L.L. MIKKELSEN1
J.K. VIDANARACHCHI1
C.G. OLNOOD1
Y.M. BAO1
P.H. SELLE2
M. CHOCT33
1School of Rural Science and Agriculture, University of New England, Armidale, NSW Australia
2 Faculty of Veterinary Science, University of Sydney, Camden NSW Australia
3Australian Poultry CRC, University of New England, Armidale, NSW, Australia
The effects of graded inclusion levels of potassium diformate (KDF) in broiler diets were evaluated in the Necrotic Enteritis (NE) challenge model. At an inclusion rate of 4.5 g /kg KDF significantly reduced NE mortalities by 58% (12.7 versus 30.0%) in challenged birds. The reductions in NE mortalities induced by KDF, however, were not associated with decreased jejunal counts of Clostridium perfringens, the causative organism. KDF did not significantly influence weight gain or feed efficiency in unchallenged or challenged birds. It is concluded that KDF appears to have promise for the control of NE in broiler chickens, particularly in the absence of antibiotics, and that further investigations are justified.
I. Introduction
Necrotic enteritis (NE) is a global poultry disease caused by the bacterium Clostridium perfringens. Proliferation of C. perfringens in the chicken intestine results in the production of toxins that cause necrotic mucosal lesions in the gut, which can result in acute or subclinical disease entities (van Immerseel et al., 2004; McDevitt et al., 2006). In its clinical form, NE causes high mortalities in broiler flocks and, in its subclinical form, NE is financially damaging because of impaired growth performance of chickens. In-feed antibiotics are routinely used to control the disease, but this practice is coming under increasingly critical scrutiny and has been curtailed in Western Europe due to concerns about the development of antibiotic-resistance in bacteria that are potential human pathogens. A prohibition of antibiotic inclusion in animal feeds is thus likely to lead to an increased incidence of NE in poultry. Consequently, there is a pressing need to identify alternative feed additives to sustain efficient chicken-meat production, without reliance on antibiotics. The inclusion of organic acids in broiler diets is one of a number of strategies to control NE in the post-antibiotic era that are under investigation (Dahiya et al., 2006). Organic acids and their salts are possible alternatives to antibiotics because of their antimicrobial and growth promoting effects. Also, lactic and propionic acids have been shown to reduce litter contamination with C. perfringens (Gornowicz and Dziadek, 2002).
Potassium diformate (KDF, Formi®) is a chemical complex, which dissociates into formic acid and potassium formate in the gut, that has shown promise in enhancing growth performance and nutrient utilisation in broilers (Selle et al., 2004). The aim of the present study was to evaluate the potential of KDF to control losses due to NE in broiler chickens caused by C. perfringens.
II. Materials and methods
A total of 1050 day-old male broiler chickens (Cobb) were purchased from the local hatchery, divided into groups of 25 and placed in floor pens with sawdust bedding.
There were 7 treatment groups of 6 replicate pens: (i) unchallenged negative control; (ii) unchallenged plus 4.5 g/kg KDF; (iii) challenged negative control; (iv) challenged positive control [100 ppm monensin, 45 ppm Zn-Bacitracin, and challenged with inclusion levels of (v) 2.25, (vi) 4.50 and (vii) 6.75 g/kg KDF.
The birds received a starter diet from day 1 to 7 and again from day 15 to 21 and the finisher diet from day 22 to 35 of the experiment. A high protein diet (50% starter diet and 50% fishmeal) was fed to the birds from day 8 to 14 to facilitate the NE challenge. All diets were pelleted and fed ad libitum. The NE challenge followed the model described by Kocher et al. (2004) with some modifications.
In the present experiment, birds were challenged with NE through infectious litter. Birds were raised on sawdust bedding re-used from a previous NE challenge experiment (Vidanarachchi, 2006) so that unchallenged pens had re-used litter from previously unchallenged birds and challenged pens had re-used litter from previously challenged birds. There was no significant difference in infection pressure between the challenged treatment groups of the present experiment, in aspects of previous NE-related deaths in pens.
