3. Growth and body composition of animals

Growth

Increase in amount of living material (cell mass)

Why is growth important?

To know how to increase lean meat and decrease fat proportion

• Understanding growth can help to maximize yield in this way

Growth curve in animals

• Consists of an S curve

• At some point maximum growth rate has been reached.

• You want to slaughter soon after this because that is when the meat is best. Afterwards the animal may change

Different growing rates in different parts

• depending on which part you might want to be best, and also having the rest be okay, affects slaughter time

• when you see graph you see that 4 is developed much later

• you want a minimum level of intramuscular fat (eating quality) vs. wanting to reduce overall fat levels.

what is the best time to slaughter the animal?

After most muscle growth but before substantial fat deposition

Growth of muscle tissue consist of two processes:

1. Hyperplasia: number of cells (myoblasts) increase by cell division

   • Before birth the most cell division happens

   • 30-40 cell divisions occur during gestation vs. to 2-4 postnatally

2. Hypertrophy: cells size increases

   1. After birth tissue mass increases with littl echange in number of cells (cell hypertrophy)

Myoblasts

Cells that will form muscle fibres

Myoblasts (each has single nucleus) →

‘fuse’ and lose separate cell membranes → multinucleated myotubes (immature muscle fibres) → elongate into mature muscle fibres

What are each of the question marks?

• Top one = muscle

• Second one = fiber bundle

• Third = fiber

What are each muscle fibres made up of?

Myofibrils

Myofibrils

Contractile organelles (contain myofilaments actin and myosin, where ‘overlap’ is the sarcomere)

What happens when the number of myofibrils increases?

The myofibrils split longitudinally = muscle is denser

Sarcomeres

• Functional units of contraction

• As muscle fibre grown in length, new sarcomeres added to ends of myofibrils

Effect of exercise on muscle growth vs larger animal species

• Exercise increases size of existing muscle fibres not number

• Whereas larger animal species have a greater number of muscle fibres in muscles, of similar fibre size.

Strong cows

• Doubled muscled animals - more and large muscle fibres in muscles than normal animals

• Lack myostatin (inhibits differentiation and growth of myoblasts before birth (a protein that normally acts as a brake on skeletal muscle growth.))

• Leads to increased muscle mass

How can muscle grown in volume?

Myofibrillar hypertrophy

• Increase number of myofibrils (muscle becomes stronger and denser)

Sarcoplasmic hypertrophy

• Increase of sarcoplasmic fluid within muscle fiber

Myostatin-related muscle hypertrophy

• Rare condition involves reduction in body fat and muscle size increases.

Fat growth

• Fat cells are also known as adipocytes

  • Size differs in different fat depots and growth stages

  • Larger in dpeots laid down earlier

  • Can reach 100 micrometer in diameter

What are four major depots of fat cells

• Subcutaneous

• Perinephric (surrounds kidneys

• Omental (fold of peritoneum connecting stomach and other abdominal viscera, caul)

• Inter and intramuscular fat.

Control of growth

Large number of hormones and growth factors implicated in control of various aspects of growth

• promote/inhibit proliferation of differentiation of cells

• Different effects in different sorts of cells

Carcass yield

• Carcass most important

• But liver, kidneys, etc. are also valuable

Carcass dressing sheep

Head removal immediately after slaughter, abdominal fat not removed, skin removed

Carcass dressing beef

Head removal immediately after slaughter, abdominal fat removed, skin removed

Carcass dressing pig

Head removal immediately after chilling and butchery, abdominal fat not removed, skin not removed.

Carcass dressing chicken

Head and feet removal before chilling and butchery, skin not removed

What is the carcass?

All animal parts that will be sold as joints/steaks

Dressing percentage

The proportion of a live animal's weight that remains as a dressed carcass after slaughter, removal of the blood, hide, head, and internal organs (viscera).

= hot carcass weight/live weight x 100

live weight = The total weight of the intact, living animal recorded prior to slaughter

hot carcass weight = The weight of the dressed animal carcass measured immediately after slaughter and dressing, before the carcass is chilled

Variation in dressing %

Can vary widely:

• Between species

• Between breeds (some have smaller organs or thicker skin)

• Different levels of fatness

• Between sexes

• Animals reared in different husbandry systems

Carcass yield from different species

Higher for pigs and broilers because smaller organs = more carcass

Carcass conformation

• Conformation = shape

• Reflects proportion of muscle to bone

• Good conformation → appearance of thicker more defined muscles

• At same level of fatness these carcasses yield more lean meat

When is most fat deposited?

• Fat is last to mature, therefore older animals are more fat

• Fat is deposited mainly from onset of pubery

• Male animals are leaner than females, they grow to larger mature size

Sex hormones important in fatness and growth control

• Androgens → actions like male sex hormone testosterone

• Oestrogens → actions like female sex hormone oestradiol

• Progestagens → act like female hormone progesterone

What does the effect of synthethetic sex hormones depend on?

The sex of the animal to which it is administered.

• Balance of male and female hormones is important

• Femal es tend to respond better to androgens

• Males to oestrogens

Castrates

Are given a combination of androgens and oestrogen to make up for the hormones they do not get anymore because the part that used to give the hormones is now removed.

Anabolic steroids

• Increase growth rate and feed efficiency

• Reduce fatness of carcasses (at least in females and castrated males)

• Lead to some reduction in amount intramuscular (marbling) fat in beef meat - unless killed when older and heavier (more mature)

• Implications in grading in countries such as North America - lowering marbling is reduced grade.

Effect of intramuscular fat on meat quality

Intramuscular fat contributes to tenderness → some anabolic implants (which lead to decreased intramuscular fat) may reduce eating quality

Regulation on anabolic sex steroids

• Use of anabolic sex steroids widespread in cattle in Europe until banned in EU

  • Concern about potential effects of steroid residues on humans - consume meat and offal from treated animals

  • When dose residues not adhered to

  • But still in use in countries outside EU

Beta-adrenergic agonists

Beta-adrenergic agonists are synthetic or natural compounds that bind to beta-adrenergic receptors to stimulate cellular signaling and alter nutrient partitioning, increasing muscle mass while decreasing body fat

• Class of pharmacological compounds → potential to alter muscularity and ratio of lean to fat in meat

• Chemical structure similar to naturally occurring catecholamines

• Chemical structure of adrenaline and found beta-adrenergic agonsists

Disadvantages of Beta-adrenergic agonists

• May produce meat darker and duller in color and tougher after cooking

• Darker color caused by reduced glycogen levles in muscle at slaughter (is higher pH)

• Toughness may result from lower activity of proteolytic enzyme systems post-mortem

• Use not permitted in EU.

Porcine somatotrophin (PST)

Giving frequent growth hormones (somatotrophin) to pigs decreases carcass fat and increases muscle development.

• Studies show increase in lean tissue growth rate and decrease in fat deposition

• Food conversion more efficient

• Unclear if there are negative effects on muscle quality but marbling fat reduced in line with overall carcass fat (might influence tenderness)

Bovine somatotrophin (BST)

Giving frequent growth hormones (BST) to ruminants

Somatotrophins can be produced by bacteria using recombinant DNA techniques to insert appropriate genes

• naturally occuring may be more acceptable to consumers than beta-adrenergic agonists

not currently permitted in EU

Antibiotics in feed

• Promote growth/feed efficiency

• Bacterial resistance concerns - use is banned in many countries

• Ionophores - organic chemicals that promote ions movement across cell membranes

  • E.g. Monensin

  • Incl. diet → improve feed efficiency by reducing amount of food eaten