Protein and energy requirements - calculations

How does the lecture estimate metabolisable protein (MP) required for growth?

Estimate the protein retained in liveweight gain, then multiply by 2 to estimate the MP required for production. Add the maintenance MP requirement.

Mp total = Mp maintainence + (P gain x2)

Pgain= protein retained in gain, in g/day when gain is expressed per day. Use the lecture's assumptions.

Calculate MP for a 300 kg medium steer gaining 1 kg/day.

Maintenance MP = 360 g/day.

Protein in gain = 148 g/kg gain × 1 kg/day = 148 g/day.

Mp= 360 + (148 ×2) = 656 g/day

Calculate MP for a 30 kg lamb gaining 100 g/day.

The lecture example gives approximately 92 g MP/day. Remember to convert 100 g/day to 0.1 kg/day before calculating the protein retained in gain.

How do you calculate energy retained (ER) from protein and fat?

ER = (P x 5.6) + (F x 9.4)

• \(P\) = protein in grams

• \(F\) = fat in grams

• Result = kcal

Convert kcal to MJ by dividing by approximately 239. The lecture also reminds you that 1 calorie = 4.18 joules.

Calculate ER for 135 g protein and 450 g fat.

\[ ER=(135 x 5.6)+(450 x 9.4) \]

\[ =4986 kcal approx 20.8 MJ

How do you calculate ME required for production from ER?

MEp = ER/k

\(ER\) = energy retained; \(k\) = efficiency of using ME for production. The lecture gives an approximate efficiency range of 13–25%. Convert the percentage to a decimal before dividing, and keep energy units consistent.

In the lecture's 400 g/day steer example, how is protein in gain calculated?

158 g/kg x 1.026 × 0.4 kg = 65g

The factor 1.026 is the lecture's correction factor for this example.

In that same example, how is ER calculated?

4.23 Mcal/kg x 0.96 × 0.4 kg = 1.62 Mcal

The factor 0.96 is the correction factor used in the lecture example.