On day 9 of the experiment, challenged birds were also inoculated by oral gavage of sporulated oocysts of Eimeria acervulina, E. tenella and E. maxima. The birds were inspected daily and dead birds were removed following registration of pen, date and bodyweight. Bird live weight and feed consumption on a pen basis were recorded on days 21 and 35. Feed conversion efficiency (kg feed/ kg weight gain) was calculated after adjustment for dead birds. Necropsies of all mortalities were conducted to determine the cause of death. On day 15 (peak of NE outbreak), two birds per pen were killed by cervical dislocation and pooled contents from the duodenum, jejunum and ileum were collected and pH was measured immediately. Jejunal contents were transferred into sterile McCartney bottles containing 10 ml of a pre-reduced salt medium. The suspension was poured into a CO2-flushed plastic bag, homogenised in a Stomacher laboratory blender and serial ten-fold dilutions were performed. Clostridium perfringens were enumerated on Tryptose Sulphite Cycloserine (TSC) agar (Oxoid, Agar base CM0587, TSC supplement SR0088 and egg yolk emulsion SR0047) using pour-plating technique and after anaerobic incubation at 39°C for 24 hours. All data were analysed according to the GLM procedure for ANOVA (SAS Institute Inc., 2001) with treatment as the main factor.
III. Results and discussion
The effects of treatment on weight gain and feed efficiency are shown in Table 1. From 1-21 days of age, unchallenged broilers, without and with KDF, and the challenged positive controls performed similarly and significantly out-performed broilers offered the challenged negative control and KDF-supplemented diets. These results indicate that the challenge caused growth depression, which was probably due to the impact of subclinical NE. Overall, while there are numerical differences, KDF was not associated with any significant alterations to growth and feed efficiency in either unchallenged or challenged birds. In contrast, monensin and Zn bacitracin significantly improved performance relative to the negative controls.
Mortality rates attributed to NE are shown in Table 2 where the challenge increased losses from 1.3 to 30.0%, which was counteracted by monensin and Zn bacitracin (2%). Medium and low inclusion levels of KDF reduced NE mortalities in challenged birds. Inclusion of 4.5 g/kg KDF, significantly reduced mortalities by 58% (12.7 versus 30.0%) and
2.25 g/kg KDF tended to reduce mortalities by 31% (20.7 versus 30.0%). While the high inclusion level (6.75 g/kg) did not influence NE mortalities, there was a quadratic response to KDF, which approached significance (r = 0.494; P = 0.053) and the regression equation indicates that the optimum KDF inclusion rate to reduce NE mortalities is in the order of 3.5 g/kg.
Counts of C. perfringens at the peak of the NE outbreak did not correspond to mortality rates as all treatments, with the exception of monensin and Zn bacitracin, had similar jejunal levels of C. perfringens. This is contrary to reports that lower gut levels of C. perfringens are found in healthy flocks (Craven, 2000) and implies that the factors that trigger acute NE are not well understood.
While there was a tendency for KDF to reduce duodenal pH in unchallenged birds it was associated with increased duodenal pH in challenged broilers and, arguably there were no meaningful pH alterations in the jejunum and ileum (Table 2). Gut pH could be indicative of the amount of the formic acid component of KDF that escapes absorption from the forestomach and enters the small intestine.
It is curious that KDF reduced NE mortalities but this was not associated with reduced counts of C. perfringens in the jejunum. It is possible, however, that the gut histopathology results (to follow) may prove instructive in this respect. Nevertheless, the significant (58%) reduction in NE mortalities generated by 4.5 g/kg KDF is a promising result.
KDF displayed promise as a ‘growth promotant' in an initial evaluation (Selle et al., 2004), but this was not the case in subsequent feeding studies (unpublished data). A review of these experiments suggests that acid binding capacities (ABC) and/or dietary electrolyte balances (DEB) may contribute to the inconsistent responses, possibly by modifying the rate at which KDF dissociates into formic acid and potassium formate in the gut. Also, anecdotally, organic acids will perform more consistently in diets with low ABC. Typical, wheat-based Australian broiler diets routinely contain an anti-coccidial drug and a xylanase feed enzyme and both agents are considered to reduce the predisposition of broilers to NE outbreaks. Therefore, it may prove instructive to determine the efficacy of KDF in the challenge model when used in combination with one or both of these agents. Under practical conditions it may be that KDF, in combination with a coccidiostat and a xylanase, could provide control against NE, which would be beneficial should the use of antibiotics be discontinued. It may also be possible to identify the most appropriate ABC and DEB to enhance the efficacy of KDF as both a growth promotant and as an agent to control NE.
References
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Vidanarachchi, J.K. (2006). PhD thesis, University of New England. NSW, Australia.
From Proceedings of the "19th Australian Poultry Science Symposium", New South Wales, Australia.
