Response of Milk Protein Concentration

Although increases in the yield of milk protein sometimes go hand in hand with increases in the concentration in the milk, this is by no means always the case. Protein yield and protein concentration can vary independently. Whether milk protein concentration increases depends on the response of lactose secretion, which determines milk yield. Since higher concentrations of milk protein are beneficial to milk processing, particularly cheese making, farmers are paid on the basis of protein content. However, despite its obvious importance, we know little of what regulates milk protein concentration.

It is widely recognized that increasing the intake of ME from carbohydrate, but not from fat, generally lifts the concentration of milk protein (Rook and Line, 1961; Gordon, 1979). However, the mechanism underlying these effects is unknown. Relationships with ME intake can be misleading because increases in ME intake are often achieved by increasing the starch content of the diet (see Thomas and Rook, 1983) and starch has a greater effect on milk protein content than either soluble sugars (as molasses) or digestible fibre (Castillo et al., 2001). Indeed, there is good evidence that graded increases in the intake of starch, at constant ME intake, leads to a progressive increase in milk protein content (Keady et al., 1998, 1999; Table 20.2). Some authors (e.g. Keady et al., 1998, 1999) have been unable to resist the temptation to attribute these effects of starch intake to an increased supply of amino acids from an

Table 20.2. Effects of starch intake on the yield and concentration of milk protein3.

Starch content in supplement (g kg"1 DM)

Table 20.2. Effects of starch intake on the yield and concentration of milk protein3.

Starch content in supplement (g kg"1 DM)






Milk yield (kg day 1)






Protein (kg day"1)






Protein (g kg"1)






Starch intake (g day 1)






ME intake (MJ day 1)






Energy balance (MJ day 1)






aVal U8S 3f8 taken either direct, or calculated from, the data of Keady et al. (1998).

aVal U8S 3f8 taken either direct, or calculated from, the data of Keady et al. (1998).

increased synthesis of microbial protein in the rumen. Moreover, this explanation has been favoured despite their own measurements showing no effects on microbial protein production (Keady et ai, 1998) or only very small increases (Keady et ai, 1999). This interpretation is even more puzzling because, although a small increase in microbial output was measured in response to an increase in starch inclusion, an even greater increment in amino acid supply from extra dietary UDP actually led to a small decrease in milk protein content (Keady et al., 1999). It seems, therefore, that effects of increasing starch intake are not due simply to an increased supply of microbial amino acids. It is important to note that increases in milk protein content occur with little or no increase in the yield of milk protein because milk yield is unaffected or even reduced (Table 20.2). A more fruitful approach to uncovering mechanisms regulating milk protein content might be to focus more on the secretion of lactose.

Older studies showed protein undernutrition to have little effect on the concentration of protein in milk, unless the undernutrition was severe (providing only around 60% of requirement) when protein concentration was reduced (see Thomas and Rook, 1983). However, it is now known that deficiencies of specific amino acids can reduce milk protein concentration even though the supply of total amino acids is well in excess of requirement (Table 20.3). Moreover, it is noticeable that supplements of limiting amino acids very often increase protein concentration as well as protein yield (Schwab et ai, 1976; Rulquin and Vérité, 1993; Choung and Chamberlain, 1995c). And the increase in protein concentration may even be enhanced by a fall in milk yield associated with a fall in lactose secretion (Rulquin and Delaby, 1997). Any suggestion that these increases in protein content are linked to shortages of glucose for lactose synthesis is likely to be countered by the view that the supplementary amino acids themselves could act as glucose precursors. But we do not know whether EAAs that are limiting for milk protein synthesis are readily available for gluconeogenesis. In the experiment of Kim et al. (2001c), even though glucose supply clearly affected milk protein content, the provision of glucose did not affect the efficiency of transfer of histidine into milk protein, which suggests that the contribution of histidine to glucose synthesis was unaffected by the availability of glucose. Estimates of the contribution of amino acids to gluconeogenesis in lactating ruminants vary from as little as 2% to as much as 40%, with little understanding of the causes of the variability (Danfaer et ai, 1995). Note also that these estimates refer to the use of amino acids in general; we have no reliable quantitative data on either the use of specific EAAs in gluconeogenesis or its regulation. Attempts to show empirically that increasing tissue supplies of glucose might increase the yield of milk protein by sparing the use of EAAs in gluconeogenesis, so making them available for milk protein synthesis, provide no clear support for the idea (Hurtaud et ai, 1998a,b, 2000). It is noteworthy that

Table 20.3. The effects of the intravenous infusion of a mixture of l-histidine, l-methionine, l-tryptophan and l-lysine on the yield and concentration of milk protein in cows eating a diet of grass silage and a cereal-based supplement containing feather meal. (From Choung and Chamberlain, 1995c.)

