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CHAPTER 2 Critical review of lactation and factors affecting lactation in domesticated goats
General introduction
The goat world population is estimated at 746 million (FAOSTAT 2010) of which 223 millions are raised in Sub-Saharan Africa with the majority (more than 90%) being raised by smallholder farmers (Rumoza Gwaze et al., 2010). In South Africa approximately 7 million goats are raised (Donkin and Ramsay, 2000); and out of nine provinces, three use goats for milk production. Goats are available to Africans, who therefore have the potential to produce their own milk in abundance (quantity and quality). Domestic goats (Capra aegagrus hircus) belong to the kingdom of animalia, class of mammalia, order of ruminantia; family of bovidae. The modern goat is a subspecies of goat domesticated from the wild goat of southwest Asia and Eastern Europe between 7000 and 6000BC; it is closely related to sheep; both of them belong to the antelope subfamily of caprinae. (Goat-wikipedia) For thousands of years, goats have been used fo milk, meat, mohair and skins production over much of the world (Mamabolo, 1998). Female goats are referred to as does or nannies; intact males as bucks or billies; their offspring are kids. The name “kid” also refers to goat meat from younger goats while the term “chevon” refers to meat from older goats. As a member of the bovidae family the goat has the ability to convert plant carbohydrates and proteins into available nutrients for human use: milk. Goats can be incorporated into a crop rotation to take advantage of nutrient cycling; they can also be used to control weeds, to harvest crop residues or fight bush encroachment (Goat-wikipedia). Goats belong to the order of “ruminantia”, which means that they are members of the group of animals equipped with a “rumen” (the first major compartment of the fourcompartment stomach that characterises the cow, the sheep and the goat). The rumen is the “furnace” chamber where microbial fermentation takes place thanks to the millions of bacteria, protozoa and fungi that inhabit the rumen. These ruminal microbes have the capacity to use the energy-rich plant parts, making them digestible for the host animal.Most of the grasslands and rangeland plants harvested by the ruminants are made of cellulose (the portion of the plant structure that comprises the walls of the plant’s cells). Cellulose is very fibrous and indigestible to monogastrics (simple stomached animals).But rumen microbes do produce an enzyme called “cellulase” which is the only mammalian secretion capable to breakdown cellulose into cellobiose and then to glucose which is digestible to the microbes and subsequently to the host animals (Rinehart, 2008).Digestion begins when an animal takes a bite from the pasture; as the animal chews, the feed is formed into “bolus” (a packet of food capable of being swallowed). Saliva is excreted, which further aids in swallowing and serves as a pH buffer in the stomach.Once in the rumen, the feed begins to undergo fermentation. Rumen microbes ingest the feed, turning out the end-products which serve as a major source of nutrients for the animal. Some of the principle products formed are ammonia (NH3) methane (CH4) carbon dioxide (CO2) and the volatile fatty acids (VFAs) namely acetate, propionate and butyrate (Church, 1979; Perry, 1980).Of the three VFAs, acetate is found in large extent circulating in the peripheral blood; in the lactating ruminant. The mammary gland is an important user of acetate for milk fat synthesis. As with acetate, propionate is largely unaltered by the rumen epithelium; it (propionate) is transported via the hepatic portal vein to the liver where it serves as a primary precursor for glucose synthesis which is also synthesized at a lesser contribution from AA, lactate and glycerol. Butyrate, which is found in much smaller quantities than acetate and propionate, is extensively metabolized within the rumen and omasum epithelial cells to form aceto-acetate and beta-hydroxy butyrate; any butyrate reaching the peripheral circulation is either oxidized or contributes to fatty acid synthesis (Sherwood et al., 2005). The other end-products resulting from the microbial activity are the large quantities of gas produced – mainly methane (CH4) and carbon dioxide (CO2),which must be expelled from the animal through the processes of respiration and eructation on a continuous basis otherwise bloating, ending in death, can occur quickly (NRC, 2007) .Ruminants require two types of protein in their diet, the protein degraded in the rumen or also the “rumen degradable proteins” (RDP) which are essentially food for rumen bacteria (when microbes die they are passed through to the stomach to the small intestines where they are digested by the animal and absorbed into the animal’s bloodstream). The second group of proteins required by the ruminants in the diet is the one that does not undergo rumen degradation, but passes straight to the abomasum or stomach for digestion; this group of proteins is referred to as “rumen undegradable proteins” (RUP). This is the group of proteins that does benefit directly to the animal body. Rumen microbes differ in preferences for nitrogen sources, with ammonia being the most preferred source of many bacteria. Ammonia is absorbed into the animal’s system through the rumen wall or is consumed by bacteria to become microbial protein.
