L. Pérochon

Modelling the effect of the stage of pregnancy on dairy cows' milk yield

The effect of pregnancy on milk yield was studied on a 601 lactation sample. One hundred and sixty-nine lactations of non-pregnant cows, managed under identical conditions, served as controls. Lactations were divided into five groups according to parity (primiparous or multiparous) and production potential (two ranks for primiparous cows, three ranks for multiparous cows). The difference between the individual milk yield of each pregnant animal, and the mean production of the corresponding controls was computed weekly from the week of conception. The reducing effect of pregnancy began to be measurable from the 20th week of pregnancy, regardless of the group studied or the week of conception. Analysing the amplitude of this effect restricted the investigation to the three following groups: (i) primiparous cows, (ii) low- and medium-producing multiparous cows, (iii) high-producing multiparous cows. The effect of pregnancy was lower in primiparous than in multiparous cows. In the latter, it was higher in high-producing cows, partly because of the greater calf weight. At the 29th pregnancy week, this effect reached -17middot;5, -2·4 and -3·6 kg/day in the three groups, respectively. In each of these three lactation groups, the following non-linear model Y = −e −a [(Pw~18)e −bpw ] was fitted, where Y is the difference between the milk yields of pregnant and non-pregnant cows at a given stage (week), Pw is the pregnancy week, and a and b are parameters. The overall milk yield losses over a complete lactation, as estimated from the integral of that equation, reached 89, 137 and 203 kg, respectively in the three groups.

Modelling an information system using the MERISE method for agricultural research: The example of a database for a study on performances in dairy cows

Abstract The MERISE method for designing the information system for a research project is described, with emphasis on the data modelling steps. The description is based on an example concerning the design of a relational database, to be used in an exploratory study of the interrelationships between production and health performances of dairy cows over their productive life. The collection of requirements, the construction of a conceptual data model, its conversion into a logical data model suited to a relational structure, and the influence of processing trends on data models, are thus illustrated. It is suggested that using standard guidelines for designing an information system is an aid to research, and the useful role of data conceptual models is pointed out.

SIMBAL: a herd simulator for beef cattle

A beef cattle herd simulator was designed and developed as a tool for testing the consequences of farm management practices on herd performance. This discrete-event simulator is stochastic and individually based. It integrates a detailed reproduction model together with parameter-settable practices. This article details the various core components of the simulator. We then used it on virtual experiments with a herd of 70 cows. The first virtual experiment compared an autumn calving system with a herd management system based on winter calving. Adapting the herd management rules made it possible for the two systems to give similar outputs. In order to show the importance of the number of mated cows and of the reproductive period length on the performance of the herd, two different strategies were tested in with two new virtual experiments. The first one consisted in reducing the number of mated cows while the second one deals with reducing the reproduction period when the bull is present. The first of these scenarios led to drop in herd size in proportion with the female reduction over the years, whereas under the second scenario, a gap is simulated and the herd was wiped out after 10 years. We also focused an in-depth analysis on the variability of the results. The model shows that an average result not only masks variations from replication to replication but also masks variations in performances from year to year.