Nursery pigs in a pen in a hog barn National Pork Board

Evaluating alternatives for antibiotics in nursery pig diets

As research on antibiotic growth promoters alternatives progresses, the credibility and impact of future studies will be improved with proper design, protocol implementation and reporting of information.

By Kristin Olsen, Nicholas Gabler, Wes Schweer, Kent Schwartz (DVM), Chris Rademacher (DVM) and John Patience, Iowa State University
It is clear in today’s pork industry that producers need to look toward alternatives to sub-therapeutic growth promoting antibiotics. The implementation of the Veterinary Feed Directive has shifted the way producers can use antibiotics for growth promotion, which adds urgency to identification of effective alternatives to antibiotic growth promoters.

With the heightened attention surrounding this topic, there have been many alternatives proposed and advertised to swine producers. Current literature is inconclusive regarding efficacy of various AGP alternative products, and under what circumstances specific products are beneficial for productivity. The National Pork Board Checkoff program funded a two-stage project addressing AGP concerns with collaborators at Iowa State University and the USDA National Animal Disease Center as outlined in a March 2017 National Hog Farmer column “Evaluation of sub-therapeutic growth promotion antibiotic alternatives” by Schweer and Gabler. Stage 1 of the project was a literature review seeking to identify components essential to studies that evaluate alternative products as well as to identify trends in the literature that may explain the variation in outcomes of AGP alternative studies.

The goals of Stage 2 were two-fold: to develop and validate a protocol for assessing AGP alternatives using the information derived from Stage 1, and to generate baseline data of the effect of AGP alternatives on pig performance. This column focuses on the second goal: providing the industry with data relating to certain AGP alternatives.

A six-week nursery experiment was conducted at a commercial wean-to-finish barn with 1,300 piglets (weaned at about 21 days of age; 13.5 pounds ± 0.39 body weight). There were four dietary treatments (Table 1) including:
1) Negative control diet (NC; no antibiotics or alternative ingredients),
2) No. 1 plus sub-therapeutic antibiotics (Positive control diet, PC),
3) No. 1 plus high levels of zinc oxide and a dietary acidifier (ZA; NC + ZnO + acidifier), and
4) No. 1 plus a direct-fed-microbial and resistant potato starch (DR; NC + DFM + RS). Diet analysis is currently under way in order to confirm presence of the specified alternative products in the feed.

Iowa State University

Table 1: Dietary treatments

Thus far, the majority of research conducted using AGP alternatives has been done in academic settings, which tend to utilize fewer pigs per pen than commercial barns. For this reason, we wanted to see whether the number of pigs in a pen would impact the effectiveness of AGP alternatives. Two different pen sizes were used: 11 pigs (small pens) or 31 pigs each (large pens). Small pens were modified to maintain approximately the same square footage per pig (4.4 square feet in large pens and 4.5 square feet in small pens) and approximately equal feeder space per pig. This avoided confounding of the experiment by floor space per pig (if floor space had not been adjusted) or by feeder access (if feeder space had not been adjusted).

During the course of the experiment, all pigs experienced two naturally occurring health challenges. During the first week, pigs experienced acute diarrhea and septicemia (Salmonella, Actinobacillus suis and Streptococcus suis was confirmed) and during Week 4 circulation of a field strain of porcine reproductive and respiratory syndrome virus was confirmed. Growth performance was likely influenced by these disease challenges, as growth rates were not as high as expected. Mortality was 1.8% and morbidity (pigs removed from the study for illness or injury) was 6.1% for the entire nursery period. The number of pigs removed from the study for any reason was not influenced by diet, however fewer pigs were removed from small pens than large pens (5% of pen removed versus 9% of pen removed, respectively; P=0.049).

For the overall six-week nursery period, pigs gained 0.61 and 0.65 pounds per day in large and small pens, respectively. Feed intake was 0.94 and 0.96 pounds per day in large and small pens, and F:G was 1.54 and 1.29 (pen size P=0.004). The PC diet, containing antibiotics, appeared to yield the most favorable response, resulting in a 19% increase in ADG, and a 12% increase in ADFI (P<0.05) compared with the NC. The DR diet did not impact ADG or ADFI for either group size (P>0.10). However, a significant interaction between diet and group size prompted a closer evaluation of the ZA diet (interaction P=0.012). This diet improved ADG in the large pens by 8%; however, ZA did not improve ADG in the small pens (Fig. 1). This response was also observed on ADFI, which was improved in large pens on the ZA diet, but not in small pens (Figure 2; interaction P=0.015). An interaction was not observed for F:G; small pens were more efficient than large pens (pen size P=0.004), and pigs fed the PC diet were more efficient than the other three dietary treatments (diet P<0.0001).

Iowa State University

Figure 1: Effects of diet and group size on ADG for six-week nursery period

Iowa State University

Figure 2: Effects of diet and group size on ADFI for six-week nursery period

In this study, the effect of ZA diet on ADG and ADFI was influenced by group size, suggesting that the number of pigs per pen should be considered when evaluating AGP alternative ingredients, comparing results across studies and in future study design. These results suggest that a diet containing high levels of zinc and a dietary acidifier may have potential to improve growth performance in an environment where pigs are slightly more stressed, such as in a barn where pigs are housed in larger groups. Our results also suggest that a positive response to certain AGP alternatives may be less pronounced in studies conducted where pigs are housed in smaller groups as is common in academic settings, particularly when pigs have high health status.

Iowa State University

Figure 3: Effects of diet and group size on F:G for six-week nursery period

As research on AGP alternatives progresses, the credibility and impact of future studies will be improved with proper design, protocol implementation and reporting of information, including: defined genetic background, health status/disease challenges present, vaccine/medication history, reporting of mortality and performance, a clear outline of experimental design (e.g. diet formulation, floor space per pig, number of pigs per pen) and a description of the study environment. These factors may all influence a study’s outcome, and thus will be important when comparing results across experiments.

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