Average daily weight gain of Iberian fattening pigs when grazing natural resources

In spite of the growing interest in organic farming and sustainable production, there is a lack of information about the performance of pigs when they feed only on natural resources. This research studied the fattening pattern during the montanera (free-range grazing on acorns and grass during autumn and winter, without any supplementary feed or mineral complement) of the Iberian pig in the dehesa (Quercus ilex open woodlands) in order to know the average daily gain (ADG) and to analyze the influence of three factors: year of montanera, age and sex of pigs. The fattening data were obtained from two flocks of castrated pigs during the first two months (from November the first) of two consecutive montaneras (2003 and 2004) on the same farm. Pigs (from 111.8± 0.9 kg BW at introduction) were individually weighed every 15 days in the early morning, with no food intake since the previous evening. The ADG for all pigs was 0.76± 0.01 kg/day; however, some variations were observed according to the year (0.74±0.02 kg/day in 2003 vs. 0.78±0.02 kg/day in 2004; pb0.05), the sex (0.78±0.02 kg/day for males vs. 0.72± 0.02 kg/day for females; pb0.01) and the age (0.71±0.02 kg/day for pigs younger than a year vs. 0.83±0.02 kg/day for pigs older than a year; pb0.001); however a factorial 3-way ANOVA does not show significant interaction between factors. Comparing the fortnights periods, the lowest ADG was measured in the first 15 days of montanera (0.60±0.02 kg/day), and the highest ADG occurred in the second one (0.85±0.02 kg/day). Later on, the datasets were adjusted with the GLM procedure for the fixed effects of the year of the montanera, the age and the sex. With the adjusted datasets, different regression models for the fattening in montanera were established; the best adjusted of those was the potential model (Y= 0.32*X 1.22 ; X= days of montanera; and Y= kg life weight gained during the montanera). According with this model pigs would need an average of 58 d to fatten 46 kg, as the Iberian Breed Quality Standards demand. Finally, the results show that the age and sex, as factors that influence the ADG, should be taken into close account to choose pigs and that it is possible to reach a high ADG grazing only with natural resources, following appropriate management strategies such as rationing in order to obtain the benefits of compensatory growth.