The microbial protein is then passed through the digestive system to be absorbed in the small intestines (Sherwood et al., 2005)
Energy is the single most important dietary component for an animal after water; energy is derived from carbohydrates, fats, proteins and from the animal’s body reserves. Energy intake maintains body functions and facilitates growth and development, including reproduction and lactation (Rinehart, 2008).
Goat milk yield and constituents and some blood metabolites associated with milk production.
Goat milk yield
Goat milk is popular in the nutrition of babies allergic to cow milk and for various therapeutic uses, including the production of up-market cheeses and powdered milk (Silanikove et al., 2008). Milk is the liquid nutrient secreted from the mammary gland of mammals for their young (Sherwood et al., 2005). It has also been defined as the normal clean and fresh secretion from the mammary epithelial cells of a healthy female mammal excluding week one pre- and post-partum (Pulina, 2002). Milk is secreted from the mammary gland which consists of glandular tissue made of mammary epithelial cells that produce milk and of the excretory ducts that take milk out of the organ. The mammary epithelial cells surround a spherical lumen called milk alveolus; when the cells surrounding the alveoli contract, the hydrostatic pressure in the alveolar lumen increases and milk is propelled out of the alveoli into milk ducts that empty themselves in a wider chamber called the cistern where milk will be stored before and between lactation ((Sherwood et al., 2005).Presumably, the role of hormones is primarily to induce and maintain the activity of synthesizing enzymes in the cells. The rate of uptake of glucose and AA is determined by the rate of synthesis in the mammary epithelial cells, and not by changes in the plasma concentration of metabolic hormones such as insulin, glucagon and growth hormone,which regulate the rate of uptake of these substrates in many other tissues. The mammary cells can take up to 25% of the glucose and up to 60% of the AA that are provided in the blood (Sherwood et al., 2005). Uptake of glucose and AA by the udder is given priority and the udder is allowed to benefit from energy sources stored in other tissues such as adipose and muscle tissues. In dairy cows use of body tissue energy for milk production can account to 82 % (Moe, 1981). Such a redirection of the utilization of nutrients in the body is called “homeorhesis” which is under hormone regulation (Baumann et al., 1983; Bell, 1995) Under homeorhetic regulation lactation uptake of glucose by the mammary gland increases considerably while uptake and utilization of glucose in muscles and adipose tissue is reduced. If insufficient amounts of AA are absorbed from the intestinal tract,muscle proteins will be broken down into AA to be utilized by the mammary epithelial cells. These changes in nutrient partitioning between organs during lactation will occur even when the concentrations of many nutrients in the blood are within the same range as in non-lactating animals (Sherwood et al., 2005). Earlier studies conducted by Sahlu et al. (2007) investigated the effect of diets in milk production and constituents on 249 pastured dairy goats; results showed that milk production and composition were affected by feeding treatment and year. In South Africa, Greyling et al. (2004) compared milk production potential of indigenous and Boer goats fed two different feeding systems. It was seen in the intensively maintained groups that feed intake was significantly (P < 0, 01) correlated to milk production irrespective of the breed. Raats (1988), who worked on the effect of supplementation on milk yield in Boer goats and found that milk production is affected by level of nutrition, reported similar results. However, reports on the effect of level of nutrition on milk constituents are sometimes contradictory. Oltner et al. (1983) found slight changes in yield according to level of nutrition and protein/energy ratio, but no obvious patterns emerged.
Goat milk constituents
Milk constituents are principally synthesized by the secretory cells of the mammary gland from the precursors adsorbed from blood circulation. These precursors derive directly or indirectly from nutrients in the diets (Pulina, 2002). Goat milk can successfully replace cow milk, especially for people who are allergic to cow milk; it is nearest to human milk in its content of fat and protein (see Table 1.1 and Table 1.2, next page) and serves as a good dietary source of minerals which makes it a complete food for neonates (Bawala et al., 2006). Goat milk is a very tasty, very delicious and very nutritious product with a slightly sweet and sometimes salty undertone.
