Goats milk is frequently used for dairy and the final quality of cheese and milk-derived products depends on milk quality.
Another interesting intended use is the human consumption of raw milk: this has a particular fatty acids content that allows it to replace bovine milk for allergic and intolerant people. In the case of cows’ milk, frequently the intolerance to milk is confused with the intolerance to lactose, probably because of similar clinical signs. Anyway, the one against milk is a specific immune reaction that causes gastroenteric symptoms but can even cause anaphylaxis. This last aspect is linked to milk protein so that it is better to talk about allergy, not intolerance. On the other hand, lactose intolerance is due to the lack of a digestive enzyme (lactase): this enzyme is present in newborns, has a peak at 34 weeks of age of the baby, then decreases its concentration at weaning. Only 30% of adult people maintain lactase (in Asia it is quite unknown after weaning!).
To consume the milk of mammals different from cows doesn’t solve the problem of intolerance because all kinds of milk have lactose as sugar (Lomers et al., 2008). But can goat milk help in the case of allergy? Milk proteins are similar for all mammals so that there are only some benefits from the consumption of goat milk (Infante et al., 2003). Anyway, goat milk is more digestible than bovine because of its lower concentration in fat, protein, and sugar. Moreover, fat contains less saturated fatty acids than bovine milk.
Milk solids are important for dairy: fat and protein are important both for dairy processing and cheese special taste. Furthermore, dairy processing modifies protein and consumes lactose so that frequently solves the problems linked to allergy and intolerance. To optimize and standardize milk quality is more difficult in goats than cows. To understand how to reach the goal, it is important to know the synthesis process for milk, especially for milk fat and protein.
Milk fat
Milk fat is composed of triglycerides. Goat milk is particularly rich in medium- and long-chain fatty acids, with lower saturation than bovine milk (Chillard et al., 2003). Short-chain fatty acids derive from rumen fermentation (acetate and butyrate) while the long-chain ones come directly from the bloodstream, and the medium-chain have mixed genesis. In goat milk, milk fat has a high concentration at the beginning of the lactation, decreases for most of it, and can increase again at end of lactation (Sauvant et al., 1991). This is due to:
- Dilution: at the beginning and the end of lactation the production is reduced;
- NEFA mobilization, especially at the beginning of lactation: blood NEFA are precursors of milk fat so that they are linked.
To recap, at the beginning of lactation: negative energy balance, high NEFA concentration, and high mammary lipid synthesis. Furthermore, kid births at the end of the winter: in spring-summer the peak of lactation is over and goats have a reduction in milk fat with even the inversion of the fat/protein ratio.
Milk protein
Casein is the most of the milk protein (75-80%), followed by lactoglobulins, lactoalbumins, and immunoglobulins. The smallest part of the nitrogen component of milk is the non-protein nitrogen (especially urea).
Milk protein synthesis happens in the udder: the mechanism is not completely known yet, but it depends on the availability of the amino acids. Amino acids can be essential or non-essential. The firsts must be added to the diet because they cannot be synthesized by the organism. If the essential amino acids intake doesn’t fulfill the requirements, protein synthesis (even for milk protein) is affected. The mammary gland needs a huge amount of essential amino acids. Quite a half of the ingested protein is used for milk production so that the udder of a lactating cow is the largest user of dietary amino acids (Lapierre et al., 2012). These amino acids can be further divided into two classes (Mepham, 1982): the first which includes amino acids that have an almost stoichiometric transfer between udder input and output (among them there are Methionine, Phenylalanine, Tyrosine, Tryptophan), and the second which includes with a higher input than the output (the branched-chain amino acids). The first class limits milk protein production, while the second is even involved in several metabolic pathways with a secondary (but not negligible) role in milk production.
Nutritional strategies
Nutritional strategies are not simple to be implemented. It is important to keep in mind that seasons considerably influence milk quality. Milk solids are higher during autumn-winter and decrease in spring-summer (when technicians are more required to intervene).
Milk fat
Short-chain fatty acids result from ruminal acetate and butyrate. Acetic acid comes from the cellulose degradation by the rumen microflora and is the most represented (about 70%) volatile fatty acid (AGV), while butyric acid has lower concentrations. The best strategy to improve fat production in the udder is to maximize NDF use in the rumen improving acetate formation.
Anyway, the most important point is fiber digestibility. Fiber is digested slower than non-structural carbohydrates and it depends on several factors. Forages are the most relevant fiber-bearing ingredients: let see how their digestibility is influenced.
