Complex sentence: The crates are usually placed side by side in rows of 20 sows per row and 100 rows per shed, the floors of the crates slatted to allow excrement to fall into a pit below.
Split into simpler sentences: The crates are usually placed side by side in rows of 20 sows per row and 100 rows per shed. The floors of the crates are slatted to allow excrement to fall into a pit below.