Nutritional strategies to improve the quality of goat milk
The goat’s milk in Italy is mainly destined for cheese making. There is a considerable variability of dairy production using pure goat’s milk, many of which “proudly” produced with raw milk . From yoghurt, robiola and all its derivatives with slow and low temperature lactic fermentation, to more or less seasoned rennet curd caciotta and tome, the quality of the finished product always depends on the quality of the starting milk .
Goat’s milk has some characteristics that make it interesting for human consumption. Thanks to the characteristic profile of fatty acids, in fact, it assumes an important nutraceutical role in human nutrition, carving out a space for itself in the complicated world of food allergies and intolerances.. Speaking of cow’s milk, milk intolerance is often associated with lactose intolerance, perhaps because the gastrointestinal clinical manifestations are quite overlapping. The first, however, concerns a specific immune reaction, which, however, is often not limited only to gastrointestinal symptoms, but which can sometimes lead to anaphylaxis. This aspect is manifested above all towards the protein component of milk; for this reason it would be more correct to call it allergy. The second, or lactose intolerance, is related to the deficiency of the enzyme responsible for the specific digestion of lactose (milk sugar), this enzyme lactaseit is found in large quantities in the lactation phase, and reaches its maximum level around the 34th week and then decreases after weaning. The presence of the enzyme in adult subjects remains in about 30% of the population with very high variability, up to almost total absence, for example in the Asian population. Consuming milk from goat or other mammals other than cattle does not solve the problem because the milk sugar is always lactose ( Lomers et al., 2008).
In the face of an intolerance, or rather in the face of an allergy to cow’s milk, can consuming goat’s milk or other mammals bring benefits? The proteins of cow’s milk are very similar to those of the milk of other mammals, including goat, therefore those who manifest allergy to cow’s milk only sometimes find benefits in consuming goat milk ( Infante et al., 2003). So why, in the common imagination, would goat’s milk seem more tolerated than bovine milk? Simply because it is more digestible . It has a lower content of fat, proteins and sugars and, as regards the lipid component, has a lower content of saturated fatty acids than cow’s milk.
The solid component of milk, however, takes on an important significance in terms of dairy transformation . In fact, it is the proteins and fat that in the dairy, in addition to conditioning the dairy yield, make the excellences we know well unique. Furthermore, the cheese-making process, by transforming proteins and consuming lactose, often solves the problems associated with intolerances and allergies. In the breeding of the dairy goat, optimizing or, better still, standardizing the quality of the milk, is much more complicated than in breeding the dairy cow . To understand which strategies can be implemented to improve the quality of milk, it is necessary to review the synthesis mechanisms, specifically of fat and protein.
The milk fat mainly consists of triglycerides. Compared to cow’s milk, goat’s milk has a rather different fatty acid profile: it is in fact particularly rich in medium-long chain fatty acids and, in absolute terms, has fatty acids with a lower degree of saturation ( Chillard et al., 2003). This aspect, we have already underlined, makes it particularly interesting for human health. Short-chain fatty acids are synthesized in the breast from precursors derived from rumen fermentation (acetate and butyrate). The long-chain fatty acids are instead transferred directly to the breast from the bloodstream. The medium-chain ones have a mixed genesis.
The percentage of fat in goat’s milk is higher at the beginning of lactation and then decreases in most of the lactation. Towards the end of lactation it may again increase ( Sauvant et al., 1991). This phenomenon depends on two reasons: the first is represented by the dilution effect which concentrates the fat more at the beginning and at the end of lactation, ie when we have lower productions; the second is linked to the mobilization of NEFAs, so it is specific to the onset of lactation.
There is a direct correlation between the blood content of NEFA and the concentration of fat in milk, these being direct precursors for breast lipid synthesis . In summary, at the beginning of lactation: negative energy balance, increase in NEFA concentration, higher breast lipid synthesis. Furthermore, in the normal reproductive season with deliveries at the end of winter, the lactation peak between spring and summer is exceeded, a period in which the lowest levels of fat in absolute are recorded, up to even reaching the fat-protein inversion: the huge puzzle for those involved in feeding the dairy goat.
