A project was designed to establish the principles under which cow nutrition and management affect milk protein composition, in particular casein composition, and milk functionality, as it relates to the processability of milk for manufacturing, particularly in the subtropical dairy systems of south-east Queensland.
The project consisted of one systems experiment and two component experiments. Comprehensive data was recorded for milk yield, body condition score (BCS), liveweight, nutrition, cow management, rumen function, blood chemistry, climate, and detailed milk composition, including true protein, fat, lactose, casein, whey protein, urea and mineral concentrations and the proportions of αS1-, αS2-, β- and κ-caseins.
Two contrasting farmlets (Feedlot and Dryland) of twenty cows each were observed over a twelve month period in order to identify the effect of stage of lactation (SOL) on milk protein composition and its interaction with production system, calving season and crossbred genetics. Casein concentration was found to be unaffected by production system, although it was significantly affected by individual feed elements, in particular, non-fibre carbohydrate and starch concentrations. Although there was no significant effect of calving season on casein concentration, cows milked over summer (i.e. spring-calved) showed significant negative interactions of casein concentration with environmental parameters (temperature and temperature and humidity index (THI)). The addition of crossbred genetics improved casein concentration. Neither calving season, crossbreeding nor production system had any effect on whey protein concentration, however in both farmlets, there was a significant effect of crude protein intake and non-fibre carbohydrate concentration on whey protein concentration. Mass fractions of all individual caseins displayed significantly different, parallel spline curves when compared by calving season. Spring-calvers produced greater mass fractions of αS2- and β-caseins and lower mass fractions of αS1- and κ-caseins than autumn-calvers. Spring-calved cows in the Feedlot farmlet had significantly lower mass fractions of β-casein and greater mass fractions of αS1-casein than Dryland farmlet cows. Crossbreeding caused a significant increase in the mass fraction of β-casein.
A randomised split-unit design experiment was conducted in order to identify how heat stress affects milk protein production, and to separate its direct physiological effects from the indirect effects of reduced VFI. Separate groups of 12 Holstein-Friesian cows were used in two runs (24 cows total). The cows were tethered in six temperature controlled rooms in pairs. The cows were randomly allocated to three treatments. In each experimental run, two rooms were subjected to each treatment. The three treatments were 1) thermoneutral environment + ad libitum intake (C); 2) ad libitum intake + imposed heat stress (H); and 3) intake restricted to that observed in H but thermoneutral environment as in C (R). Heat stress caused significant reductions in milk yield beyond those attributable to reduced VFI only. There was no additional effect of heat stress on milk protein concentration, and it was concluded that intake was the most important factor in determining milk protein concentration. Casein number, casein:whey ratio and casein concentration in milk were all significantly affected by heat stress beyond the effects of diet restriction. Heat stress caused changes in casein and whey protein composition, which were not found with restricted intake alone. The proportion in total casein of αS1-casein increased and the proportion of αS2-casein decreased. The proportion of blood serum albumin (BSA) in total whey protein increased under the effects of heat stress. Heat stressed and restricted intake cows appeared to mobilise different tissues as endogenous sources of energy under paired feeding. When heat stress and restricted intake conditions were removed, milk composition, casein and whey protein profiles of H and R quickly returned to those of C.
Immediate and residual effects of two lengths of low plane of nutrition (PON) on the synthesis of milk protein and protein fractions were studied in a randomised design experiment with three treatments (twelve cows each treatment). All cows were fed on a base diet of 7.0kg DM/day ryegrass pasture, plus supplemental silage, which was removed from the diet to reduce PON for either three or nine weeks. The experiment covered three sequential periods 1) covariate – high PON (five weeks), 2) period of low PON for either three weeks or nine weeks, and 3) period of high PON to assess ability of cows to recover any production lost as a result of treatments (five weeks). There was a significant effect of duration of low PON, with L9 causing reductions in milk true protein, fat, lactose, casein and urea concentrations. With the exception of urea, there was a significant residual effect on the synthesis of these components even after PON was improved. There was no significant effect of low PON of either duration on casein composition.
Through the suite of experiments in this thesis, it was concluded that milk protein composition and milk casein composition are highly variable between cows and over time. The most significant finding of this thesis program was that heat stress caused changes in the metabolism of the lactating cow and in the mammary synthesis of protein and casein components, beyond that attributable simply to reduced (VFI). It would appear that factors associated with the day of sampling, for example ambient temperature, played the most significant role in causing variation within protein composition. An intriguing finding of this suite of experiments was that there would appear to be correlation between sample day (and all its associated environmental factors) and crude protein, true protein, casein and whey protein concentrations, but yet no effect on casein mass fractions. It was concluded that the potential for nutritional interventions designed to alter milk protein composition, and especially milk casein composition, towards a specific profile remains very limited at this time.