Simulation of a dairy herd over a period of 15 years was used to examine replacement strategies among six operationally practical systems of culling cows which whould maximize income over feed costs. The strategies or criteria used to remove the lower ranking cows to a relatively constant herd size were: (1) Mature Equivalent (M.E.) milk production, (2) M.E. gross milk income, (3) actual milk production, (4) actual gross income, (5) income over feed cost, and (6) present value of expected gross income of cow and her subsequent replacements. Two each of prices of milk, fat differential, feed, and operational costs totaled 2 4 x 6 x 2 replications per trial or 192 replications. The following were treated as stochastic variables: (a) variation in milk production and milkfat percentage between cows and between lactations of the same cow, (b) chance of a calf being a heifer or bull, (c) chance of involuntary death or removal of cows and youngstock, and (d) chance of month of the year of involuntary removals. The mean and variance of the base herd approximated the average Michigan DHIA Holstein population in 1966. The sire value, approximating the best bulls used in A.I. was identical for any given year through all strategies and replications, but improved over time at the rate of 130 1b milk per year. Culls or cows removed because of low in each respective strategy were removed at the most profitable point to cull in their respective lactations by equating the milk income of the marginal month with the sun of the month's feed costs and monthly operational charges. Practical use of this simple method of determining when to cull cows within a lactation was demonstrated. A complete factorial design to analyze the generates data showed no significant differences between strategies under alternative combinations of prices in average income including salvage over feed cost per cow discounted to the present. This income was affected by the level of milk price (P

We construct a vintage capital model of economic growth in which the decision to replace old technologies with new ones is modeled explicitly. Depreciation in this environment is an economic, not a physical concept. We describe the balanced growth paths and the transitional dynamics of this economy. We illustrate the importance of vintage capital by analyzing the response of the economy to fiscal policies designed to stimulate investment in new technologies. A revised version of this paper is published in the Journal of Monetary Economics, v. 40, no. 3(December 1997):457-499.

This book contains 28 chapters with emphasis on the interactive nature of the relationships between the soil, plant, animal and environmental components of grassland systems, both natural and managed. It analyses the present knowledge and the future trends of research for combining the classical view of grasslands, as a resource for secure feeding of an increasing human population, with the more recent perspective of the contribution of grasslands to the mitigation of environmental impacts and biodiversity erosion as consequences of human society activities. The chapters are organized within five sections dealing with the different functions and the main ecosystem services expected from grasslands: (i) domestic herbivore feeding and animal production; (ii) the regulation of biogeochemical cycles and its consequences for the environment; (iii) dynamics of biodiversity hosted by grasslands; (iv) integration of grasslands within sustainable animal production systems; and (v) interactions of grassland areas with other land use systems at the landscape level.

Air Emissions from Animal Feeding Operations: Current Knowledge, Future Needs discusses the need for the U.S. Environmental Protection Agency to implement a new method for estimating the amount of ammonia, nitrous oxide, methane, and other pollutants emitted from livestock and poultry farms, and for determining how these emissions are dispersed in the atmosphere. The committee calls for the EPA and the U.S. Department of Agriculture to establish a joint council to coordinate and oversee short - and long-term research to estimate emissions from animal feeding operations accurately and to develop mitigation strategies. Their recommendation was for the joint council to focus its efforts first on those pollutants that pose the greatest risk to the environment and public health.