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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 for 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).
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
6.3 Effect of phenotype characteristics on blood metabolites.
6.4 Predicting the goat’s milking capacity from PTS
6.5 Conclusions
CHAPTER 7
General conclusions
BIBLIOGRAPHY
ADDENDUM: RAW DATA