The milk protein is mainly represented by casein (75-80%) followed by lactoglobulin, lactalbumin and immunoglobulin above. A small amount, to complete the nitrogen component of milk, consists of non-protein nitrogen of which the main one is urea . Milk protein is synthesized in the breast, although the mechanism of synthesis is not yet fully understood. The way of synthesis of the protein in the breast derives from the availability of its constituents, that is the amino acids. It should be remembered that amino acids are divided into essential, semi-essential, i.e. those that must be taken with the diet because the body is unable to produce them, and non-essential, that is, those that the body is able to produce. It is quite clear that the absence of essential amino acids affects normal protein synthesis in general terms.
The mammary gland is also a very greedy organ for essential amino acids. Milk protein accounts for about half of the dietary protein. This means that, in a lactating animal, the mammary gland represents the major user of amino acids in the diet ( Lapierre et al., 2012). At the breast level, the issue is further complicated. The essential amino acids are in fact further divided into two classes ( Mepham MP, 1982). ; these are typically the branched ones. More simply, some essential amino acids, ie those of the first group, are limiting for the synthesis of milk protein . Those of the second group, having also significance for other metabolic processes, could have a lower role in terms of limiting factors but in any case not negligible.
The nutritional strategies to improve the quality of milk are not simple, but the solutions derive from the concepts set out above. It should be remembered that the season substantially affects the quality of the milk . Titles tend to be higher in autumn-winter and tend to drop in spring-summer when the professional is asked to intervene. Let’s see how:
The lipid component
We have seen that short-chain fatty acids derive from acetate and butyrate produced at the rumen level. Acetic acid derives from the degradation of cellulose by the rumen microflora and represents the most present share of volatile fatty acids (AGV) (about 70%). Butyric acid, on the other hand, is present in much lower quantities. The best strategy to increase mammary fat synthesis is to favor the formation of acetate at the ruminal level and therefore optimize the use of fiber (NDF) .
In addition to the fiber content of the diet, an even more decisive role is determined by the degradability of the fiber . Compared to non-structural carbohydrates, the degradability of fiber is much slower and depends on many factors. Speaking of forages, which certainly represent the fiber-bearing foods par excellence, let’s try to understand what are the factors that influence their degradability.
The use of NDF depends not only on the microflora present in the rumen, but also on some intrinsic characteristics of the fiber itself, namely the ability it has to stimulate ruminative activity (chewing and rumination), a characteristic which is referred to as peNDF . It should be remembered that in order to have an effective ruminative activity at least 2/3 of the NDF must be peNDF!
Regardless of the type of forage (polyphite lawn, alfalfa, ryegrass), and also from the cutting period (first, second, third cut), the fundamental condition for preserving its nutritional qualities is represented by mowing at the right vegetative stage , or the pre-flowering that , at a glance, means the moment when you see about 10-15% of flowers in the lawn to be mowed. The longer it delays, the more the NDF moves towards ADF and ADL or, more simply, the fibrous component becomes less and less degradable. This simple and banal concept still represents today the most insidious limit to the efficient use of the fibrous component.
However, unpredictable climatic conditions often prevent intervention at the right time. For this reason, the use of bandaged fodder is also growing in the breeding of dairy goats . However, it should be remembered that the same wrapped or hay forage will have different characteristics that must be taken into consideration. For example, a polyphite meadow hay, if wrapped, would have the same content in NDF, but about 20% less than peNDF, with an increase in the albeit low fraction A of carbohydrates (AGV and simple sugars) and B1 ( starches, but above all soluble fiber) and obviously a reduction in the amount of B2 (the available insoluble fiber). In summary,the forage ensiling process improves the degradability of the fiber but at the same time reduces the peNDF .
To maximize the lipid concentration of milk it is therefore necessary to focus on the quality of the fiber in all the aspects just considered. When the quality of the forage is not optimal, an effective strategy could be to provide alternative, equally noble, degradable fiber, such as beet pulps and distillers. However, we will guarantee rumen functionality for example by introducing straw which, in the face of a slightly degradable fiber (fraction B and C), assumes a primarily functional significance.
As for the lipid component of non-rumen derivation (medium and long chain fatty acids), we have seen that it directly depends on the lipid content of the diet. Increasing the lipid content of the diet can be a useful strategy . Some studies ( Inglingstad et al., 2017) have effectively shown that even in the goat the addition of hydrogenated fats has contributed to increasing the concentration of milk fat (as it is already widely known for the dairy cow ( Rabieeet al., 2012)). However, the profile of fatty acids also shifts towards medium and long chain ones and above all with a lower degree of saturation. This scenario could represent an added value for the marketing of milk as a food. During the transformation phase, however, this particular aspect of the fatty acid profile represents a problem for the structural characteristics of the product.
