Research shows fly larvae can improve leg health

The use of black soldier fly larvae triggered intrinsically motivated behaviours that can potentially promote activity and reduce leg problems, thereby improving broiler welfare.

Studies show that in 2017 approximately 90-95% of European fast-growing broilers obtained a weight of 2-2.5 kg within 6 weeks. What we see is that rapid growth and leg problems have a serious impact on activity and behaviour, as fast-growing broilers are mostly inactive. Fast growth promotes the development of broiler lameness, directly by impairing leg bone development and indirectly through limiting broiler activity. As moisture and ammonia accumulate in the litter over time, broilers that spend more time resting are prone to develop contact dermatitis. Environmental enrichment that triggers intrinsically motivated behaviours can potentially promote activity and reduce leg problems, thereby improving broiler welfare.

Scattering larvae in the litter

The study investigated the effect of scattering fly larvae in the litter on broiler behaviour, contact dermatitis, lameness, and performance. Broilers received either 5% or 10% of the estimated daily dietary DM intake as BSF larvae (hereafter referred to as A5 and A10, respectively), either twice or 4 times a day (F2 and F4, respectively) which, apart from the control treatment, resulted in treatments A5F2, A5F4, A10F2 and A10F4. Broilers received BSF larvae at set times each day (08:00 and 14:00 hours for the F2 treatments and 08:00, 11:00, 14:00 and 17:00 hours for the F4 treatments). The larvae were provided by scattering them across the litter throughout the pen.

Leg health

Both foot pad dermatitis and hock burn were scored using a 5-point scale, with 0 equal to no lesions and 4 representing severe lesions, while gait (indicative of lameness) was scored on a scale ranging from 0 (normal, dextrous, and agile walk) to 5 (incapable of walking). The prevalence and severity of hock burn and lameness was significantly less in broilers receiving 10% of their dietary DM as BSF larvae compared to controls, and the severity of lameness was lower in broilers receiving 5% of the dietary DM as larvae 4 times a day compared to controls. Foot pad dermatitis was also reduced in all the larvae groups. Since leg problems can be painful and inhibit natural behaviour, this suggests that the BSF broilers experienced improved welfare. The study researchers expect to see more beneficial effects on leg health from providing larvae under commercial conditions where broilers can benefit more from enrichment. However, the applicability of providing BSF larvae in commercial settings remains to be investigated.

Broilers received either 5% or 10% of the estimated daily dietary DM intake as BSF larvae, either twice or four times a day. Photo: Koos Groenewold

Behavioural changes

Several studies have indicated that promoting activity from a young age onwards promotes leg bone development and even increased activity levels later in life, where prolonged elevated activity levels indicate improved leg health. In addition, increased activity means better litter mixing which helps the litter to dry more easily. A drier litter reduces the risk of contact dermatitis. Thus, environmental enrichment that facilitates activity in broilers may help to improve broiler welfare.

The researchers found that all broilers receiving BSF larvae increased the amount of time spent walking, standing idle, ground pecking and foraging, while their time spent resting was reduced compared to controls. Overall activity – any behaviour except resting and perching while sitting – was lower over several weeks in the control broilers compared to all the BSF groups. Between the larvae groups, the 10% (provided 4 times a day) broilers were more active after receiving larvae than the 5% (provided twice or 4 times a day) broilers. From week 2 onwards, the controls showed a decline in ground pecking and total foraging behaviour, while the prevalence of these behaviours remained relatively high and constant in all larvae groups. Time spent in a standing position was lower in the controls than in all the BSF groups throughout the study period; the controls spent more time sitting. In general, time spent standing decreased from week 2 onwards in the controls, and from week 3 onwards in the 5% BSF broilers and from week 4 onwards in the 10% BSF broilers.

Performance

The weight gain was generally smaller (but not significantly so in most cases) for the 10% broilers compared to controls. Previous studies suggest that chitin plays a role in the reduction of broiler growth as it is partially digested by broilers. In this study, the researchers found that providing 10% of the dietary DM as BSF larvae twice or 4 times a day resulted in high larvae intake. They suggest that the digestion-inhibiting effect of chitin could have been strongest in these broilers, resulting in the observed diminished performance.

To be more specific, the relative consumption of BSF larvae was slightly higher than anticipated (approximately 6% and 12% instead of the expected 5% and 10% of dietary DM) which could have caused a slight imbalance in amino acid uptake, thereby affecting broiler growth. The researchers noted that other studies have found that BSF larvae inclusion in the diet can increase broiler growth and feed intake and increase the broiler’s T helper cell frequency and serum lysozyme activity, improving nonspecific immune responses. In the current study, the FCR based on DM was higher in the 4 times a day than in the twice a day frequency, while the FCR of the control group was in between these 2.

Amount and frequency

The researchers concluded that in their study the largest amount of larvae provided at the highest frequency (10% of the dietary DM as larvae provided 4 times a day) was most effective in promoting activity and lowering the prevalence of hock burn and lameness, while the final weight of these broilers was not significantly reduced compared to the controls. The elevated levels of foraging behaviour and general activity was greatest and most consistent for broilers receiving BFS larvae in the highest amount and frequency. Overall, broilers receiving 5% or 10% of their dietary DM as BSF larvae 4 times a day showed improved leg health. This study demonstrates that by facilitating natural behaviour and activity through providing larvae as part of the dietary DM can reduce leg health problems and thus benefit broiler welfare.