Grow ruminants

A critical requirement for integrating a ruminant growth model into the larger CLEM framework was that it be both simple and sufficiently precise to predict livestock production outcomes under local field conditions. While many models exist for predicting ruminant live-weight gain, most require information on forage passage rates through the rumen or were developed for European cattle breeds. The former data is not readily available for many of the feedstuffs commonly used by smallholders and the latter models could not be confidently applied to many tropical breeds which are small in comparison to European breeds. For example, Bali cattle are well adapted to heat, can work up to 5 hours per day without apparent physical distress and survive well on poor forages. Moreover, Bali cattle have higher fertility rates than many other cattle breeds and buffalo when raised under similar conditions, but have poorer milk production and suffer higher calf mortality rates. Nevertheless, the key determinant of animal growth, reproduction and mortality rates is animal nutrition. Forage quality, measured by digestibility and protein content, commonly limits production, but smallholders have an array of different feed sources of varying quality available at intermittent intervals; e.g. native and introduced grasses and legumes, field crop residues, plantation residues (leaf, stem, fruit), tree leaves etc.

The ruminant model combines published data and field data relating to animal live-weight, live-weight gain, milk production, age at first calf and calving interval, as well as the quality, composition and quantity of the various sources of feed. The model is largely based on published energy functions with coefficients adjusted for various breeds, with additional intake restrictions based on estimated crude protein (CP) requirements. Energy coefficients for different breeds are included, currently, calving and mortality rates are primarily based on published data for Bos taurus and Bos indicus breeds. The model is sufficiently robust to capture responses to both grazing and ‘cut and carry’ systems, cope with distinct wet and dry season conditions, and the feeding of crop residues.

Data input is restricted to the quantity and quality of available forage, with annual pasture, sown forage and crop residue biomass, nitrogen content and date of harvest sourced from the database created from APSIM output (or other crop model, published data, or research results).

Intake is determined from the age and current or previous highest weight of the animal. This growth rate is adjusted for the effects of available forage (for grazing), forage quality, and whether the animal is lactating. Protein requirements are calculated on the basis of the adjusted intake, which, if insufficient, is reduced linearly in relation to crude protein required and crude protein supply. The digestibility and calculated intake determines the digestible and metabolisable energy intake which is partitioned into energy for maintenance and for growth. Calving interval, age at first calf and calf mortality rates are related to the condition of cows, based on published survey data and field observations for the particular breeds.

For more information and equations see the Details section below.

The gross energy content of forage (MJ per kg dry matter) for fodder. This is a standard parameter (18.4) and should not need to be changed.

Methane produced by ruminants while consuming food will be calculated and reported to a Greenhouse gas type store for methane. This entry is a drop-down list of available Greenhouse gas type resources of the Greenhouse gases available for this simulation.

A Ruminant herd with at least one Ruminant type. The ruminant herd must have an initial population defined using Ruminant initial cohorts

This activity will produce methane emissions that will be provided to a Greenhouse gas type in the Greenhouse gases store either specified above or by assuming a store named Methane if provided.

Potential intake is the amount of food an individual can consume in a time-step and is based on the live weight at the time of intake, determined based on the normalised animal weight (coefficient) and the highest weight recorded for the individual. The metabolic live weight (live weight raised to the power of 0.75) is used for sucklings and weaners.

 

Where LiveWeightForIntake is the highest weight this animal has achieved, and IntakeCoefficient and IntakeIntercept are coefficients provided in the Advanced/Diet parameters in Ruminant type. Example values for IntakeCoefficient are 1.7 for cattle and 1.9 for sheep and goats, and for IntakeIntercept 0.024 for cattle and 0.033 for sheep and goats. These values can be changed, but this should be done with caution.

The potential intake will be reduced accounting for the effects of available pasture, forage and tropical forage quality, and competition (see Graze ruminants). Changes in forage intake associated with supplements are affected by the digestibility and protein concentration of the forage and the supplement, the type of forage, the type of supplement and the intake of supplement.

The potential intake will be increased by a lactation multiplier for any lactating females as a function of the day of lactation (dol) (Figure 6 ADD FIGURE).

 

Milk production (L/day) is also calculated in this activity as it is critical to calculating feed intake of suckling individuals and female energy demands

where the milk curve value is determined from supplied values based upon whether the female is being suckled or milked.

This value is capped at zero and uses an adjusted energy balance that considers the energy balance in the previous timestep in calculating milk production.

with the daily energy required to produce milk calculated by

Protein also affects the animal intake. The model uses the ratio of crude protein (CP) required by the ruminant to CP supplied by the feed consumed to reduce intake, equivalent to reduced digestion, before calculating energy provided from the intake.

