Comprehensive Animal Science: Meat Production, Utility Assessment, and Breeding Methodologies

Meat Utility and the Fattening Process

Meat production represents one of the most significant utility properties in animal husbandry. Meat, along with milk and eggs, is considered the most suitable food of animal origin for human consumption. Unlike some other utility traits, meat production is not strictly tied to a specific species, breed, or sex. The total amount of meat produced is directly related to the growth intensity of the animals and their fertility. Meat encompasses all edible parts of slaughtered animals, though these parts differ in nutritional, technological, and organoleptic characteristics depending on the species and age of the animal. Fattening (výkrm) is the process by which a breeder intentionally influences the organic functions of the animal to ensure sufficient meat production of optimal quality.

Two critical terms used to evaluate this process are výkrmnost (fattening capacity) and vykrmenost (fatness). Výkrmnost is a genetically determined trait referring to the intensity of body weight gain under optimal nutrient conversion, essentially the efficiency of converting nutrients into tissue. In contrast, vykrmenost refers to the current degree of muscularity and fat cover at a specific time. Assessment of the animal's progress involves "butcher's touches" (řeznické hmaty) to evaluate fatness, focusing on the root of the tail, subcutaneous fat, and internal fat. Slaughter maturity (jatečná zralost) is defined as the optimal slaughter weight, expressed as the relationship between the animal's age and the achievement of an optimal ratio between muscle development and fatness. This represents the ideal time to end the fattening process and is genetically limited by body size and breed. Ending the process too early prevents the full utilization of growth potential, while extending it leads to an undesirable increase in the amount of fat.

Carcass Value and Standardized Processing

Carcass value is a set of indicators assessing the quality of the cold carcass weight (jatečně upravené tělo or JUT) and the meat itself. It is expressed through slaughter yield—the weight of the animal after processing—and the net gain, which is the ratio of JUT weight to the animal's age. For cattle, JUT is defined as two prepared halves or four quarters without skin, head, distal parts of the limbs, or the tail (separated before the first caudal vertebra). Internal organs and the contents of body cavities are removed. In cows, the udder and udder fat are removed; in bulls, the fat above the topside and scrotal fat are removed. Halving is performed along the spinal canal to avoid damaging the spinal cord. It is critical to note that the spinal cord and brain are classified as highly dangerous waste due to the risk of Bovine Spongiform Encephalopathy (BSE), caused by prions.

For sheep, the JUT definition is similar to cattle, but for animals older than $12$ months, the spinal cord must be removed; kidneys and kidney fat remain in the carcass. Porcine (pig) JUT consists of two halves with the head (excluding eye and ear cutouts), including the skin but without bristles or internal organs; the spinal cord and brain are also removed. Poultry JUT is defined as the body without feathers, head, and the distal parts of the pelvic limbs separated at the hock. Slaughter weights vary by category, and the quality of JUT for cattle, sheep, and pigs is evaluated using the SEUROP system across the European Union. This system includes a subjective determination of six muscularity classes (S, E, U, R, O, P, where S is the highest) including potential sub-classes, and fatness classes ranging from digits $1$ to $5$ with sub-classes and super-classes. In pigs, JUT bodies usually range from $60 imes 120\,kg$. The SEUROP classification is fundamental for determining the price of slaughter animals.

Meat Quality Indicators and Defects

Carcass value incorporates several indicators assessing meat quality, which can be categorized into nutritional, technological, sensory, and hygienic-toxicological qualities. Nutritional quality focuses on the nutrient composition, while technological quality covers aspects such as water-binding capacity, color, and $pH$ levels. Sensory quality involves appearance, aroma, and juiciness. Hygienic-toxicological quality ensures the meat is free from residues of undesirable substances, microorganisms, or molds. Factors influencing meat utility include internal factors—such as species, nutrient composition, utility type, breed, sex, age, heritability, and health status—and external factors like nutrition, feeding systems, transport, fasting before slaughter, and the aging process. Meat aging is essential for improving technological and sensory properties, increasing tenderness, juiciness, and flavor, with different aging durations required for different species.

Normal meat quality can be compromised by metabolic defects such as PSE and DFD. PSE (Pale-Soft-Exudative) meat is characterized by a light color, soft texture, and high water loss. This condition is caused by a high volume of lactic acid, leading to a $pH$ lower than $5.8$, and is often triggered by stress factors or genetic predisposition. DFD (Dark-Firm-Dry) meat is dark, tough, and dry. This occurs when an animal is exhausted before slaughter, resulting in a $pH$ higher than $6.2$ within one hour post-slaughter.

