Animal Nutrition

Definitions

• Nutrition: series of processes by which an organism takes in and assimilates food for growth, tissue maintenance, and production (milk, eggs, wool)
• Nutrient: any chemical element or compound in the diet that supports growth, reproduction, work, lactation, or maintenance of life processes
• Essential nutrient: removing it from the diet causes an abnormality; adding it back eliminates the abnormality (e.g. Ca deficiency → milk fever; Ca/P/Vitamin D deficiency → rickets)
• Feed = food for farm animals; Diet = mixture of feedstuffs supplying nutrients; Ration = supply of feed at a feeding or daily

Six Main Classes of Nutrients

Water

• Animals need ~2× as much water as dry feed intake; can survive much longer without food than without water
• Makes up ~60% of body; muscle tissue ~75% water, fat tissue ~15% water
• Sources: drinking water, water in food, metabolic water (from C₆H₁₂O₆ → H₂O + CO₂ + ATP)
• Water loss: urine (~60%), feces, skin and lungs (15–60%)
• Intake increased by: lactation (dairy cow needs ~0.9 kg water per kg milk), dietary salt, heat stress

Carbohydrates

• Primary source of energy; made of repeating CH₂O units; derived from plants (except lactose)
• Structural carbs: cellulose, hemicellulose (high fiber, not digestible by monogastrics without hindgut fermenters or rumen)
• Non-structural carbs: sugars, starch (corn = 85% carbohydrate)

Lipids

• Soluble in organic solvents; provide 2.25× more energy than carbs/proteins (9.4 kcal/g vs ~4 kcal/g)
• Fats: long-chain, saturated, solid at room temp (e.g. tallow)
• Oils: long-chain, unsaturated, liquid at room temp (e.g. soy oil)
• Functions: energy, solvent for fat-soluble vitamins (A, D, E, K), source of essential fatty acids, palatability

Protein

Most expensive nutrient added to diets. Contains C, O, H, and N (nitrogen). Source of essential amino acids. Molecular makeup involves chains of amino acids linked by peptide bonds.

Vitamins

Fortification = adding nutrients not already present. Enrichment = replenishing nutrients lost during processing.

Minerals

Minerals must be fed in proper balance — over or underfeeding causes serious problems. Cannot be decomposed or synthesized by the body.

Forages vs Concentrates

Feed Efficiency

Ability of an animal to convert a unit of feed into a unit of body mass. Major determinant of cost.

Factors affecting efficiency: diet quality, age (mature animals less efficient), composition of gain (fat vs lean), endocrine status, environment, genetics.

GI Tract Types

Three forces acting on food in the GIT: mechanical (chewing, peristalsis), chemical (HCl, bile), enzymatic (amylase, lipase, proteases).

Stomach — Three Regions

• Cardiac region: mucus secretion
• Fundic region: HCl (acidifies), zymogens (pepsinogen, prorennin), mucus
• Pyloric region: mucus, gastrin (hormone controlling gastric juice flow)
• Zymogens are inactive enzyme precursors activated by hydrolysis (e.g. pepsinogen → pepsin)
• Rennin: complex of enzymes in stomach of young pre-ruminants; curdles milk by coagulating casein into curds → retains milk longer for digestion
• Lingual lipase: secreted by tongue glands; initiates fat digestion in neonates

Small Intestine

• Three sections: Duodenum (digestion), Jejunum (digestion + absorption), Ileum (mostly absorption)
• Villi increase surface area for absorption. Malnutrition causes villus atrophy.
• Secretin (triggered by low pH) and Cholecystokinin (CCK) (triggered by lipid + peptides) secreted by duodenum

Absorption Mechanisms

Hindgut Fermenters (Horses)

• Large cecum (blind pouch) + large colon provide microbial fermentation of fiber
• Produces short-chain VFAs (Volatile Fatty Acids), water-soluble vitamins, and protein
• Only VFAs and water-soluble vitamins are absorbed from the cecum/colon
• VFAs can provide over 70% of horse's energy requirement

Avian GI Tract

• Crop: food storage and moistening
• Proventriculus: glandular stomach (HCl + enzymes)
• Gizzard: muscular stomach; mechanical breakdown using grit (sand and small stones). No sphincter separating it from duodenum.
• Ceca (2): microbial fermentation, water absorption
• Cloaca: common chamber for GI tract, urinary tract, and egg laying
• Phytate-phosphorus (main plant P storage form) requires phytase enzyme for release

Ruminant Animals

True ruminants (Bovidae, Cervidae, Giraffidae): cattle, sheep, goats, bison, moose, reindeer, giraffe, wildebeest, antelope. Pseudo-ruminants (Tylopoda): camels, llamas.

Key characteristic: consume large amounts of fibrous material quickly, then rest and ruminate (chew cud). Microbial fermentation in a multi-chambered foregut makes otherwise indigestible plant cellulose available — animals cannot make cellulase, but rumen microbes can.

