Ruminant milk production

Milk production of lactating females is calculated in the Grow ruminants activity as it is related to the current location in the lactation period, quality and quantity of intake, and energy demands. This activity is a required activity for the ruminant model and also controls the suckling individual’s milk intake.

During the calculation of Potential intake, if an individual is lactating, her potential intake will be increased by a lactation modification factor that is a function of the number of milk days (days since start of lactation, i.e. birth) and her body condition. This accounts for additional dietary intake to meet the energy demands to produce milk. The values in this equation are either tracked values associated with the individual (e.g. dayOfLactation) or parameters of the Breed Parameters (Ruminant type) associated with the individual.

Also, during the calculation of potential intake, milk production (see calculation below) is estimated for the time step assuming an average feed quality or the quality values of the intake from the previous time step if available. This is required to calculate the potential intake of any suckling offspring prior to determining the actual intake for the time step. This is made possible because potential intake is calculated for all individuals in descending order of age such that a mother’s milk production will be known by the time it is needed for suckling offspring potential intake calculations.

Any feeding (including grazing) will next take place before the RuminantGrow model calculates the energy demands and individual weight gain (CLEMAnimalWeightGain event). Given the quantity and quality of the individual’s monthly intake is known from what was eaten, the model can adjust breeders’ milk production accordingly before the milk is provided to suckling offspring and any milking activities.

Calculation of milk production

To determine the amount of energy (MJ) required for lactation the value of k for lactation (kl) is calculated using the metabolic energy from the current dietary intake. For this, a lower limit of 50% dry matter digestibility is assumed if no diet dry matter digestibility (%) is available.

where energy metabolic equals

The subsequent potential milk production is calculated based on one of two curves: (1) that for lactating females with a suckling offspring, or (2) that for lactating females being milked.

From the potential milk production, we can calculate the energy required to produce the milk (Jouven et al, 2008)

The energy available for milk production is then modified based on the current growth and energy budget of the individual. This is achieved by considering the energy balance (adjustedEnergyBalance = energyIntake - energyMaintenance - energyLactation - energyfetus) from the previous time step with a lower limit set to

The daily milk production is then calculated using

The milk produced for each individual is then calculated by multiplying MilkProduction by the number of days in the timestep (30.4 for monthly timestep) with the energy used equal to