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Biomedical subjects

A M Azzam

Publications and source records attributed to A M Azzam.

4 recordsLinked to original sources
PubMed1991

A Markovian decision model for beef cattle replacement that considers spring and fall calving.

In certain production environments, beef cows are mated during a breeding season that starts in early summer. Cows found not to be pregnant at the end of the breeding season could either be culled or retained and remated the following summer. Alternatively, nonpregnant cows could be mated in the winter. This option would result in having both a spring and a fall calving herd. The purpose of our study was to determine the optimal replacement policy (maximizing long-run average net returns) for a specific production environment by determining for each age of cow, reproductive status (nonpregnant or pregnant), and season of pregnancy checking (spring or fall) whether the cow should be retained to the next breeding season (summer or winter) or be replaced by a pregnant heifer. The problem was formulated as a Markovian decision process and the optimal policy was found by linear programming. The optimal policy was one in which nonpregnant cows were always culled and replaced by heifers in the summer breeding herd, resulting in spring calving only.

Animals↗
PubMed1990

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.

Age Factors↗
PubMed1979

Pulmonary function in heterozygotes for alpha,-antitrypsin deficiency: a case-control study.

In this paper we present the initial cross-sectional data from a prospective study of lung aging in heterozygotes for alpha 1-antitrypsin deficiency. Using a case-control design, our cases included 37 heterozygotes for alpha 1-antitrypsin deficiency, protease inhibitor phenotypes MZ and MS, selected because they were parents of children with homozygous alpha 1-antitrypsin deficiency identified in a statewide newborn screening program between 1971 and 1974. All of the heterozygotes were less than 40 yr of age. Our control subjects were selected from a random sample of a working population participating in a longitudinal study of lung aging, using a 2:1 match of control subjects to cases, matching age, sex, ethnic origin, and smoking. Using a respiratory symptom questionnaire, spirometry, and the single-breath N2 test, we found no significant difference between heterozygotes and control subjects in terms of respiratory symptoms or pulmonary function data. We conclude that to 40 yr of age, the heterozygous phenotype (Pi MZ and MS) is not a risk factor for impairment of pulmonary function.

Adult↗