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Heat instability or popularly referred to as milk flocculation may result due to various not well understood reasons. It does appear if cow dietary composition may be involved. It also appears if cleaning in milk lines is implicated as protease activity is sometimes found. Furthermore, metabolic and enzymatic alterations occur when milk is heated, especially with long-life products. In this contribution I examined the evidence available in cow dietary composition. The first study was by Dr Reid and co-workers who examined the effect of dietary crude protein and phosphorus in a pasture-based system. This has implications for our south-eastern coastal pasture belt. The study was published in the Journal of Dairy Science, Volume 98 of 2015, pages 517 to 531, with the title: The effect of dietary crude protein and phosphorus on grass-fed dairy cow production, nutrient status, and milk heat stability.
In their study, the authors examined the effect of supplementary concentrates with different dietary crude protein (CP) and phosphorus (P) concentrations on blood nitrogen (N) and P status and on milk yield, composition and heat stability. They fed on a dry matter (DM) basis 4kg concentrates per cow daily to grazing dairy cows that consumed about 13 kg grass DM during early lactation. In the trial design they allocated 48 spring-calving dairy cows to one of four concentrate treatments which were fed for 8 weeks. The concentrate compositions consist of: high CP, high P (HPrHP), which were 30.2% CP and 0.68% P; medium CP, medium P (MPrMP), which were 20.2% CP and 0.47% P; low CP, medium P (LPrMP), which were 10.1% CP and 0.51% P; and low CP, low P (LPrLP), which were 10.1% CP and 0.006% P.
The results show that N excretion in the urine was significantly higher in cows fed the HPrHP and MPrMP concentrate treatments, whereas P excretion was significantly lower in animals fed the LPrLP concentrate treatment. By reducing the level of P in the diet (LPrLP) blood P concentration was lowered, whereas milk yield and composition (fat and protein) were not affected by either CP or P in the diet. Milk urea N varied in line with CP concentration, thus reflecting the expected positive correlation between dietary CP and milk non-protein N. By increasing supplementary CP and P in the diet lower milk heat stability at pH 6.8 resulted, the effect of HPrHP being significant.
In summary, the findings show: (1) Increasing dietary CP caused a decrease in milk heat stability, which reduced the suitability of milk for processing; (2) Increasing dietary CP increased milk urea N and milk non-protein N. In both cases though this, however, will depend on the ratio of soluble and rumen by-pass protein in the CP; (3) Increasing dietary P increased faecal P excretion. These are important considerations for milk processors and producers, especially as CP, P and maybe other important minerals can fluctuate on pastures.
In the second study Dr Martins and colleagues studied the effect of dietary cation-anion difference (DCAD) on the stability of milk proteins. Their study was also published in the Journal of Dairy Science, Volume 98 of 2015, pages 2650 to 2661, with the title: Effect of dietary cation-anion difference on performance of lactating dairy cows and stability of milk proteins.
Previously, it was found that casein micelle stability in the milk is negatively correlated with milk concentrations of ionic calcium. It is also known that ionic calcium in the milk may change according to the metabolic and nutritional status of the cow. The study of Dr Martins and co-workers aimed to evaluate the effect of DCAD on concentrations of casein subunits, whey proteins, ionic calcium, and milk heat and ethanol stability. For that purpose they allocated 16 Holstein cows to four contemporary 4 × 4 Latin square (change over) designs, which consisted of four periods of 21 days and four treatments according to DCAD treatments: 290, 192, 98, and −71 milli-equivalents (mEq) per kg of dry matter (DM).
They found that the milk concentrations of ionic calcium and κ-casein were reduced as DCAD increased, whereas the milk urea nitrogen and β-lactoglobulin concentrations were increased. As a result of these alterations, the milk ethanol stability and milk stability during heating at 140°C were increased linearly with increasing DCAD. In addition, 3.5% fat-corrected milk and fat, lactose, and total milk solids contents were linearly increased by 13.52, 8.78, 2.5, and 2.6%, respectively, according to DCAD increases from −71 to 290 mEq/kg of DM, whereas crude protein and casein content were linearly reduced by 4.83 and 4.49%, respectively. In conclusion, the results suggest that control of metabolic changes in lactating dairy cows to maintain blood acid-base equilibrium should be an important consideration to keep milk stable to ethanol and during heat treatments. This is important in both pasture-based and TMR production systems.