S. M. Azzam

Markov chains as a shortcut method to estimate age distributions in herds of beef cattle under different culling strategies.

Markov chains were used to estimate the age distribution at equilibrium and average age in herds of beef cattle and the age distribution and average age of culled cows. These estimates are essential in systems simulation and economic analyses of beef production alternatives because the biological input and output from a beef production system depend on the age structure of the cows in the system. Ten sets of data on age-specific probabilities of culling for different reasons were found in the literature and used in simulation of different culling strategies. Three different culling strategies were simulated using these data. In addition to culling on maximum age, culling was either based on health alone, health and reproductive failure in one year (not pregnant) or health and reproductive failure in two consecutive years (not pregnant twice). The average herd age in the data sets reviewed under the actual culling strategies ranged from 4.58 to 6.73 yr and the average age of culled cows ranged from 5.41 to 9.94 yr. Description of the culling process as a Markov chain enables calculations of the age distribution at equilibrium by simple matrix operations, an advantage over the methods presently used. The scarcity of estimated age-specific probabilities of health and reproductive failure points to the need for more population analyses of beef cattle.

Modelling reproductive management systems for beef cattle

Abstract Management alternatives for beef herds that varied in level of reproductive efficiency were studied by computer modelling. This paper describes the stochastic dynamic model developed to describe reproductive processes in beef cattle. The model is compared with other stochastic models of reproduction in cattle. Results from modelling different management alternatives (length of breeding season and breeding heifers 21 days ahead of cows in spring-calving herds) are discussed. Proportion of females in the breeding herd that became pregnant increased with length of breeding season and by breeding heifers ahead of cows. Proportion of females in the breeding herd that weaned a calf was mostly affected by the pregnancy rate but also by survival of calves to weaning. Average age of calves at weaning increased with increasing first-service conception rate and decreased length of breeding season. With high first-service conception rate, length of breeding season had relatively little effect on age and weight at weaning.

A network model that determines the optimal path of breed-crossing decisions to maximize net returns

Abstract A model for finding the sequence of breed-crossing decisions that maximizes the expected discounted net returns over a given number of generations is presented. The method is based on network analysis. The assumptions are that there is an initial breed of females, and that all females, in a generation will be mated to one of a given number of alternative breeds. The optimal path of breed-crossing decisions depends on the revenues expected from additive direct effects, maternal effects. breed-specific individual and maternal heterosis effects, and the cost associated with choosing a particular breed of sires. A simple numerical example is presented.

Evaluating net income from different durations of breeding seasons in beef production using a deterministic simulation model

Abstract A deterministic computer simulation model was used to evaluate the economic efficiency of reproductive management decisions in beef production: length of breeding season (45, 70 or 120 days) and whether to initiate breeding of heifers 3 weeks earlier than the mature cows. Four conception rates at first service (an indicator of reproductive efficiency) were also simulated (50%, 60%, 70%, 80%) with each length of breeding season. The economic model organizes and summarizes herd performance data, feed requirements and prices to calculate the net income for one year of production. A description of the economic model and a discussion of how net income is influenced by length of breeding season and conception rate at first service are included. The optimum length of breeding season was found to depend upon conception rate at first service. In cows, length of the breeding season has little influence on net income when conception rates at first service were higher (70%, 80%), but a longer breeding season (120 days) increased net income when conception rates at first service were low (50%, 60%). Initiating breeding of yearling heifers 3 weeks before the cows generated greater net incomes when conception rates at first service were low (50%, 60%), but only slight differences in net income were evident with higher conception rates at first service (70%, 80%).