The NDF use is linked both to the rumen microflora and the ability to stimulate chewing and rumination (the physically effective NDF, peNDF). For a good stimulation, the peNDF must be 2/3 of the total NDF. Independently from the forage type and cutting period, mowing at the right vegetative stage is essential to preserve the nutritional qualities of the ingredient: the best is just before blooming when only 10-15% of the field has flowers. The later is the mowing, the more the fiber becomes indigestible. Weather conditions are unpredictable and often prevent mowing at the right time. For this reason, in dairy goat nutrition haylage is increasingly used. It is important to take into account that the composition and characteristics of the hay change if it becomes haylage: i.e. the haylage from mixed-grass meadows hay will have the same NDF, but 20% less peNDF, a (little) increase in carbohydrates fraction A (VFA and simple sugars) and the B1 (starch, especially soluble fiber), and reduced amount of the B2 (the insoluble fiber). The forage ensiling process improves fiber degradability but reduces the peNDF. All the cited aspects of fiber quality must be considered to improve milk fat concentration. It could be useful to give alternative noble degradable fiber, i.e. beet pulps or distillers, to compensate for the low quality of the forage. In this case, rumen functionality will be allowed with undegradable fiber (i.e. straw) with a primarily functional meaning.
The milk fat that doesn’t derive from rumen fermentation (medium and long-chain fatty acids) comes directly from the diet: the increase in dietary lipids can be a useful strategy. Some studies (Inglingstad et al., 2017) demonstrated that hydrogenated lipids addition to goats’ diet contributed to increase milk fat (as known per dairy cows, (Rabiee et al., 2012)). However, the fatty acids profile also shifts towards those with medium and long chains and, above all, with a lower degree of saturation. This scenario could represent an added value for the marketing of milk as a food. On the other hand, during the transformation phase, this particular aspect of the fatty acid profile represents a problem for the structural characteristics of the product.
Dairy goats’ diets lipid content is generally 4-4.5% of the DM to meat animal energy. We must pay attention not to exceed in NSC and starches because of the consequent pathologies (SARA), as well as in lipid content because of the negative impact on rumen microflora.
To recap, the strategies to improve milk fat are to optimize dietary fiber quality and to add dietary lipids (fractionated and bypass).
Milk protein
Milk protein depends on amino acids availability and the essential ones are the limiting factor. Moreover, casein is an animal-origin protein and contains a lot of indispensable amino acids: this means that the amino acids that reach the udder must be of high biological value. Nutritional strategies to increase milk protein concentration can be unsuccessful and often the only thing that we can do is to avoid their decrease, especially during the summertime.
The generic evaluation of the dietary protein is no more sufficient to predict their metabolic fate, but it is useful to think about metabolizable protein (MP). MP is composed by:
- Bacterial protein, the best high-value amino acid source ever;
- Rumen undegraded protein.
The first point is to improve bacterial protein synthesis by giving an adequate amount of degradable and soluble protein. In this case, the only limit is the rumen microflora ability to degrade dietary protein: it depends on the number of microorganisms in the rumen and we can only give an adequate energy intake to support the best microbial efficiency. The CNCPS® system ranks in the same way (fraction A, B, or C) both carbohydrates and protein based on their rumen degradability. The best scenario is the one where the fractions of both these molecules with the same degradability are represented in the same amount. As an example, dietary soluble protein (B1) or non-protein nitrogen (A) must be accompanied by an equal intake of starch (B1) or sugars (A).
Point #2 represents the undegradable protein that bypasses the rumen. As just said, udder protein synthesis depends on the essential amino acid availability. The nutritional strategy should be to improve the biological value of the undegradable protein. This is a daunting task but there are some vegetable ingredients with an interesting amino acid profile: sunflower and its by-products, corn gluten, beer threshers, and distillery by-products.
Lastly, rumen-protected amino acids can be useful even for dairy goats to balance the dietary intake. There are just a few scientific pieces of literature about small ruminants and the effect on milk protein is at odds. Some authors (Flores et al., 2009) indicated an improvement of milk protein due to rumen-protected methionine addition to the diet, while others (Alonso-Mélendez et al., 2016) did not see significant changes.
Even lysine and choline (this is not an amino acid but has a metabolic fate strictly related to methionine) seem to improve milk yield, but there is no evidence about its quality. It would therefore be necessary to investigate the topic.
Conclusions
Dairy goats have unique nutritional requirements, so that very often the interventions to improve the milk quality are disappointing. It is not possible to intervene by increasing non-structural carbohydrates to satisfy energy needs. This goal can be achieved by giving the right amount of fat. However, this could affect the rumen microflora which is also responsible for protein degradation. There is a close relationship between the nutritional principles provided with the diet and all the various mechanisms involved in the genesis of the lipid and protein content of milk. In conclusion, to improve milk quality, the concepts just discussed must be applied from an overall perspective.For more information: marketing@vetagro.comOriginal article here.