Control Infusion3 sed

DM intake (kg day 1)









Milk yield (kg day 1)




Milk protein (g kg 1)




Milk protein (g day 1)




aThe infusion contained (g day 1): histidine, 9.7; methionine, 9.1 ; tryptophan, 2.6; lysine, 30.6.

although additional glucose was without effect, an isocaloric addition of propionate did increase the yield of milk protein (Hurtaud et al, 1998b). This effect of propionate resurrects earlier ideas (see Thomas and Rook, 1983) on a role for propionate in the mechanism whereby an increase in the starch content of the diet leads to an increase in milk protein content (see above). Further comparisons of the effects of propionate and glucose itself on the use of amino acids in gluconeoge-nesis are needed.

A positive relationship between energy status of the cow and milk protein content emerges from an analysis of a large number of feeding trials (Coulon and Remond, 1991). Increases in energy intake and energy balance would be expected to be associated with increased secretion of insulin (Vernon, 1988; Lalman et al, 2000). In this connexion, the reported effect of insulin on milk protein concentration in experiments using the hyperinsuli-naemic-euglycaemic clamp (Griinari et al., 1997) is interesting. However, the results are difficult to interpret. Apart from the unphysio-logical levels of insulin and glucose infused, the effects of insulin are hopelessly compounded with the effects of an increased ME intake (Griinari et al, 1997) or, in the case of Mackle et al (1999), with the known additive effects of ME intake and amino acid supply (Gordon, 1979). Moreover, there is other evidence that argues against a general relationship between milk protein content and insulin or energy status. Infusing casein postruminally in increasing doses progressively increases the concentration of milk protein (0rskov et al, 1977; Whitelaw et al, 1986; Choung and Chamberlain, 1993a). However, in all these experiments, the increases in milk protein content went hand in hand with a decrease, rather than an increase, in energy balance, regardless of whether the energy balance was substantially positive (Choung and Chamberlain, 1993a) or markedly negative (0rskov et al, 1977; Fig. 20.7). Hence the postruminal supplements of casein redirected nutrient use towards the udder at the expense of tissue deposition when the cow was in positive energy balance and stimulated mobilization of body tissue when the cow was already in negative energy balance. Neither action is compatible with an increased

100 200 300 400 500 600 700 800

in infused (g day-1) Fig. 20.7. The effect of infusion of casein into the abomasum on energy balance in lactating dairy cows. The range in energy balance over all experiments was +20 to -41 MJ day-1. (Data of 0rskov etai., 1977; Whitelaw et al., 1986; Choung and Chamberlain, 1993a.)

secretion of insulin (Vernon, 1988). The apparent relation between energy status and milk protein concentration (Coulon and Remond, 1991) seen across a wide range of feeding experiments may again reflect the increased starch intake that usually follows an increase in concentrate allocation.

But insulin can affect milk protein concentration indirectly by regulating the availability of glucose for synthesis of lactose. When insulin was infused intravenously into dairy cows and no attempt was made to control blood glucose, the concentration of milk protein increased as the secretion of lactose, and hence milk yield, decreased (Thomas et al, 1987; Table 20.4). Conversely, arterial infusion of glucose in the goat increased milk yield and reduced the concentration of milk protein (Mepham and Linzell, 1974). These two experiments confirm what would be expected from simple theory: variations in the relative availability of glucose and amino acids might lead to changes in milk protein concen-

Table 20.4. Effects of intravenous infusion of insulin in dairy cows on the yield and composition of milk. (From Thomas et ai., 1987.)

Control Infusion Control

Protein (g day 1) 669 668 680

Lactose (g day 1) 972 762 969

tration. Kim et al. (2001c) further tested this hypothesis by infusing dairy cows intravenously with the first-limiting amino acid for the secretion of milk protein, given with or without glucose. They showed that, without glucose, the infusion led to an increased yield and concentration of milk protein whereas, when glucose was included in the infusion, the same increase in the yield of milk protein was accompanied by an increase in lactose secretion and milk volume, with the result that the concentration of protein was unchanged. It is worth noting that, in the experiment of Kim et al. (2001c), the effects of glucose on the secretion of lactose occurred when blood glucose was within the normal physiological range (hence the supply of glucose would be judged to be adequate). Moreover, circulating levels of insulin were unaffected by infusion of glucose, showing that milk protein content varied independently of insulin.

This is not to imply that glucose status is the key to understanding all the nutritional effects on milk protein content. Although simple relationships emerge in some experiments (e.g. Kim et al., 2001c), results of other experiments discussed above show the mechanisms, or at least their interactions, to be more complex. Progress in unravelling the causes of the effects is hampered by the difficulty in defining nutrient status in enough detail; in the absence of this information, any suggested mechanisms must remain speculative.

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