Organic Broadcaster

Researchers evaluate pastured broilers as means to reduce flies in organic dairy pastures

By Hannah Phillips, Roger Moon, Ulrike Sorge, & Bradley Heins

The face fly (Musca autumnalis), horn fly (Haematobia irritans), and stable fly (Stomoxys calcitrans) are common pests on pasture-based dairy farms. These flies can transmit diseases and cause cows to exhibit an increase in fly avoidance behaviors—head throws, tail swishes, foot stomps, and crowding—which can decrease the time spent grazing and lower milk production. Thus, flies are a nuisance to both cattle and humans.

To prevent flies, producers commonly use organic-approved repellents. However, repellents are only temporarily effective after topical application on the cow. The most effective method of fly control is to break their life cycle and reduce the population to below the pest level.

Adult female flies lay their eggs in fresh cow manure. The eggs hatch and the maggots develop into larvae, which feast on the nutrient-rich manure inside the safety of the manure pat. Once the larvae mature, they burrow under the manure pats to pupate and finish developing into adult flies. The cycle from egg to adult fly takes about 10 to 20 days.

Fly larvae may sound like a repulsive snack to humans; however, the high protein content and digestibility of fly larvae makes it a potential excellent addition to the broiler chicken diet. Some producers believe that chickens will consume fly larvae from cow manure pats, and that grazing chickens and dairy cattle in succession is an effective method of fly control. However, no scientific studies have determined whether chickens can successfully reduce the number of flies that emerge from cow manure. Thus, the objective of this study was to determine if broiler chickens reduce the number of fly larvae from cow manure pats on pasture.

The experiment was conducted from June to August 2018 at the West Central Research and Outreach Center (WCROC) in Morris, Minnesota. The WCROC has a 250-cow research dairy farm consisting of a conventional low-input herd and an organic-certified seasonal grazing herd.

Fresh manure from the organic grazing herd was collected to form one-liter pats. The manure pats were planted with 100 lab-raised face fly maggots each. The pats (n = 33) were randomly assigned to be raised in the following treatment groups: 1) greenhouse in buckets with sand substrate (environmental control; n = 3); 2) pasture without broilers (pasture control; n = 10); 3) pasture with broilers at 25 ft2 per bird (low stocking density; n = 10); and, 4) pasture with broilers at 5 ft2 per bird (high stocking density; n = 10).

Fifty mixed-sex Freedom Rangers (Welp Hatchery, Bancroft, Iowa) were raised in a brooder until 4 weeks of age. At 4 weeks of age, broilers were randomly assigned to one of the two broiler stocking density treatment groups (25 birds per pen), and were housed in a floorless mobile shelter divided into two equally sized areas with outdoor pasture access respective to their stocking density treatment group. When the birds were approximately 7 weeks of age, the manure pats were randomly placed in the outdoor pasture space of their pens 1 to 3 feet apart. Broilers (approximately 4.3 lbs per bird) had pasture access from dawn to dusk (approximately 5 a.m. to 10 p.m.) and were fed once nightly (0.33 lbs per bird [20% crude protein]).

Behavior observations were recorded on the second and third replicates of the study twice daily in the morning (between 9 and 11 a.m.) and afternoon (between 1 and 5 p.m.) for the duration of the study. Prior to each behavior observation, the number of birds in the outdoor pasture space was recorded. Behaviors were recorded in continuous one-minute observations on 10 identified focal birds per pen (i.e., the same birds were recorded at each observation using Animal Behaviour Pro© app). Behavioral states were recorded as durations and behavioral events were recorded as binary outcomes. The manure pats were exposed to their treatments until the larvae matured (3 to 4 days). The study was concluded in three complete replicates using a total of 99 manure pats and 150 broiler chickens.

Broilers spent their time sitting awake (54 ± 7.6%), sleeping (21 ± 5.5%), or upright (24 ± 5.6%, either standing, walking, or running. The most performed behavioral event was preening (28 ± 9.2% of birds) followed by foraging (21 ± 5.7% of birds). These behavior results suggest that broilers spent the majority of their time resting. Only 14 ± 9.6% of the birds were observed in the outdoor pasture space where the manure pats were placed. However, when broilers were actively foraging in the pasture they were not observed foraging specifically in manure pats.

Figure 1. Raw data and trend line showing the percent of birds in their outdoor pasture in relation to heat index

Low pasture use may be partially explained by heat index (Figure 1). The optimum temperature that maximizes pasture ranging has been described around 62 degrees Fahrenheit for laying hens (Hegelund et al., 2005). As described in previous studies, broilers prefer shade to range (i.e., tree cover, tall grasses, and shade structures) (Dawkins et al., 2003; Fanatico et al., 2016; Stadig et al., 2017). Therefore, it was not surprising that as heat index increased, birds spent more time inside the mobile shelter protected from the heat of the sun.

Figure 2. Percent of face fly pupae recovered from manure pats between treatment groups

The broilers had no effect on the number of pupae recovered from the manure pats compared to the pasture control group in this study (Figure 2). The overall recovery of pupae from pasture treatments (pasture control, low-density broilers, and high-density broilers) was quite low compared to the pats raised in the greenhouse (environmental control).

Future studies using a similar experimental methodology should consider planting manure pats with greater than 100 maggots to ensure a higher recovery and should explore ways to increase the duration that birds are exposed to the manure pats into the pupae stage of flies. Furthermore, layer hens should be considered in future studies since they are likely better suited for reducing flies because of their older age and increased experience foraging.

Hot weather negatively affected pasture ranging and foraging behaviors necessary to reduce flies on pasture. Therefore, the broilers had no impact on the number of fly larvae in manure pats in this study. Previous knowledge and results from the current experiment suggest that producers can maximize ranging and foraging behaviors by providing broilers shade in their ranging environment, especially when it is hot. Fly management for grazing dairy herds is difficult to address using only one strategy. Therefore, a multifactorial approach should be used to reduce flies on the farm. More information on fly management can be found on the WCROC website:

Hannah Phillips is a University of Minnesota (UMN) graduate student in animal science. Roger Moon is in UMN’s entomology department. Ulrike Sorge is with Bavarian Animal Health Services in Poing, Germany. Bradley Heins is in UMN’s department of animal science.


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