There are two kinds of flies that harm dairy cattle, annoy farmers, and reduce profitability. Blood feeding stable flies slow calf growth while non-biting house flies spread pathogenic bacteria. Both flies develop as maggots in moist organic debris and are readily found in calf bedding. Two research studies conducted by the University of Minnesota looked to see which source of bedding was a better choice for fly control.

The objective of both studies was to determine if bedding source affects production of filth flies and abundance of beneficial fly killing wasps in bedded-pack pens. Nine pens (seven heifers per pen) were first cleaned and then bedded with straw, pine shavings or hardwood sawdust for 12 weeks. Additional bedding was added as needed to maintain a dry pack surface. This was consistent for both studies.

Bedding use averaged 317 pounds straw, 268 pounds shavings and 291 pounds sawdust per pen per week. When the second study was conducted in 2010 the amount of bedding varied slightly, but the trend was the same.

In both studies heifer growth and cleanliness were not affected by bedding source and heifers gained an average of 2 pounds per day. Bedding pack temperatures did not vary among bedding sources.“When we conducted the second study the bedding pack temperatures were not significant, however, the straw was warmer in both of the study years,” notes Jessica Starcevich, a graduate student at the University of Minnesota. Pack depths were different with straw being deepest (10 inches), shavings intermediate (9 inches) and sawdust being shallowest (8 inches); this was consistent for both studies.

Samples from the bedding packs indicated pens bedded with pine shavings and hardwood sawdust contained fewer developing house flies and stable flies than pens bedded with straw. "In the 2010 study pine shavings produced significant numbers of stable flies, however sawdust still produced the fewest,” notes Starcevich. 

Surplus from straw would have been much greater were it not for naturally occurring beneficial fly-killing wasps, which were more active in straw. “This statement remains consistent for both years, although shavings produced a lot of stable flies in 2010, beneficial wasps were still more active in straw,” says Starcevich. Differences in fly abundance may have been due to porosity and compaction. If this is indeed the reason, then corn stalks are likely to be as bad, if not worse than straw.

“The take home message is that it is best to use sawdust whenever possible, but if it is more economically feasible to use straw, then fly production can be reduced by conserving or augmenting (adding more) natural beneficial wasps,” notes Starcevich.

Further research is needed to understand why straw produced more flies, why beneficial wasps were more active in straw, and whether dairy producers can depend on naturally occurring wasps being present at their calf production facilities.