There were three main stages involved in the conduct of this study. These were a pilot assay followed by two full-scale replicated slope ratio assay trials designed to evaluate the availability of CM from Boree, Riverland and Melbourne and one CSM from Narrabri. The pilot study established the lower, middle and upper dose levels needed to ensure a linear response to standard lysine and to the test proteins. The values of lysine levels selected in the pilot work to be used for the slope ratio bioassays were (g lysine/kg): 4.7, 5.4, 6.1 and 6.8. However, the first slope ratio assay failed to be statistically validated, with low bird feed intake, inappropriate feed texture and low dietary amino acid concentration in the diet being the major causes for failure. Thus, a modified slope ratio assay was carried out which included steam-crumbled diets and a higher supply of amino acid, which improved by nearly 40% the overall bird feed intake and live weight gain (LWG).

Central to the project was to prove and validate that the observed responses of the chickens were due to the amino acid under test. Therefore, the test of validity of the bioassay as described by Finney (1971) was statistically analysed for LWG and for feed conversion ratio (FCR). For LWG the slope ratio assay was conducted within the linear portion of the birds’ LWG response to lysine, and the intersecting lines regression model was found to be statistically valid. On the other hand, the validity test imposed for FCR failed due to curvature present in the response to lysine, particularly in the basal diet points. Therefore, the bioavailability results based on FCR should be rejected. However, in spite of the failure of this FCR validity test, the results have been included in this report in a similar way that the bioavailability figures obtained with the LWG have been included. Explanations for this option are given within the report.

The lysine bioavailability estimates for the four test proteins meals as assessed by chick slope ratio assay using either LWG or FCR as the criteria for availability were (LWG, FCR): CSM 0.555, 0.609; CM Boree 1.114, 0.919; CM Riverland 0.905, 0.878; CM Melbourne 0.828, 0.876. In general the estimates of availability when using FCR were in close agreement with the LWG figures.

CSM lysine availability determined in these studies was lower by 24% and 30% for LWG and FCR, respectively, than values reported by Major and Batterham (1981). This discrepancy may be attributable to differences in plant cultivars, processing methods for oil extraction and the type of condensed tannins present in the meal.

Among the three CM, Boree exhibited the highest lysine availability compared with the other CM sources. Since Boree CM is an extruded extracted meal, this suggests that processing conditions may be the main factor affecting lysine availability in these meals.

In these studies, the availability of lysine from various CM was 49-100% higher than that of CSM.

Plant processing conditions for oil extraction (particularly heating conditions), and differences in the type of condensed tannins which are present in both protein meals are probably responsible for the difference in lysine availability. Further experiments are needed to confirm this.

One of the main objectives of this research was to compare the slope ratio assay for availability as determined in this work with the apparent ileal digestibility method undertaken during a previous study at QPRDC (Perez-Maldonado, 2003). For CSM, both methods agreed well (slope ratio assay 0.555 c.f. ileal digestibility method 0.515). Therefore, ileal digestibility of lysine appears to provide a reasonable estimate of lysine availability in the case of CSM from Narrabri.

The lysine availability of CM from Boree, Riverland and Melbourne were 1.114, 0.905 and 0.828, respectively, which were 51%, 25% and 20% higher than the lysine ileal digestibilities obtained on similar CM samples using the ileal digestibility method. These results suggest that the two methods compare poorly for CM. This outcome is difficult to explain and further investigation is needed.

However, a recent review (Sales and Janssens, 2003) indicated that the use of markers such as chromium oxide may be the cause of low digestibility determinations. Therefore, CM digestibility can not be relied upon as a measure of the lysine availability, but more evaluations of the lysine availability in other protein meals may give insight into the relative merits of the two methods.

In conclusion:

• A chick slope ratio assay methodology for determining the availability of amino acids in feedstuffs was established.
• Three major assays trials were conducted including a pilot assay and two full-scale replicated bioassays to establish the bioassay and to determine lysine bioavailability in four protein meals.
• Of the protein meals tested, cottonseed meal had much lower available lysine than did the canola meals.
• There are biologically significant differences in available lysine among the three canola meals tested which are most likely due to the processing conditions used to extract oil to produce the CM.
• For CM, the ileal digestibility method underestimated lysine availability compared with the slope ratio bioassay and more research is needed in this area.
• Condensed tannins, poliphenolic compounds found in CSM and CM, may play an important role when assaying the digestibility and availability of lysine. Further research using other protein meals is needed to confirm this.

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Last updated: 19 December 2004  Copyright RIRDC
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