temperature of steam distillation over the range 138–197°F., but with acetoin there was a progressive reduction in extent of removal with decrease in distillation temperature.

6. Steam is most effectively used if it is applied in separate stages instead of in one stage, i.e., a multiple Vacreator uses steam more efficiently than a single-unit machine. This effect is more marked with diacetyl, which has a high equilibrium coefficient, than with acetoin, which has a low coefficient.

7. The equilibrium coefficient computed from steam treatment of diacetyl solutions in the Vacreator is of the same order as that determined by the laboratory continuous equilibrium vaporization apparatus.

8. The efficiency of cream-deodorization equipment may be lowered by condensation of vapour back into the liquid, e.g. by use of excess cooling water in the Vacreator jackets.

9. There are indications that removal of diacetyl from cream is somewhat slower than from water.

The thanks of the author are due to Mr J. K. Scott for helpful discussion, to Mr E. R. Elley who did the analytical work, and to Mr B. Le Heron for assistance in the operation of the Vacreator and collection of samples.

1. Records were collected from a herd where many cows were producing relatively high daily yields of milk at the time of drying-off. Animals giving 7 lb. of milk or less daily were dried off by intermittent milking, and those giving over 7 lb. daily were dried off by ‘stop’ milking. The data from 113 dry periods were analysed so that a comparison could be made of the new infection rate in the early dry period according to level of production at drying-off.

2. The new dry-period infection rate was found to increase with yield at drying-off. This was not due to age differences between the groups of cows.

3. The rise in total infection rate with yield was due mainly to an increase in Str. agalactiae infections.

4. A high proportion of clinical quarters were detected in the dry period by the cowmen.

5. The yield in the lactation following the dry period studied was not related to yield at drying-off.

6. New dry-period infections were significantly (P <0·01) more numerous in quarters of cows that had been hand-stripped during lactation compared with quarters of machinestripped cows.

Experience in the use of a rapidly growing culture of Streptococcus thermophilus for the assay of traces of penicillin in milk is described.

1. The effect of disinfecting the skin of the teats at drying-off on the incidence of new infection in the early dry period has been studied. After the last milking of lactation the udders of alternate cows to go dry were washed with sodium hypochlorite solution (800 p.p.m.), dried, and each teat immersed in a 5% iodine tincture for 20 sec., and then again immersed 24 hr. later.

2. There was significantly (P <0·05) less new infection in cows that had teats disinfected than in control cows. This difference in infection was highly significant (P <0·01) for cows that were not infected on drying-off.

3. The treatment effect was due to a highly significant (P <0·01) reduction in the incidence of infection by Staph. aureus. New infection caused by Str. uberis was not affected.

1. During 1950–53 the milk of 814 Ayrshire cows was sampled six times during the lactation at intervals of approximately 5 weeks, starting towards the end of the first month of lactation. The weight of milk at successive evening and morning milkings was recorded and samples from each mixed in proportion to the yield. The milk was analysed for total solids, fat, S.N.F. by difference, crude protein (N×6·38), casein and lactose.

2. The weighted lactation average was calculated for each constituent of the milk of each cow and used in genetic studies (see Part II), and in assessing the effect of age on milk composition. The analyses of the individual samples were used to determine the separate effects of stage of lactation and of season on milk composition.

3. Advancing lactation caused the following changes in milk composition:

(a) Milk yield was highest 45 days after calving and then fell regularly to the end of lactation.

(b) Total solids, S.N.F., fat, crude protein and casein contents fell rapidly for 45 days, with fat and total solids continuing to fall for a further 30 days. The concentrations of all these constituents then increased continuously for the remainder of the lactation, rising more rapidly after about 200 days.

(c) The changes in lactose content were opposite from those of fat and protein and smaller in magnitude. The changes bore a marked resemblance to those for yield. The value rose to a maximum after 45 days, fell slowly until about 165 days after calving and then more quickly. The lactose content of milk from cows in their first lactation fell much more slowly with advancing lactation than that in the milk of older cows.

4. Seasonal effects were of smaller magnitude than those arising from advancing lactation and caused the following changes in milk composition:

(a) Yield rose steadily from January to the May-June period, and then fell to a minimum during October-November.

(b) Fat content was at a maximum in October, falling steadily to a minimum in June.

(c) Crude protein and casein contents rose to a peak in May-June and again in September. Lowest values occurred from January to March.

(d) Lactose content was at a steady high level from January to June, falling to a lower level by August at which it remained for the rest of the year. Again there was some similarity in the pattern of change in lactose content and in yield. The range of variation in the values for lactose was less than those for protein and fat, and the variations were mainly opposite in sign.

(e) Total solids and S.N.F. were at a minimum in March and April, but whereas S.N.F. reached their highest level in May-June, total solids were highest in October.

5. Increasing age of the cow resulted generally in increasing yields of poorer quality milk. The difference in composition between milk from the first and the grouped ninth and later lactations was fat 0·19%, S.N.F. 0·34%, crude protein 0·08%, casein 0·21% and lactose 0·25%.

6. On the evidence of cell counts, a number of the samples came from cows with some inflammation of the udder. The average casein numbers, however, did not indicate any serious incidence of mastitis, and it was concluded that although lactose contents may have been lowered slightly, the effect of disease on the average composition was small.

7. Although the figures are not closely comparable, it appears that there has been little change in the composition of the milk of Ayrshire cows in Scotland since 1921–22.

8. The number of individual cow samples deficient in fat was 2% and in S.N.F. 16% of the total. In 560 samples of farm bulk milk none were deficient in fat and only 1·6% deficient in S.N.F. May, June and July were the months with most fat-deficient samples, and the period from January to April provided the largest number of samples deficient in S.N.F., closely followed by the period July to October.

9. Possible causes of the seasonal changes in milk composition are discussed. These changes are thought to be caused mainly by feeding although this does not give a complete explanation of all variations.