In spite of domestication, sows are still genetically programmed to perform nesting behaviour close to farrowing. In order to facilitate nest building, a method for a strategic use of large quantities of straw has been developed by Swedish piglet producing farmers. The objectives of the present study were to quantify the effect of strategic use of 15–20 kg of chopped straw given once 2 days prior to expected date of farrowing, compared to small daily amounts (0.5–1 kg) and 2 kg close to farrowing (controls), on the nest building behaviour and the duration of farrowing. The behaviour from 18 h pre-partum until 1 h after birth of first piglet and the duration of farrowing was continuously observed in 138 video recordings from 4 commercial farms. On each farm, 20–34 sows (parity ≥ 2) were studied during one or two consecutive lactations. Compared to controls, strategic use of straw triggered the sows to start nest building earlier and increased the total time spent nest building pre-partum by 19% (p = 0.039). Sows given large amounts of straw also performed less nesting behaviours during the first hour after birth of the first piglet. This shows that nest building is affected not only by the presence of straw, but also by the quantity of straw provided, and that 2 kg of chopped straw seems to be too little to make the sow terminate nest building well in advance of farrowing. There was no significant effect of treatment on the duration of farrowing but a strong negative association was found between time spent nest building pre-partum and the duration of farrowing regardless of treatment. The model predicted a 1-h increase in total nest building time pre-partum to be associated with a 12% (95% CI = 4–19%) shorter duration of farrowing (p = 0.004).