Effects of dietary methionine on broiler flock uniformity - By S. Mack, A. Lemme, G. Irish and J. Tossenberger, Degussa Australia Pty Ltd - It is well documented that increasing levels of essential amino acids when starting from a deficiency increase broiler performance. The response typically follows the law of diminishing returns, which means a non-linear dose response approximating an asymptote.

Summary

Graded levels of DL-methionine (DL-Met) (0, 0.4, 0.8, 1.2 g/kg) were added to basal starter, grower and finisher diets deficient in methionine and cysteine in order to evaluate the effect on body weight and breast meat uniformity of male broilers. Birds were individually weighed at days 5 (start of experiment), 17, 35 and 42. Final body and breast meat weights increased from 1831 to 2836 g and 362 to 707 g, respectively, with increasing dietary methionine content.

The coefficient of variation for body weight and breast meat weight decreased with increasing dietary methionine level. Model calculations revealed that if 1800±100 g body weight and 420±25 g breast meat weight were set as production targets at 42 days of age only 26% and 30%, respectively, of the flock would meet these targets when the basal deficient diets were fed whilst 58% and 59%, respectively, would meet these targets at the highest inclusion level.

Introduction

Such equations are appropriate for determining the optimum dietary amino acid levels for maximum performance and can be combined with parameters such as feed cost and revenues from meat marketing in order to determine economically optimum dietary amino acid levels (Hoehler, 2000; Pack and Schutte, 1995). Such calculations have been based on the potential dietary effects on the average performance of a flock, but have not considered the effects of dietary amino acid imbalance on uniformity or variability within a flock. Therefore, the purpose of the present study was to examine the effect that the addition of graded levels of DL-Met to a diet deficient in methionine plus cysteine (Met+Cys) has on the variability of body weight and breast meat yield in broilers. In addition, model calculations will demonstrate the potential impact of variability on profitability assuming that variability of a delivered flock would be considered in the pay-out system.

Material and Methods

A total of 640 five-day old male Ross 308 broiler chicks were used in the experiment. Birds were equally distributed to 20 pens with 32 birds each. Average body weight and coefficient of variation (CV) within each pen were 95 g and 5.7%, respectively. Eight pens were assigned to the basal treatment while four pens were assigned to each of the remaining treatments. Maize-wheat-pea-soybean meal based basal starter (day 6 to 17), grower (day 18 to 35) and finisher (day 36 to 42) diets were formulated to be deficient in digestible Met (2.6 g/kg, 2.3 g/kg, 2.2 g/kg, respectively) and Met+Cys (5.5 g/kg, 5.1 g/kg, 4.9 g/kg, respectively). The digestible lysine contents were 12.0 g/kg (starter), 11.3 g/kg (grower) and 10.7 g/kg (finisher).

The calculated digestible Met+Cys to lysine ratio was 0.46 while the ratios of threonine, tryptophan and arginine to lysine were 0.68, 0.18 and 1.06, respectively. Calculated amino acid levels were confirmed by analysis. Three levels of DL-Met (0.4, 0.8, 1.2 g/kg) were supplemented to the diet to achieve graded levels of Met+Cys. Feed and water were offered for free consumption throughout the whole experiment.

Birds were individually weighed at days 5, 17, 35 and 42. At 42 days of age all birds were slaughtered in a commercial slaughterhouse, processed and breast meat weights were recorded for each bird. Broiler performance data were checked for outliers, which were defined as values outside 2 standard deviations of the treatment mean. In case of outliers not only the single value, but the whole data set of the bird was excluded from the statistics. Average body weights, breast meat weights and CVs were calculated for each treatment (Table 1). ANOVA was applied using individual birds as experimental units. Comparison of means was performed according to Scheffé (SAS Institute, 1987) and P<0.05 was considered statistically significant.

As the achieved average maximum body weight of birds on the basal treatment at day 42 was only 1831 g, 1800 100 g was chosen as the production target weight for the model calculations and CVs for 1800 g body weight were calculated using the aforementioned equations for average body weights and CVs. By means of integral calculations the proportion of the flock meeting the target window of 1700 to 1900 g body weight was determined.

Since there was only one point in time for recording carcass data after termination of the experiment at day 42, model calculations could not be standardised to a certain breast meat weight as explained for body weight. Therefore, variability found at day 42 for the four treatments (Table 1) was applied. Since across all treatments breast meat yield was on average 23.3% of empty body weight, an average breast meat weight of 420 g was assumed for a bird of 1800 g live weight.

Results and Discussion

The development of the average body weights over time was well described by Gompertz equations [y = a * exp(-b * exp(-c * (days -5))); r2 = 0.86-0.99] for each of the four treatments while the development of the CVs over time followed exponential functions [y = a + b * (1-exp(-c * (days-5))), r2 = 0.60-0.89]. Dietary treatments resulted in an additional weight gain of 1005 g leading to an average body weight of 2836 g at day 42 in birds fed the highest DL-Met supplementation level (Table 1). The latter performance indicates excellent experimental conditions suggesting that the respective CVs might also represent relatively low variability. The responses of body weight at days 17, 35, and 42 to increasing Met+Cys levels were non-linear. However, maximum performance was probably still not achieved at the highest supplementation level.

There was a very significant increase in CV with age from 5.6% at day 5 to 16.8% at day 42 in birds fed the basal diet whereas the CV increased only slightly in birds of the highest Met+Cys level (Table 1). Thus, CV decreased from 16.8% to 6.7% with increasing dietary Met+Cys at the termination of the trial.

Therefore, the magnitude of the effects on variability appears to be dependent on both the degree and the duration of the amino acid deficiency. However, the variability in body weights at day 42 decreased with increasing body weight. Under practical conditions the broiler producer has to meet a defined average target weight plus or minus a certain deviation.

Average bodyweights outside the target range result in lower revenues. Current routine in many broiler production systems is to achieve the target weight on average for the whole flock without considering variation within the flock. If variation is considered, the question about the proportion of birds meeting the defined target window arises. According to the present data, the CVs standardised to an average body weight of 1800 g decreased non-linearly from 17.0% to 6.7% with increasing Met+Cys levels. This is similar to the final CVs at day 42, encouraging the use of the final CVs for breast meat for further model calculations (Table 2). Only 26% of the birds on the basal treatment would meet the target range of final body weight whereas this percentage would increase to 58% in birds fed the highest DL-Met inclusion level.

Assuming a monetary deduction of $0.01 per kg live weight for birds outside the production target of 1800±100 g in a 10,000 birds flock, this would mean $58 higher revenue at 1.2 g/kg supplemental DL-Met compared to the basal diet. Under practical conditions the effect can be expected to be of a smaller magnitude since the Met+Cys content would not be as low as in the present basal diet. On the other hand, the variability as such might be higher in reality compared to that obtained with the experimental conditions in the present trial. This could be reduced by balancing the amino acid profile.

A similar effect would occur if revenues were based on breast meat yield (Table 2). Defining a production target of 420±30 g only 30% of a flock would meet the target when fed the basal diet while 59% of the flock would meet it at the highest Met inclusion level. Setting the financial penalty at $0.05 per kg breast meat outside the target range for a 10,000-bird flock would lead to $61 higher revenue at the 1.2 g/kg supplemented DL-Met level compared to the basal diet.

References

Hoehler, D. (2000). AminoNews™ Degussa AG, 1 (1): 9-16.
Pack, M. and Schutte, J.B. (1995). Poultry Science, 74: 488-493.
SAS Institute (1987). SAS User's Guide: Statistics, Version 6 edition, Cary, NC, USA

Source: University of Sydney - Published on ThePoultrySite - April 2005