4-Compartment Stomach

Rumen Microbes & Fermentation

• Strictly anaerobic environment. >60 bacterial species and 20 protozoal species are normal inhabitants.
• Three niches: sugar fermenters (~3% of typical diets), fiber digesters (cellulose/hemicellulose), starch fermenters (grain diets)
• Protozoa: prey on bacteria, engulf starch granules, important in N recycling
• Heat of fermentation: rumen is warm; up to 10% of total energy lost as heat. Asset in cold stress, liability in heat stress.
• Methane: 5–10% of ingested energy lost as CH₄. Highest with forage, lowest with grain diets. Cattle in US contribute ~20% of all atmospheric CH₄.

VFA Production & Use

VFAs absorbed through rumen papillae → portal vein → liver. Propionate is primary gluconeogenic substrate in ruminants.

Protein Digestion in Rumen

1. Bacteria break down soluble protein to peptides and ammonia (enzymes on bacterial surface)
2. Bacteria synthesize microbial protein from the peptides and ammonia
3. Microbial protein digested in the abomasum (true stomach)
4. Excess ammonia → liver → urea → recycled in saliva or excreted in urine

Acidosis

• Caused by: accidental grain overload, too-rapid diet change to high grain
• Lactic acid accumulates (10× more acidic than VFAs) → rapid pH drop
• Sub-acute ruminal acidosis (SARA): pH 5.0–5.5; Acute: pH ≤ 4.5
• Below pH 6: cellulolytic and methanogenic bacteria decrease, amylolytic bacteria increase, Lactobacillus spp. increase
• Prevention: ≥10% roughage in finishing rations, gradual diet transitions, buffers (NaHCO₃), ionophores

Neonate Ruminants

Born with non-functional rumen. Cleaned by mother. Rumen nearly functional at ~60 days. Suckling causes esophageal (reticular) groove to close → milk bypasses rumen directly to omasum. Diet drives rumen development: grain promotes papillae growth more than hay alone.

Definitions

• Non-nutritive additives: any compound added to the diet for reasons other than nutrient supply
• Drugs: substances intended for treatment, prevention, or diagnosis of disease — require FDA approval
• GRAS (Generally Recognized As Safe): substances with published safety/utility information; efficacy must still be proven for each new product

Non-regulated Additives

Regulated Additives

Antibiotics

• Bacteriostatic (prevent growth) or bactericidal (kill bacteria)
• Used as growth promotants: control sub-clinical disease, nutrient-sparing effect, metabolic effect (more ATP available)
• Problem: antibiotic resistance and potential transfer to human pathogens
• FDA Guidance 209/213: strict withdrawal times before market (monitored by APHIS)
• Common: Oxytetracycline, Chlortetracycline, Tylosin (Tylan), Carbadox (swine)

Ionophores (Ruminants)

• Allow ions to move through cell membranes of gram-positive bacteria → selectively inhibit acetate-producing and lactic acid-producing bacteria
• ↑ propionate, ↓ acetate in rumen → improved feed efficiency
• Also reduce bloat, acidosis, coccidiosis risk, and methane production
• Used in ~95% of feedlot cattle diets (fed at 10–13 g/ton)
• Monensin (Rumensin): 80% market share. ↑ feed efficiency 5–10%, ↑ weight gain 2–7%
• Lasalocid (Bovatec): 15% market share
• Warning: Horses DO absorb ionophores — very toxic, no treatment available

Beta-Agonists (Ractopamine)

• β-adrenergic agonist — shifts energy partitioning from fat to lean muscle tissue
• Not an antibiotic, not a hormone. No withdrawal period.
• Paylean (pigs): last 28 days before market, 4.5–9 g/ton. ↑ carcass wt 10.6 lbs, ↑ feed efficiency 7–15%, ↑ daily gains 6–20%
• Optaflexx (cattle): 200–400 mg/head/day for 28–42 days. ↑ carcass wt 15–20 lbs, ↑ feed efficiency 10–15%

Buffers, Probiotics & Enzymes

• Buffers: NaHCO₃ at 0.75% DM most common for dairy cattle and feedlot cattle during diet adjustment. MgO also used. Prevent rumen acidosis on high-grain diets.
• Organic acids: Citric acid most common for young pigs and chicks. Lowers stomach pH → improved protein digestion, reduced pathogen incidence.
• Phytase: most common added enzyme. From Aspergillus or E. coli. Releases phytate-bound phosphorus from plant feeds; ↑ P absorption 5–7%, ↓ P excretion 50–75%. Primarily for non-ruminants.
• Probiotics (direct-fed microbials): live Lactobacillus, Streptococcus, Bacillus, yeast cultures. Most effective in stressed, newly weaned, or relocated animals.
• Prebiotics: non-digestible ingredients (food for good bacteria); promote lactic-acid bacteria, suppress E. coli. Primarily in pet foods.
• Yeast (S. cerevisiae): source of B vitamins and mannan-oligosaccharides. In dairy cattle: higher rumen pH, ↑ cellulolytic bacteria, ↑ fiber digestion, ↑ microbial protein synthesis.

Bloat

Frothy (pasture) bloat: caused by soluble proteins and saponins from legumes (clover, alfalfa) → insufficient saliva → slime production → foam traps gas. Sudden death risk. Prevention: poloxalene (Bloat Guard) = non-ionic surfactant that consolidates gas bubbles → eructation. Mineral oil also used.