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SummaryResin acid composition (RAC) has previously been shown to inhibit the growth of the Gram-positive bacterial species Clostridium perfringens in vitro and to modulate the ileal microbiota of broiler chickens. The following trials examined the effect of RAC on broiler chickens in two experiments. In experiment 1, 1400 one-day-old Ross 308 broilers were divided into two coccidiostat treatments: chemical (CC) and ionophore (IC), which were further divided into two RAC dosages: 0 and 0.5 g/kg. All diets were supplemented with xylanase, β-glucanase and phytase feed enzymes. The birds were raised in a commercial-type environment without additional microbial challenge during the 42-day trial. RAC improved the body weight gain by 3.3% and feed conversion ratio by 5.7% with CC, and improved footpad lesion scores with IC but had no effect on the litter quality. Experiment 2 was a 35-day subclinical necrotic enteritis (NE) challenge trial with 510 male Ross 308 chickens. The dietary treatments included a non-challenged, non-supplemented control and four NE challenged treatments with dietary RAC supplementation at 0, 1, 2, and 3 g/kg. The birds were challenged with Eimeria maxima on day nine and C. perfringens on day 14. While RAC at 1 g/kg significantly increased bird weight gain during the challenge, it did not affect the microbial or short chain fatty acid (SCFA) profiles. In contrast, RAC at 3 g/kg reduced the abundance of the Lactobacillus group and tended to reduce the abundance of genus Bifidobacterium and the total numbers of eubacteria. These experiments suggest that dietary RAC at a moderate dose positively affected broiler performance. However, changes in caecal microbiota populations may not have influenced the observed performance effects of RAC.

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According to the resource allocation theory, animals have to make a trade-off between resource-demanding life traits to obtain maximal fitness. Artificial selection toward efficient producing farm animals, however, may have created animals that have an impaired ability to divert resources to maintenance processes, such as responding to immune challenges. Residual feed intake (RFI), defined as the difference between observed feed intake (FI) and expected feed intake based on metabolic BW and growth, was used as a measure for feed efficiency. Individual BW and FI of 352 pullets were recorded weekly from 4 until 14 wk of age to estimate RFI. The top 50 efficient R- and the top 50 nonefficient R+ birds were selected. BW and BW gain in both groups were similar. FI and RFI, however, were significantly higher in R+ birds. Thirty animals out of every group were randomly allocated to 1 of 3 treatments: immunization with keyhole limpet hemocyanin (KLH), Mycobacterium butyricum, or heat-inactivated Salmonella enteritidis bacteria. Antibody titers against KLH, M. butyricum, or Salmonella lipopolysaccharide did not differ between R+ and R- birds. The antibody titer against Salmonella protein was higher in R+ birds. We concluded that a population of chickens from a commercial breed shows considerable variation in RFI. Specific antibody production against KLH, M. butyricum, and S. enteritidis lipopolysaccharide, however, is not influenced by efficiency in terms of RFI. R+ animals may have a higher level of nonantigen specific antibodies, as indicated by the higher antibody response to Salmonella protein.

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Mycotoxin exposure is common in the poultry industry. Deoxynivalenol (DON) is usually detected at levels below the maximum threshold (5000 ppb), but depending on diet and age, broiler performance can be affected. We evaluated the effects of 900 ppb and 2300 ppb DON on the performance, intestinal morphometry, and lesion scores of broiler chickens. One-day-old male Ross broilers (n = 736) were divided into 4 treatments with 8 replicates each, and a pen containing 23 birds was the experimental unit. The animals were fed diets naturally contaminated with two levels of DON: 900 (Low DON—LD) or 2300 (Moderate DON—MD) ppb, with or without activated charcoal, over 28 days. After this, all birds were fed a marginally DON-contaminated diet without charcoal. During the first 28 days, body weight gain (BWG) and feed conversion ratio (FCR) were significantly impaired when broilers were fed a MD diet without activated charcoal. Even after feeding a marginally contaminated diet from D28–35, birds previously fed the MD diet presented a significantly lower performance. The villus height:crypt depth (VH:CD) ratio was significantly higher in the ileum from 14-day-old broilers fed the MD when compared with the LD diet. At D28, the MD diet caused decreased villus height (VH) and increased crypt depth (CD), affecting VH:CD ratio in both intestinal segments, with higher levels in the jejunum from 28-day-old broilers fed a non-supplemented LD diet. Broiler production was negatively affected by DON, even at moderate levels (2300 ppb).

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