Modeling weight gain or growth of livestock is an important tool for optimizing management decisions and production systems.Amongst considered livestock were cattle, sheep, chicken, ducks or turkeys.There exist also studies dealing with the growth patterns of intensive piglet production or a pig breed used as laboratory animal . But organic livestock farming differs from intensive livestock farming in many respects. Animal husbandry, feeding, weaning age and drug administration are the main aspects of that difference. Thus piglets from organic farming are kept under different rearing conditions than piglets in intensive production systems. It is of interest if this difference finds expression in a different growth pattern. There-fore, the aim of this study is the analysis of weight gain of piglets from organic farming. There are two types of growth models: models with a rather mathematical motivation, such as data fitting , and models with a biological motivation.

The latter models explain growth e.g. from balancing anabolism and catabolism . Intermediates between these types are unified approaches, which in essence describe the biologically motivated models by differential equations that generalize the logistic growth model. As the models differ by the number of parameters, with more parameters enabling a better fit, the model selection is based on the Akaike Information Criterion AIC , which combines the good fit with a penalty for the number of parameters. Amongst the considered models, only two fitted the piglet data properly: the initial growth phase with only minimal human influence could best be described by the biologically motivated Bertalanffy model, while the long term growth with different phases of human interaction was best described by the two-linear model. The applied data were recorded in one research center in Upper Austria. In that facility piglets are bred and raised under organic conditions until they reach an age of about 100 days. On that farm there are about 30 breeding sows kept for piglet production. The sows are enabled to litter in farrowing pens. Thus, group suckling is implemented. All piglets were born and raised on that farm.

A total of 49,699 live weight data were recorded between January 2007 and December 2013 from 5188 piglets aged between 0 and 105 days. The piglets usually were weighed at the day of their birth, at the latest on the first or second days of live, and then once a week. The body weight of each piglet was recorded with a weighing scale with smallest range of 20 g pre-weaning and with an accuracy of 500 g post-weaning and entered in an Excel chart. A macro was programmed to convert the chart in an optimal pattern for editing and analyzing the data. Some piglets are so-called runts, meaning that they are backward in development and die within the first days or weeks of live. In such a case just a few recordings per piglet exist. Since such animals usually increase less in weight and therefore have a lower body weight than their same-aged conspecifics, a dataset including just the piglets with 6 or more body weight recordings was generated. This dataset contains 48,284 data points of 4393 piglets. In both datasets, the body weight mean values of each day of live were calculated. The mathematical modeling was performed using these body weight mean values. Data fitting was done in Excel using the Solver add-in and the least squares method.The von Bertalanffy model describes weight gain of piglets in the first weeks of live until weaning best. Considering the whole piglet rearing the most natural nutrition is given during the suckling period. Provided that the piglets remain with the sow the ingested amounts of milk as well as the frequency of suckling are not influenced by the animal owner. The biologist von Bertalanffy developed his model for all animal species under natural living and nutritional conditions.

The model concept is based on physiological processes that are responsible for growth. Growth is defined as a consequence of anabolism and catabolism and will take place as long as building processes outweigh degradation processes. In young animals the primary function of the ingested food is rapid growth, thus it can be seen as a natural limitation of growing capabilities. In this sense the model concept is suited to describe growth development in this early period of live. This model is based on the idea of growth as a biological process until achieving a species-specific weight in adult age. By contrast, during the fattening period the hogs are supplied with much more energy than they would need for natural growth in order to reach as high weight gain as possible in a minimum of time. Thus, the results of the paper confirm that this model appears to be optimal for close to natural conditions.