Effects of Antibiotic Growth Promoters in Simple or Complex Diets Fed to Weanling Pigs

The most complex diet in this experiment at Hans H. Stein Monogastric Nutrition Research Laboratory at the University of Illinois increased the feed intake by weaner pigs, which led to faster growth rate but reduced feed efficiency. The addition of an antibiotic growth promoter increased daily gain only in one phase and impaired feed efficiency overall in this study.
calendar icon 17 July 2014
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Recently, the Food and Drug Administration announced that the number of antibiotic growth promoters that are available to be used by pigs will be reduced to reduce the risk of transferring antibiotic resistance from animals to humans.

Only antibiotics that are not used in human medicine may be used as antibiotic growth promoters by animals in the future. To accommodate this change, it may be necessary to re-evaluate the use of antibiotic growth promoters in diets fed to pigs. It is possible that antibiotic growth promoters may be eliminated from swine diets if the complexity of the diets is increased.

To test this hypothesis, an experiment was conducted to determine the effects of using an antibiotic growth promoter in diets formulated to vary in complexity.

Experimental Design

Six diets were formulated in each of three phases. Days 0 to 11 comprised phase 1, days 11 to 25 comprised phase 2 and days 25 to 39 comprised phase 3.

In phase 1, the low-complexity diet was based primarily on corn and soybean meal, with three per cent plasma protein and 25 per cent whey. The medium-complexity diet was similar but also contained five per cent fish meal, mostly at the expense of soybean meal. The high-complexity diet contained no fish meal but contained 7.5 per cent plasma and 10 per cent lactose, mostly at the expense of corn.

In phase 2, the whey concentration was decreased in all diets, while plasma was eliminated from the low and medium diets and reduced to three per cent in the high diet. Lactose was removed from the high-complexity diet in phase 2 as well.

In phase 3, whey was removed from the low-complexity diet and reduced to five per cent of the medium- and high-complexity diets. Fish meal and plasma protein were removed from all diets in phase 3.

For each diet in each phase, three additional diets were formulated by adding an antibiotic growth promoter (Pulmotil in phases 1 and 2 and Mecadox in phase 3) to the diet, resulting in 18 total diets.

A total of 294 weaner pigs with an average initial body weight of 6.32kg were randomly allotted to the six dietary treatments. Pigs were weighed at the beginning of the experiment and at the conclusion of each phase. Daily feed allotments and uneaten feed were recorded and used to calculate average daily feed intake and gain:feed ratio.


Pigs fed the high-complexity diet had a tendency toward increased bodyweight on day 11, and increased (P<0.05) bodyweight on days 25 and 39 than with pigs fed medium- and low-complexity diets (Table 1).

Table 1. Effects of diet complexity and antibiotics on growth performance of weanling pigs
No AGPWith AGPP-value
Item High Medium Low High Medium Low Complexity AGP Complexity
Bodyweight, kg
Day 11 8.79 7.98 8.22 8.95 8.47 8.19 0.07 0.43 0.72
Day 25 15.58 14.37 14.54 16.52 15.11 14.99 <0.01 <0.05 0.83
Day 39 25.15 23.85 23.93 25.95 24.47 23.62 <0.05 0.47 0.63
Av. daily gain, g/day
Day 0-11 224 150 174 237 194 171 <0.01 0.09 0.17
Day 11-25 483 456 452 540 474 485 <0.01 <0.01 0.29
Day 25-39 671 678 672 675 670 618 0.27 0.24 0.34
Day 0-39 483 449 452 503 465 444 <0.01 0.28 0.35
Av. daily feed intake, g/day
Day 0-11 263 205 216 288 218 204 <0.01 0.36 0.27
Day 11-25 751 645 672 86 692 698 <0.01 <0.01 0.50
Day 25-39 1091 1036 1054 1164 1125 1055 0.10 <0.05 0.36
Day 0-39 735 679 680 799 717 686 <0.01 <0.05 0.28
Gain:feed ratio
Day 0-11 0.848 0.773 0.809 0.825 0.837 0.849 0.45 0.22 0.23
Day 11-25 0.679 0.708 0.674 0.655 0.685 0.696 <0.05 0.32 0.05
Day 25-39 0.607 0.661 0.629 0.576 0.597 0.587 <0.01 <0.01 0.35
Day 0-39 0.654 0.684 0.666 0.630 0.650 0.648 <0.01 <0.01 0.58

Pigs fed the high complexity diet also had greater (P<0.05) average daily feed intake in all phases and overall than pigs fed the low-complexity diet, as well as greater (P<0.05) average daily gain in phase 1, phase 2 and overall.

However, the gain:feed ratio in the pigs fed the high complexity diet was lower (P<0.05) in phase 2, phase 3 and overall than in pigs fed the low- and medium-complexity diets.

Pigs fed diets containing an antibiotic growth promoter had greater (P<0.05) bodyweight at day 25 than pigs fed no antibiotics. Average daily feed intake was increased (P<0.05) among pigs fed an antibiotic growth promoter in phase 2, phase 3 and overall. Average daily gain was also greater (P<0.05) in phase 2 for pigs fed the antibiotic growth promoter. In phase 3 and the overall experimental period, the gain:feed ratio was decreased (P<0.05) in pigs fed an antibiotic growth promoter.

Key Points

The high-complexity diet increased feed intake in weanling pigs, which resulted in greater gain. However, feed efficiency was decreased in pigs fed a high-complexity diet.

Adding an antibiotic growth promoter to diets of any level of complexity increased feed intake and increased gain in phase 2. However, in this experiment, the antibiotic growth promoter had no effect on overall gain, and adding antibiotics decreased feed efficiency.

Addition of the antibiotic growth promoter to diets used in this experiment resulted in less improvement in pig growth performance than had been observed in previous experiments. The pigs used in the present research had an excellent health status, which may be the reason for this observation.

This report is based on unpublished data by Yanhong Liu and Hans H. Stein.

July 2014

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