Energy Value of Fats and Oils and Their Application in Animal Nutrition

1. Energy Value of Animal Fats and Vegetable Oils: DE, ME and NE Systems

Pure fats and oils are the most digestible feed ingredients, with an apparent digestibility of up to 95%.

Fats in cereals or oil-extraction/starch by-products have lower digestibility because they are encapsulated in plant cell walls and require cell wall breakdown before utilization; cereal germ (especially corn germ) fat has higher utilization efficiency.

As energy sources, fats have higher metabolic efficiency than carbohydrates and proteins:

• Animal fat ME/DE = 99.4%
• Corn 97.6%, Wheat 96.8%
• Soybean meal 91.1%, Fish meal 90.5%, Blood meal 89.4%

In fat metabolism, energy losses due to fermentation and ammonia excretion are nearly zero.

1.1 Energy Utilization Efficiency of Different Nutrients (Determined by C/O Ratio)

The ATP production capacity of nutrient oxidation depends on the carbon/oxygen (C/O) ratio:

• Glucose (carbohydrate): C/O = 1.0
• Amino acids (protein): C/O = 3.0
• Fatty acids (oil): C/O = 9.0

Thus:

• Fat NE/ME = 89%
• Starch NE/ME = 82%
• Protein NE/ME = 57% (large losses via urea/uric acid excretion)

→ Fats have the lowest heat increment, offering significant advantages in heat stress during high temperatures.

1.2 Deviations in Different Energy Systems (DE/ME/NE)

• Digestible Energy (DE, common in English-speaking countries): Overestimates high-protein ingredients and underestimates high-fat ingredients;
• Metabolizable Energy (ME, poultry standard): Fat ME/DE ≈ 99.4%, closest to actual values;
• Net Energy (NE, swine standard): Most accurate, with fats showing the highest efficiency.

Table 1: Energy Values of Nutrients in Different Systems (MJ/kg, relative to starch in parentheses)

表格

Nutrient	GE	DE	ME	NE	NE/ME
Starch	17.5	16.5	16.2	13.3	0.82
Crude Protein	23.7	20.0	18.2	10.4	0.57
Pure Fat	39.3	37.3	37.1	31.5	0.89

(Noblet, 2006)

1.3 Effects of Fats on Production Performance (Net Energy Advantage)

Apple et al. (2009) conducted trials with 28–114 kg pigs:

Corn-soybean meal basal diets supplemented with 5% tallow/poultry fat/soybean oil, formulated to equal ME/Lys, showed no differences in daily gain or feed ratio but significantly increased dressing percentage and carcass fat; no such differences occurred when formulated to equal NE/Lys.

→ The ME system underestimates fat value due to higher net energy efficiency.

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2. Metabolizable and Net Energy Values of Fats and Oils (Feedstuff Table Comparison)

2.1 Ranking of Animal Fats by ME (MJ/kg)

Fish oil > Poultry fat > Lard > Tallow

2.2 Ranking of Vegetable Oils by ME (MJ/kg)

Rapeseed oil/Soybean oil/Sunflower oil > Coconut oil > Palm oil

2.3 Variations Across Databases

• INRA: Uniform values for animal/vegetable oils (no source differentiation);
• CVB (Netherlands): ME for supplemental fat in laying hens upregulated by 15% (close to net energy); adjusted for broilers based on saturated fatty acid ratio;
• NRC (1998): Lard ME correlated with linoleic acid content (6% linoleic acid: 38.5 MJ/kg; 9%: 40.9 MJ/kg).

Table 2: Energy Values of Fats and Oils in Different Databases (MJ/kg, AME/NE)

Fat/Oil	INRA (Layers)	CVB (Layers)	NRC (Swine)	NE (Swine)
Fish oil	37.7	38.5	35.4	31.5
Poultry fat	37.7	38.5	37.7	31.5
Lard	37.7	38.5–40.9	36.0	31.5
Tallow	37.7	36.0	34.0	31.5
Soybean oil	37.7	38.5	37.7	31.5
Palm oil	35.0	35.0	35.0	31.5

(Sauvant et al., 2004; CVB, 2005; NRC, 1998)

2.4 Industrial By-Products (PFAD/Fatty Acid Blends)

• Palm Fatty Acid Distillate (PFAD): High FFA content, energy value 10%–18% lower than intact oils;
• Fatty acid blends: ME ≈ 32–35 MJ/kg, NE ≈ 28–30 MJ/kg (lower than triglycerides).

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3. Key Factors Influencing Fat Energy Value (U/S Ratio, FFA, Age)

3.1 Fatty Acid Composition: U/S (Unsaturated/Saturated) Ratio

Higher U/S = Higher digestibility (Wiseman Equation):

• Chicks/Weanling pigs (U/S 1–2.25): +5% digestibility per 1 U/S increase;
• Older broilers/Growing pigs (U/S 2.25–3.5): +2.5% digestibility per 1 U/S increase.

Typical U/S Values:

• Tallow: 1.1 (high saturated fat, low digestibility)
• Lard: 1.4
• Poultry fat: 2.4
• Soybean oil: 5.0
• Palm oil: 1.2

→ Optimal dietary U/S ≥ 2.25; below this threshold, saturated fat digestibility drops sharply.

