Short-term choice of fattening pigs for additional plant-based materials

Highlights

• We measured pigs’ preference for different plant-based enrichment materials.

• Structure matters: Pigs prefer pelletized materials over chopped and powdered ones.

• Size and flavour matters: Maize kernels in straw were most attractive for pigs.

• Structure, size and flavour should be considered by choosing enrichment material.

• Adding nutritive additives can further stimulate exploration and interest of pigs.

Abstract

A major problem in conventional pig production systems is the occurrence of abnormal behaviours such as tail biting. Tail biting is a multifactorial problem, but it seems that a lack of proper enrichment materials prevents the animals from performing normal species-specific behaviour, e.g., rooting, nosing or chewing, and thus increases the prevalence of tail biting. The aim of this study was to identify plant-based enrichment material and nutritive additives that are attractive to pigs and that can be used as enrichment in practice. Therefore, we carried out two choice tests to investigate (i) which plant-based enrichment materials, differing in structure and flavour, pigs prefer and (ii) what kind of nutritive additives can be used to further increase the attractiveness of plant-based enrichment materials. In each choice test, pigs had the choice of six offered options. Pigs were tested individually in their familiar environment in an arena for 150 s. The first choice test revealed that pelletized materials, i.e., lucerne pellets and straw pellets, were preferred over all other options tested (Friedman test, n = 55, p < 0.001). In the second choice test, the pigs’ interest was highest for the largest nutritive additive, i.e., maize kernels, compared to the other options tested (Friedman test, n = 65, p < 0.001). When choosing enrichment materials for pigs, structure, size and flavour should be considered. Pelletized plant-based enrichment materials or non-pelletized materials with nutritive additives can be recommended to stimulate exploration. Further studies should examine the interval at which enrichment material needs to be altered to keep pigs’ interest at a high level.

Introduction

Tail biting is a common problem in conventional pig production systems. The causes are multifactorial (Brunberg et al., 2016; Schrøder-Petersen and Simonsen, 2001). However, it is often assumed that one of the most important risk factors for the occurrence of tail biting is the lack of appropriate enrichment materials hampering the pigs from performing species-specific exploration and foraging behaviour, such as rooting, nosing, and chewing (EFSA, 2014; Špinka, 2017; Stolba and Wood-Gush, 1980; Wood-Gush and Vestergraad, 1989). Plant-based or non-plant-based materials are used as enrichment materials to facilitate exploration. Plant-based enrichment materials such as straw are often deemed incompatible with slurry systems. Therefore, farmers often provide durable, non-plant-based enrichment materials to pigs, e.g., metal chains or plastic items, to distract the animals and to lower the risk for the occurrence of tail biting. Furthermore, non-plant-based materials are often low cost and require lower time investments. However, these materials mostly have only short-term enrichment effects, as rearing, fattening and finishing pigs habituate relatively quickly to them (Scott et al., 2006, 2009). To achieve long-term enrichment effects that may be better suited to reducing the risk of tail biting, the respective enrichment materials probably need specific characteristics. It has been shown that growing and fattening pigs show high levels of exploration towards plant- or non-plant-based enrichment materials that are compound, changeable, manipulable, chewable, edible or odorous (Fraser et al., 1991; Jensen and Pedersen, 2007; Studnitz et al., 2007; Van de Perre et al., 2011; Van de Weerd et al., 2003). Most plant-based materials fulfil these specific requirements and are thus recommended as the most suitable enrichment materials for pigs.

When given a choice, finishing pigs prefer plant-based materials over non-plant-based materials (Scott et al., 2009). In a choice test, growing pigs preferred pens littered with peat, mushroom compost and sawdust compared to pens littered with sand, wood bark, straw, and non-littered concrete floor (Beattie et al., 1998). These results suggest that pigs favour textures similar to earth. This was supported by a study in which eighteen materials from six categories were examined in a choice test (Jensen et al., 2008).

Plant-based materials can be provided in different ways to pigs, which is closely related to the management of the stable. In existing conventional housings with fully slatted floors, long materials, such as hay or straw, can plug the slurry system once they have passed through a slatted floor. Thus, for reasons of practicability, smaller materials, which may be chopped or pelletized, should be used. These smaller materials also bear the advantage that they can be automatically dispensed into the pens. The attractiveness of plant-based materials for growing and finishing pigs can be further increased by using different nutritive additives (Jensen and Pedersen, 2007; Zwicker et al., 2013). Nutritive additives obviously increase the rooting behaviour of pigs by imitating natural conditions in which pigs root to find food and by enhancing the reward of rooting. The feed intake of pigs is influenced by the nutrient composition (Kyriazakis and Emmans, 1995; Whittemore et al., 2001), which affects the flavour and taste of the feed (Mizushige et al., 2007; Mori et al., 1991). This suggests that different nutritive additives may cause differences in the pigs’ preference and thus in the time they spend exploring and ingesting it.

In our study, we further investigated two successive short-term choice tests: (i) which plant-based enrichment materials, differing in structure and flavour, do pigs prefer; and (ii) what kind of nutritive additives can be used to further increase the attractiveness of plant-based enrichment materials?