Finishing pigs need up to 400g of straw per pig per day to meet their behavioural needs for manipulation and rooting, according to a study at Aarhus University. But satisfaction of behavioural needs is not the only benefit of straw – pigs also have a higher growth rate and the risk of gastric ulcers is reduced.

In 2010, the Ministry of Justice and Ministry of Food, Agriculture and Fisheries jointly commissioned a study from Aarhus University to clarify how much whole straw is needed to satisfy the rooting and manipulation requirements of pigs under practical conditions. The results have been published in a new report from DCA – Danish Centre for Food and Agriculture.

"Danish and EU legislation stipulates that pigs must have permanent access to a sufficient quantity of straw or other manipulable material to enable proper investigative and manipulative behaviour – but there are no specific figures for how much straw is needed," explained the main author of the report, senior scientist Lene Juul Pedersen from Aarhus University.

Pigs Need Activity and Novel Stimuli

Pigs use much of their active time investigating their surroundings. The behaviour may be motivated by hunger but finishing pigs, which typically have free access to feed, are primarily motivated by information-seeking and curiosity. Pigs examine a substrate or an environment by behavioural patterns such as sniffing, rooting, pushing, chewing or biting.

With this behaviour, the pigs glean information about their surroundings, such as potential food sources, nesting materials or suitable resting places. Pigs are focusing their investigative behaviour on materials that are novel, changeable and/or malleable. In Nature, this new information will be stored and used later, for example if food resources become scarce, and such behaviour therefore increases the pig's chances of survival and reproduction.

The investigative behaviour is governed by an instinctive motivation to seek, find and explore stimuli that potentially contain information. Therefore, the pigs in an intensive production situation with free access to feed still need access to manipulable materials (rooting and enrichment materials) that can fulfil this motivation.

If the animals are short of stimuli, they can redirect their normal exploratory behaviour towards the other pigs in the pen, and especially against their ears and tails. The recipient of such attention may be injured by, for example, tail-biting. The degree of exploratory behaviour re-directed at pen mates can be used as a measure of how well the environment satisfies the pigs' requirements for rooting and enrichment materials.

In the project, the scientists therefore used changes in the incidence of exploratory behaviour re-directed at pen mates as an indicator of a lack of fulfilment of the behavioural need. In the study, the following secondary variables were also registered:

• injuries to the tail
• daily weight gain
• quantity of clean straw remaining in the pens (the amount of straw that is left before the next allocation of straw)
• targets for indoor climate, and
• gastric health.

Finishers should be given straw in amounts up to 400g per pig per day to satisfy their needs for exploration and occupation, according to a report from DCA.

Pigs Need up to 400g of Whole Straw to Meet Their Needs

In the first phase, the scientists examined pig behaviour when they received 10, 500 or 1,000g straw per pig per day. This showed that there was no difference in the exploratory behaviour re-directed at other pigs for pigs given 500g and 1,000g straw, but for pigs receiving only 10g there was more exploratory behaviour re-directed at pen mates.

In the second phase, the scientists gave daily allocations of straw per pig of 10, 80, 150, 220, 290, 360, 430 or 500g. The results showed a significant linear correlation between the amount of straw given and the amount of time the pigs spent on investigative behaviour re-directed at pen mates, since a daily allocation of 500g per pig reduced the active time spent on this behaviour from 10.5 per cent to 8.1 per cent compared to the 10-g allocation.

In the attempt to set a lower limit for what constitutes a biologically relevant difference, scientists concluded that under the given experimental conditions, the pigs must be allocated at least 400g of whole straw per pig per day to meet their needs.

The welfare impact of providing straw in such quantities was supported by the secondary variables measured. The results were as follows:

• A significant linear decrease in the percentage of animals with injuries to the tail when allocating increasing amounts of straw (5.9 per cent and 2.8 per cent, respectively, for 10g and 500g straw)
• A significant increase in growth rate with increasing straw allocation (1,012g and 1,060g per day at 10g and 500 g straw, respectively)
• Significantly fewer pigs with gastric ulcers when allocating 500 or 1,000g straw per pig per day compared to 10g per pig (seven per cent compared to 33 per cent).

Under the given experimental conditions, which involved two weekly muck-outs and omission of data collected in the two hottest months, the amount of straw allocated did not lead to increased soiling in the pens, dirtying of the pigs or deterioration of the quality of the climate in the pens (temperature and ammonia).

Measuring the amounts of clean straw remaining in the pens showed that under the given conditions, minimum allocation per pig is approximately 80g of straw until they weight about 50kg, around 150g until 70 kg and thereafter, they need more than 290g straw per pig to ensure permanent access to straw.

"Now we have factual knowledge of how much straw is needed to meet pig requirements. Allocation of straw can apparently also improve growth and help solve problems with gastric ulcers," explained Dr Pederson.

"Where results are implemented in practice, it will be necessary to design new housing and manure systems that can handle larger amounts of straw," she added.

^{The report (in Danish) 'Hvor meget hel halm udgør tilstrækkeligt beskæftigelses- og rodemateriale til svin”, DCA report no. 30, September 2013 can be found by clicking here.}

October 2013