However, it should be remembered that, to meet the energy needs already in normal situations, the typical rations of lactating goats are formulated with a fat concentration ranging between 4-4.5% of the SS. This is because in the goat there is an easy risk of exceeding in starches or NSC, inexorably resulting in the pathology (SARA). At the same time, however, an excess of the lipid component has a negative action on the rumen microflora. For this reason, we must be very careful.
In summary, to increase the milk fat content, the strategies to be exploited are: optimize the fibrous component of the diet and add a certain amount of fat, preferably fractionated and by-pass .
The protein component
We have seen that the concentration of milk protein depends on the availability of amino acids and that, of these, the essential ones are limiting factors for the synthesis of milk. It is also useful to remember that casein, being a protein of animal origin, has a profile rich in essential amino acids. This means that the amino acids that reach the breast must be of high biological value. Nutritional strategies to increase milk protein concentration do not always yield the desired results. Often the only achievable goal is to avoid a decline, especially in the most difficult period of the year which is summer.
The generic evaluation of the dietary protein is no longer sufficient to fully predict its metabolic fate, but it would be better to think in terms of metabolizable protein (MP) .
The MP is composed of:
- bacterial protein, which is by far the best source of high biological value amino acids;
- protein that is not degraded in the rumen.
Point 1 is to favor the synthesis of bacterial protein : it is necessary to provide an adequate amount of degradable (and soluble) protein. Paradoxically, there is no theoretical limit to the amount of degradable protein. The only limitation depends on the ability of the rumen microflora to degrade all the protein supplied and is represented by the quantity of bacteria capable of processing all the administered protein. We cannot do anything about this. However, we can put the rumen microflora in the conditions to be as efficient as possible through an adequate energy intake .
It is interesting to observe how the classification of protein fractions and carbohydrates through the CNCPS system has the same nomenclature (A, B, and C) in relation to the degradability coefficient they have within the rumen. It is therefore intuitive to understand how the components of the same group are closely related to each other. Therefore the optimal scenario is the one in which the various fractions of proteins and carbohydrates with the same degradability coefficient must be represented in equal measure . By way of example, providing soluble protein (B1) or even non-protein nitrogen in the form of urea (A) must be accompanied by a corresponding supply of starches (B1) and sugars (A).
Point 2 represents the non-degradable protein content that bypasses the rumen . We have seen how the production of protein by the breast depends on the quantity of essential amino acids, especially those belonging to the first group. Therefore, the nutritional strategy should be to ensure that a large part of the non-degradable protein is of high biological value . Unfortunately, however, the plant world is not so generous in this respect. Some foods, however, have an interesting amino acid profile. Among these we remember the sunflower and its by-products, corn gluten, beer threshers and distillery by-products.
Finally, even in the dairy goat it may be useful to supplement the ration with an intake of amino acids in a rumen-protected form. The bibliography in this regard is modest and the effects on milk quality are not in agreement. Some authors ( Flores et al., 2009) have shown that the administration of protected rumen methionine in dairy goats results in an improvement in both milk production and protein content. Others ( Alonso-Mélendezet al., 2016), on the other hand, did not show significant improvements. Lysine and choline (which is not an amino acid but whose metabolic fate is closely linked to methionine) also seem to determine an improvement in quantity but there is no evidence on the quality of milk. The subject should therefore be investigated further.
The dairy goat has unique characteristics, also in terms of nutritional needs . For this reason, very often the interventions that are put in place to improve the quality characteristics of the milk are disappointing. We have seen that it is not possible to intervene by increasing non-structural carbohydrates to meet the energy needs . This can be achieved by providing the right amount of fat. However, this could have a negative effect on the rumen microflora which is also responsible for the degradation of the protein. All this to say that there is a close relationship between the nutritional principles supplied with the diet and all the various mechanisms involved in the genesis of the lipid and protein content of milk.
In conclusion, in order to improve the quality of milk, the concepts just discussed must be applied in a timely manner from an overall perspective