3.2 Free Fatty Acid (FFA) Content

• FFA < 20%: Digestibility ≈ 90%–95%;
• FFA = 20%–50%: Digestibility reduced by 5%–10%;
• FFA > 50%: Digestibility reduced by 15%–20%, with poor palatability.

→ Recommended FFA ≤ 35%; high-FFA products like PFAD should be limited (≤50% of total fat).

3.3 Animal Age and Species

• Chicks/Weanling pigs: Low fat digestibility (especially saturated fat), requiring U/S ≥ 2.5;
• Older broilers/Growing pigs: Improved digestibility, U/S ≥ 2.25 sufficient;
• Layers (high-calcium diets): Calcium forms calcium soaps with saturated fatty acids, inhibiting C16:0/C18:0 absorption, requiring higher U/S.

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4. Core Functions of Fats and Oils in Animal Nutrition

4.1 Efficient Energy Supply, Improving Production Performance

• Weanling pigs: Supplement 2%–5%, daily gain +14.5%, feed ratio –8%–10%;
• Growing-finishing pigs: 3%–5%, gain +12%–15%, feed ratio –8%–10%;
• Broilers: 2%–4%, improved gain and reduced heat stress mortality;
• Layers: 0.5%–1.5%, increased egg weight/yolk weight, maintained feed intake in high temperatures;
• Lactating cows: 3%–5% (rumen-protected fat), milk yield +10%–30%, improved milk fat percentage.

4.2 Provision of Essential Fatty Acids (EFA)

• Linoleic acid (ω-6), α-linolenic acid (ω-3): Cannot be synthesized by animals, must be obtained from feed;
• ≥2% dietary fat meets requirements;
• Regulate meat/egg fatty acid composition to produce high-linoleic acid eggs/low-fat meat.

4.3 Promotion of Fat-Soluble Vitamin Absorption

• Vitamins A/D/E/K require fat for solubilization and absorption;
• Exogenous fat reduces variability in raw material fat, ensuring consistent absorption.

4.4 Improvement of Palatability and Feed Quality

• Fats enhance flavor and lubrication, increasing feed intake by 3%–5%;
• Reduce dust, minimize mechanical losses, improve pellet quality.

4.5 Reduced Heat Increment, Alleviating Heat Stress

• Fat metabolism produces the least heat (NE/ME = 89%);
• Replace part of carbohydrates/protein in high temperatures, reducing heat stress mortality by 20%–40%.

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5. Application Strategies for Fats and Oils in Different Animals

5.1 Swine (Net Energy System Priority)

• Growing-finishing pigs: 3%–3.7% exogenous fat; prefer poultry fat or poultry fat + PFAD (≤1%) for lowest cost;
• Sows (late gestation): 5% vegetable oil, piglet survival rate +9%;
• Sows (lactation): 5%–10%, milk yield +10%–30%;
• Formulation Key Points:
  • Dietary U/S ≥ 2.25;
  • Lard requires 25% soybean oil supplementation (to increase U/S);
  • PFAD ≤ 50% of total fat, FFA ≤ 35%.

5.2 Poultry (Metabolizable Energy System, Recommended U/S = 2.5–3:1)

• Broilers: 2%–4%; 1/3 animal fat + 2/3 vegetable oil (poultry fat + soybean oil) for optimal U/S (2.5–3) and digestibility;
• Layers: 0.5%–1.5%; CVB system ME upregulated by 15%, closer to net energy;
• Contraindication: High palm oil (U/S = 1.2) used alone causes loose stools and low digestibility.

5.3 Ruminants (Rumen-Protected Fat/Calcium Soaps)

• Lactating cows: 3%–5% rumen-protected fat (calcium soap/hydrogenated fat) to avoid inhibiting rumen microbes; improves milk fat percentage and yield;
• Beef cattle (finishing): 2%–3%, improves carcass quality and daily gain.

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6. Fat Selection and Formulation Economic Principles

1. Energy value prioritized over unit price: Select ingredients with lowest cost per unit energy (yuan/MJ), not lowest unit price;Cost-performance ranking: Poultry fat > Lard > Palm oil > Fatty acid blends.
2. U/S balance is core: Dietary U/S ≥ 2.25; use vegetable oils (soybean/rapeseed oil) to adjust U/S, animal fats (poultry/lard) to boost energy value.
3. FFA control: Mixed fat FFA ≤ 35%; PFAD ≤ 50% of total fat.
4. ME vs NE system differences:
   • Swine (NE): Fat inclusion +3%, slightly lower cost, higher efficiency;
   • Poultry (ME): Fat inclusion –2%, slightly higher cost, simpler formulation.

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7. Conclusion

1. Fats are the most efficient energy source, with energy value 2.25 times that of carbohydrates/proteins; they offer low heat increment, anti-stress effects, promote vitamin absorption, and improve palatability.
2. Key determinants of energy value: U/S ratio > FFA content > Animal age; maximize digestibility with U/S ≥ 2.25 and FFA ≤ 35%.
3. Optimal formulation strategy: Animal fat (poultry/lard) + Vegetable oil (soybean oil) blend, balancing energy value, cost, and fatty acid profile.
4. Energy system selection: NE for swine, ME for poultry, combined with digestible amino acid systems for precise formulation and improved profitability.