Ruminant conception

Determining whether a breeding female conceives when mated is critical to understanding and modelling breeding and herd dynamics and will in turn dictate birth rates, weaning rates and herd sales. Just as there are a number of methods used to report conception rate, there are a number of approaches available to model this aspect of breeding.

Ruminant conception is determined based on two processes; (1) the determination of whether the female is able to conceive, and (2) the calculation of a conception rate (or the probability of conceiving) based on her breeding history and current body condition. The interaction of these two processes and the body condition of the individuals (based on feed and pasture quality through time) will determine the breeding interval for each individual.

The following conditions must all be met for a female to be able to conceive:

  • Her age must be greater than the minimum breeding age (see Ruminant parameters breeding)
  • Her age must be less than or equal to the maximum breeding age (see Ruminant type when using Controlled mating)
  • Her weight as a proportion of Standard Reference Weight must be greater than the specified amount (see Ruminant parameters breeding)
  • She must not be currently pregnant
  • It must be greater than or equal to the minimum number of days since last birth (or loss of fetus, see Ruminant parameters breeding)
  • If uncontrolled mating is being used
    • There must be bulls (adult males able to breed) present with the breeding female (see Breed ruminants) and the number present must be able to service all breeders.
    • It must be at least the calculated Inter-Parturition period since last conception (if using conception curve method)

If these conditions are met, we move on to calculate the conception rate based on the following methods and then compare the calculated rate with a randomly derived value between 0 and 1 to determine if conception was successful.

The first method uses an equation to estimate conception rate and apply this rate to all females of breeding condition (of breeding age and weight, see Ruminant conception curve). This approach can be used to ensure the desired birth rates are achieved to match field measures and works well with a single controlled mating performed once per year. This approach was used in the IAT model and is calculated for each cohort of females. Multiple curves can also be used (as in NABSA model) with each curve representing the number of offspring the mother has had and her age at first mating which have been found to influence the success of mating (see Ruminant conception advanced).

A problem arises when we try to use the curve based conception calculations across multiple time-steps (e.g. months) as occurs with natural mating and controlled mating where males and breeding females are brought together for a number of months. This is because the annual conception rate cannot be applied to individuals more than once. For example if a female has an 80% of conceiving based on her current condition, 80% of individuals in this group would conceive in the first month (given access to males or artificial insemination). This is effectively where this approach should stop to ensure the correct number give birth. But, if we apply this approach in a second time-step (month), 80% of the remaining 20% (who did not conceive in the first month) will conceive in the next month resulting in 96% of individuals conceiving. As more matings take place the conception rate further increases and does not represent the desired value. For breeding over multiple months we need to consider an alternate approach, or reduce the conception rates calculated by the conception curve to account for the known number of matings possible.

The individual based herd approach used by CLEM allows us to consider the likelihood of any breeding female conceiving on any mating occasion. We assume that the likelihood of conceiving is related to the condition of the female (see Ruminant conception by condition). CLEM currently uses a simple approach where we use the individual's current weight divided by the nominal weight as an index of condition. A minimum condition cut-off is provided below which conception does not occur. This method means that mating can occur all year round (natural mating, given males and females are located together) and mating will only be successful when the females reach a set body condition. This approach provides the functionality to limit breeding as body condition declines seasonally based on forage quality and quantity and females recover after birth and the energy demands of lactation. Further functionality provided in the model can limit the number of females that can be serviced in a time-step. A maximum conception rate is also provided as a property to account for the binary 0:1 nature of this approach and provide maximum rates known for a simulation. Future version of this approach can improve the condition calculation or probability of conceiving based upon condition.

Some breeds will not conceive while lactating and allowing suckling to remain with the mother can be used an a means of timing mating. The probability of conception during mating (Ruminant parameters breeding) is multiplied by the calculated conception rate to determine a final conception rate. A value of 0 for this property will prevent conception during lactation and a value of 1 will use the calculated conception rate with no effect of lactation on conception.

Any breeding herd must have a conception style supplied and associated parameters provided. This is achieved by adding the desired conception component below the Ruminant type.

See also