Three groups of Friesian cows, one of cows in early lactation, one of cows in mid-lactation and one of cows in late lactation, received over successive 4-week periods according to a double changeover Latin square design, rations providing a low, a normal and a high plane of energy nutrition. With the group of cows in early lactation the treatments were repeated again in mid-lactation and in late lactation.

In all experimental groups there were increases and decreases in the treatment mean values for milk yield and solids-not-fat, protein and lactose contents, in response respectively to overfeeding and to underfeeding. The responses in lactose content in early and mid-lactation were small (overfeeding, 0·00 to +0·06 percentage units; underfeeding, −0·01 to −0·08 percentage units) and less than those in protein content, whereas in late lactation they were larger (overfeeding, +0·09 to +0·14 percentage units; underfeeding, −0·13 to −0·19 percentage units) and of the similar size to those in protein content.

The teats of quarters which were milked, and those in which milking had ceased, of 16 cows were exposed to bacteria twice daily for 2 weeks by dipping them in a suspension of streptococci and staphylococci.

Of 32 quarters in which milking had ceased 22 became infected in this period. Eighteen infections were with Streptococcus dysgalactiae, 2 with Streptococcus uberis, 1 with Staphylococcus aureus and 2 with organisms not used in the culture. Only 3 of 32 milked quarters became infected in the experimental period: 2 infections were with Str. aureus and 1 with an organism not used in the culture. Animals differed in their susceptibility to infection.

Quarters which had been unmilked for 2 weeks were as susceptible to new infectionas those in which milking had just ceased.

In this experiment, factors such as intramammary pressure, leakage of milk, concentration of bacterial inhibitors, composition of secretion and cell counts of the secretion did not appear to influence the rate of infection. Yield at drying-off, milking rates and teat patency were not related to the differences in susceptibility between animals. It is suggested that the main reason for the lower new infection rate in the milked quarters is that pathogens entering the streak canal or teat sinus are usually flushed out with the secretion during milking and do not cause infection.

A versatile continuously-stirred fermentation apparatus, suitable for batch and continuous culture studies, has been constructed from glassware normally available in the laboratory. With the aid of normal workshop facilities an effective stainless steel stirrer gland was constructed. This apparatus has been used successfully for batch and continuous culture studies on the growth of lactic streptococci in skim-milk medium.

Investigations on the growth characteristics of Streptococcus lactis ML8 in skim-milk medium have indicated that agitation of the medium may decrease the growth rate of the organism by decreasing the availability of CO2 (CO2 effect) and of soluble nitrogen (soluble nitrogen effect). The former effect was observed only when the CO2 concentration was depleted by heat sterilization; the latter occurred when a certain threshold level of agitation was exceeded. Results suggested that the soluble nitrogen affect was caused by an alteration in the rate of diffusion of substances formed in the immediate vicinity of the bacterial cell by the proteolytic enzyme system.

The behaviour of Streptococcus cremoris AM1 and AM2, and Streptococcus lactis ML8 in continuous growth in reconstituted skim-milk medium under controlled conditions of pH has been investigated.

At 20°C under aerobic conditions small quantities of oxygen were found to stimulate the growth rate of Str. lactis ML8 between pH 5·05 and 5·45. Growth between pH 5·45 and 6·35 became erratic due to the accumulation of hydrogen peroxide. Above pH 6·35 hydrogen peroxide could not be detected in the continuously growing culture and below pH 5·05 the growth rate of the culture was not affected by small quantities of oxygen. An effect of direct oxygen toxicity on growth, distinct from that of hydrogen peroxide accumulation, was observed over the limited pH range investigated. After 48 h growth in the presence of small concentrations of oxygen the effect disappeared.

At 30°C under anaerobic conditions a region in the dilution rate curves was observed with Str. cremoris AM1 and AM2 and Str. lactis ML8 which suggested the occurrence of a growth-limiting component. This region of slow growth could be correlated with diminished soluble nitrogen values, and was eliminated by enrichment of the medium with soluble nitrogen.

An unusual feature of the investigation was the dilution rates, indicative of extremely fast specific growth rates, experienced for Str. lactis ML8. It was thought unlikely that wall growth contributed appreciably to the dilution rate as a similar phenomenon was not observed with Str. cremoris AM1 or AM2.

Redox potential changes at 30°C in anaerobic reconstituted skim-milk cultures of Streptococcus cremoris AM1 and AM2 and Streptococcus lactis ML8 have been investigated in batch cultures and continuous cultures maintained at constant pH by the addition of fresh medium. Str. lactis ML8 was more strongly reducing than either Str. cremoris AM1 or AM2. The positive Eh values which can arise in dense continuous cultures of lactic streptococci are not likely to affect either the lactic acid formation or proteolytic activity of the resultant bacterial concentrate. An electrolytic reduction cell is described which allows variations in redox potential in cultures to be controlled simply and effectively.

The effects of daily supplements of 300 g native and 2 hydrogenated cod-liver oils (CLO) to the diets of lactating cows have been examined. Native CLO supplements increased the proportion of propionate and decreased the proportion of acetate in the rumen and also decreased the yields in milk of fatty acids synthesized within the mammary gland and those derived from plasma triglycerides. These effects were reduced or eliminated when the hydrogenated oils were given instead of the native CLO. With all 3 supplements about 15% of the dietary C20and C22 acids was secreted in milk.

Both native and hydrogenated CLO supplements increased the concentrations of the cholesteryl ester and phospholipid components of the α-lipoproteins in the blood plasma. These components contained most of the C20 and C22 acids of the CLO supplements that were incorporated into the blood plasma lipids. With the native CLO supplement about half of the C20 and C22 acids present in the blood plasma were polyunsaturated.

There were no differences in the effectiveness of high-density plasma lipoproteins as activators of triglyceride emulsions for lipoprotein lipase hydrolysis whether the lipoproteins were from cows receiving the native or the hydrogenated CLO supplements or the control diets. Significant rates of hydrolysis by lipoprotein lipase could not be demonstrated using as substrates low- or high-density plasma lipoproteins or activated emulsions of their extracted lipids.

Additional in vitro experiments using activated triglyceride emulsions as substrates for mammary lipoprotein lipase showed that replacement of a soybean oil emulsion by an emulsion of native CLO reduced the rate of hydrolysis considerably, whereas an emulsion of hydrogenated CLO was without effect unless it was present at levels in excess of 50%.

It is concluded that the change in rumen fermentation induced by the polyunsaturated fatty acids of CLO is implicated in the decreased intramammary synthesis of fatty acids, but that additional information is required before the reason for the reduced mammary uptake of plasma fatty acids can be established.