Methods of Breeding and Genetic Selection

Breeding (plemenitba) is defined as directed reproduction combined with purposeful selection. It encompasses all zootechnical, organizational, and economic measures leading to the versatile increase of utility in farm animals through intentional mating. Breeding can be classified into classical methods and genetic methods. Classical breeding uses the breed as the basic taxonomic unit, with the primary method being purebred breeding. This includes inbreeding, line breeding, and the refreshing of blood. Genetic methods are categorized based on their technical approach: methods utilizing the resemblance between parents and offspring (including inbreeding and various crossing types), methods utilizing the heterosis effect with selection for combining ability (crossing special lines or reciprocal recurrent selection), and methods utilizing the heterosis effect without selection for combining ability (utility crossing).

Purebred breeding is the most common method, involving the continuous and thoughtful selection and mating of members of the same breed to achieve higher uniformity of traits characteristic of that breed. A specific application within purebred breeding is the "refreshing of blood," which involves a one-time mating with an unrelated individual of the same breed and utility type from a different population (often imported) to prevent overbreeding or stagnation in the original herd.

Inbreeding and Line Breeding

Inbreeding involves the mating of related individuals, typically within the $1^{st}$ to $5^{th}$ generation. It is used to stabilize and strengthen specific traits and has played a positive role in the formation of modern cultural breeds. However, inbreeding increases homozygosity for undesirable factors, which can lead to "inbred depression"—a reduction in viability and utility. Pigs are known to be particularly sensitive to inbred depression. The main goal of inbred breeding is to maintain a specific desirable trait in the herd.

Line breeding (liniová plemenitba) aims to maintain a relationship between a specific sire (the line founder) and his offspring across several generations. The objective is to increase the proportion of the founder's superior genotype in the progeny while keeping the intensity of inbreeding ($Fx$) at a moderate level, specifically below $37.5\%$. To achieve this, the line founder is mated with high-quality unrelated females, and later potentially with his best daughters. It is essential to choose successors among the sons early to ensure the breeding goals are met.

Crossing Methods and the Lauprecht System

Crossing (křížení) involves mating individuals of two different breeds within a species (bastardization) or members of different species (metization). Crossbreeds are named by joining names (e.g., Anglo-Arab) or using breed letters (C for Czech Simmental, A for Ayrshire, H for Holstein). The Lauprecht system is used to identify blood proportions, such as $C50H25A$ ($50\%$ Czech Simmental, $25\%$ Holstein, $25\%$ Ayrshire). Parental generations are denoted as $P$, and offspring as $F_0, F_1, F_2$, and so on.

Improving crossing (zušlechťovací křížení) aims to improve specific traits while maintaining the production capabilities of the original breed. The simplest form is "blood infusion," a one-time mating with an improving breed. A more intensive version is "melioration crossing," where the improving sire is used for $2 imes 3$ consecutive generations followed by strict selection. Combinational crossing involves at least two breeds to create an entirely new breed with a new genetic foundation, such as Brangus (Angus and Brahman) or Beefmaster. Conversion crossing (převodné křížení) involves repeated mating of purebred sires of an improving breed to females of a local breed for $4 imes 6$ generations. This eventually results in a population that is roughly $98\%$ identical to the improving breed, effectively replacing the original population.

Utility Crossing and Heterosis

Utility crossing aims to produce animals for production purposes by utilizing the heterosis (hybrid vigor) effect. Discontinuous utility crossing includes simple crossing (two populations mated to produce an $F_1$ generation with a $50:50$ ratio), back-crossing ($F_1$ females mated back to a parental breed), and three-breed utility crossing. In the Czech Republic, three-breed crossing is typical for pigs, where two maternal breeds ($P_0$ and $P_1$) produce fertile $F_1$ females, which are then mated with a sire of a specialized paternal breed ($P_2$) to produce $F_{12}$ slaughter offspring. Four-breed crossing is also utilized.

Continuous utility crossing includes alternating crossing (switching between two breeds, $A imes B$) and rotational crossing (using three or more breeds in a repeating sequence). These methods are designed to maintain high levels of heterosis across generations without the need to constantly replace the entire maternal base.

Interspecies Crossing (Bastardace)

Interspecies crossing involves mating members of different species to obtain utility animals for special purposes. Historically and biologically significant examples include:

• Donkey ($\sigma$) + Mare ($\rho$) = Mule (mula)
• She-donkey ($\rho$) + Stallion ($\sigma$) = Hinny (mezek)
• Bison + Cattle = Beefalo
• Wisent + Cattle = Zubron (zubroň)
• Yak + Cattle = Dzo (found in the Himalayas, approx. $1\,m$ at the withers)
• Sheep + Goat = Geep
• Zebra crossbreeds with Horses or Donkeys = Zorse, Zebroid, or Zonkey
• Camel + Lama = Cama
• Male Lion + Female Tiger = Liger
• Female Lion + Male Tiger = Tygon
• Leopard + Lion = Leopon
• Grizzly Bear + Polar Bear = Grolar or Pizzly
• Jaguar + Lioness = Jaglion