 

The crude protein (CP) requirements of a ruminant (Protein_coefficient, as grams CP per kilogram dry matter) are converted to CP required per kg feed consumed by multiplying by the proportion digestible (Dry Matter Digestibility as proportion) of the monthly intake.

The supply of protein from the diet is converted to grams CP supplied per kg of feed accounting for protein degradability.

where PercentNitrogen_Intake (as percentage, 0-100) is converted to Total N (g/kg feed) by divide by 100 to convert to proportion N, and multiply by 1000 to give grams, or multiply by 10. This total N per kg feed is multiplied by the N to CP factor of 6.25. A minimum supplied crude protein is set as 0.001 to calculate an intake adjustment factor when supply is less than required

This adjustment factor ranges from 0.5 when there is no crude protein supplied, to 1 when supply equals required. This factor is used to reduce the recorded intake as a function of crude protein supply to demand ratio, thus .

The ‘protein coefficient’ is generally set at 130 g/kg of digestible dry matter, although there are indication that Bali cattle require only 110 or less (D. Poppi, pers. comm.). The work of Schlecht et al. (1999a, b) suggests that west African zebu breeds may be similar to Bali cattle. The values are adjusted assuming the protein is 90% degradable. The protein concentration is the protein content of the available feed supply, taking into account any supplements fed, and the proportion of the ruminant diet taken from each of the pasture pools. The protein coefficient is set in the ‘Ruminant_coefficients’ section of the Ruminant type parameters.

Functions for determining energy intake, energy use efficiency for maintenance, growth and lactation, energy requirements for maintenance etc, have been taken from NRDR (2007).

Gross energy (E_G) content of feed is set at 18.4 MJ/kg DM, and the proportion of digestible energy that is metabolisable (E_M) is set at 0.81. The digestible energy is determined by multiplying the gross energy by the dry matter digestibility (DMD) of the diet (DMD_diet reported as proportion not percent).

Efficiency of use of energy for maintenance (k_m), growth (k_g) and lactation (k_l) are:

where the values for the coefficients and intercepts for K_me, k_m, k_g and k_l are set in the parameters of Ruminant type and start with EMaintenace, EGrowth and ELactation for k_m, k_g and k_l respectively. Changing these values should be performed with caution. The first value in each equation above is the EfficiencyCoefficient and the last value is the EfficiencyIntercept (including the negative sign if appropriate). These parameters are provided in the provided in the Ruminant type parameters . (i.e. EGrowthEfficiencyCoefficient is the coefficient used in the equation for growth, k_g) .

Energy required for maintenance (E_maintenance) is given by:

where Weight_live is the live-weight (kg), age is the age of the animal in days, K_me is a breed factor, S_me is a gender factor, and EM_intake is the total intake of metabolisable energy (MJ).

The value for ‘S_me’ is set to 1 for females (and castrated males) and 1.15 for males.

Energy required for milk production (E_milk) is given by:

The feeding value of the balance of energy (E_bal), which is the metabolisable energy intake minus the energy required for maintenance and milk production, is adjusted for the rate of loss or gain.

If E_bal > 0 then

Otherwise

The megajoules (MJ) of energy required per kg empty body gain (E_ebg) is given by:

Liveweight change is determined by calculating empty body change from E_ebg and E_bal, multiplied by a GrowthEfficiency which increases growth by 9% for Bos indicus breeds.

GrowthEnergyIntercept1 (a) and GrowthEnergyIntercept2 (b) are properties of the Ruminant type with details available in Table 1.11 (page 37) Nutrient Requirements of domesticated ruminants.

If E_bal > 0 then

Otherwise

There are studies which suggest that, for some tropical breeds, the efficiency of use of energy during weight loss may be better than that indicated by the equation above, and hence the actual weight loss is less.

Manure is produced by each individual and placed in a store associated with the animals location (e.g. paddock or yards). Manure is produced at a rate of intake times the non-digestible proportion of the feed

Where DMD is the Dry Matter Digestibility of all intake presented as a percentage.

For details on managing manure see Collect all manure

Methane is produced by each individual and placed in a greenhouse gas store you have named "Methane" using the methane production coefficient (Charmley, XXXX). This

The ruminant growth model provides three forms of individual mortality, a base mortality rate (adult and unweaned rates), death through excessive weight loss, and prenatal mortality.

This activity ages all surviving animals in the herd at the end of the time-step.

This activity will perform natural weaning of sucklings when their age equals the specified (Natural weaning age, months) for the species, or will use gestation length as this value if natural weaning age is set to 0. This ensures sucklings naturally wean if no other weaning activity has been provided.