DECLARATION
ACKNOWLEDGEMENTS
ABSTRACT
SUMMARY
LIST OF ACRONYMS
CONTENTS
LIST OF TABLES
LIST OF FIGURES
LIST OF PICTURES
LIST OF GRAPHS
CHAPTER 1 Introduction
1.1 Project theme
1.2 Project titlle
1.3 Motivation
1.4 Statement of the problem
1.5 Aims
1.6 Objectives
1.7 Hypotheses
1.8 Research questions
CHAPTER 2 Critical review of lactation and factors affecting lactation in domesticated goats
2.1. General introduction
2.2 Goat milk yield and constituents and some blood metabolites associated with milk production.
2.2.1 Goat milk yield
2.2.2. Goat milk constituents
2.2.2.1 Lactose content of goat milk
2.2.2.2. Protein content of goat milk
2.2.2.3 Lipid content of goat milk
2.2.2.4 Milk urea nitrogen (MUN) content of goat milk
2.2.2.5 Somatic cell counts (SCC) of goat milk
2.2.3. Selected blood metabolites associated with milk production
2.2.3.1 Blood glucose concentration in goats
2.2.3.2 Blood urea nitrogen (BUN) concentration in goats
2.2.3.3 Blood Cholesterol concentration in goats.
2.2.3.4 Blood free fatty acids (FFA) concentration in goats
2.3. Phenotype score (PTS)
2.3.1 Body condition scoring (BCS) in goats
2.3.2 Breed
2.3.3 Udder characteristics
2.3.4 Age
CHAPTER 3: Materials and Methods
3.1 Introduction
3.2 Experimentation
3.2.1 Location
3.2.2 Animals
3.2.3 Experimental design
3.2.4 Research plan
3.3 Data collection
3.3.1. Milk sampling
3.3.2 Blood sampling
3.3.3. Recording of phenotype characteristics
3.3.3.1 Measurement of udder size and udder attachment
3.3.3.2. Assessment of BCS
3.3.3.3 Age determination
3.4 Biochemical analyses.
3.4.1 Milk analysis
3.4.1.1 Somatic cell count
3.4.1.2 Lactose
3.4.1.3 Milk proteins
3.4.1.4 Milk fat percentage
3.4.1.5 Milk urea nitrogen
3.4.2 Blood analyses
3.4.2.1 Glucose analysis
3.4.2.2 Plasma urea concentration
3.4.2.3 Plasma Cholesterol
3.4.3 Feed analyses
3.4.3.1 Gross energy (GE)
3.4.3.2 Crude protein (CP)
3.4.3.3 Calcium (Ca)
3.4.3.4 Phosphorus (P)
3.5. Statistical analyses.
CHAPTER 4: Results and discussion: Effect of goat breed on milk yield and components
4.1. Milk Yield
4.2. Lactose concentrations
4.3 Concentrations of milk proteins
4.4. Milk fat concentrations
4.5 Milk urea nitrogen (MUN) concentrations
4.6. Milk somatic cell count
4.7. Effect of breed on milk yield and components
4.8 Conclusions
CHAPTER 5 Results and discussion: Effect of goat breed on selected blood metabolites associated with milk production.
5.1 Blood glucose concentrations
5.2. Blood urea nitrogen (BUN) concentrations
5.3 Blood free fatty acid (FFA) concentrations
5.4 Blood cholesterol concentrations
5.5 Effect of goat breeds on selected blood metabolites associated with milk production
5.6 Critical component analysis of milk yield and milk constituents in different breeds of goats.
5.6 Conclusions
CHAPTER 6: Results and discussion: Effects of phenotype characteristics on blood metabolites, milk yield and constituents.
6.1 Introduction
6.2 Effects of phenotype characteristics on milk yield and constituents
6.2.1: Effect of BCS on milk yield and constituents
6.2.1.1: BCS and Milk yield
6.2.1.2: BCS and milk constituents
6.2.2: Effect of udder size on milk yield and constituents
6.2.3: Effect of udder attachment on milk yield and constituents
6.2.4 Effect of age on milk yield and constituents.
6.3 Effect of phenotype characteristics on blood metabolites.
6.3.1. Effect of BCS on blood metabolites
6.3.2 Effect of udder size on blood parameters
6.3.3: Effect of udder attachment on blood parameters
6.3.4 Effect of age on blood metabolites.
6.4 Predicting the goat’s milking capacity from PTS
6.5 Conclusions
CHAPTER 7 General conclusions
BIBLIOGRAPHY
ADDENDUM: RAW DATA
