Improved characterization of protein sources and implications on evaluation of rations for dairy cows

Venter, Louis Johannes (2017-03)

Thesis (MScAgric)--Stellenbosch University, 2017.


ENGLISH ABSTRACT: This study looked at a recent development in a new laboratory assay used to determine the unavailable nitrogen (uN) fraction in non-forage feeds. The need for this new assay was brought about as the current methods used to determine the uN fraction provide inconsistent results with varying amounts of AA being liberated. Furthermore, the need to improve nitrogen utilization efficiency (NUE) in dairy cows is necessary to reduce harmful waste to the environment and decrease feed costs. For that reason an effective approach to improve NUE, as well as improve protein characterization, is by improving the estimation of protein fractions such as the unavailable one. However, due to the assay being a novel procedure the implementation of the procedure in our laboratory required slight modifications in order to replicate the procedure. Nonetheless, successful results were obtained on 19 protein sources that are commonly used in South Africa with comparable values being achieved in different laboratories. Although the procedure was developed to determine the uN fraction, it became apparent that the rumen degradable (RDP) and undegradable protein (RUP) fractions need to be determined in order to quantify uN. Consequently, the new assay has provided a drastic improvement in past procedure to determine the uN fraction with comparable values for both RDP and RUP in the literature. As for the uN fraction, the new assay reports significantly higher values than previously presumed ranging from 34.53g kg-1 CP in sunflower meal to 447.70 g kg-1 CP in feather meal. These values indicate that there has been a drastic underestimation in the uN fraction, which has resulted in high levels of N excretion as well as an unnecessary expense for the farmer. The uN fraction, and relative assay, has also been recently implemented in the Cornell Net Carbohydrate and Protein System (CNCPS) which in addition to the former acid detergent insoluble nitrogen (ADIN) method, allows for a comparison to be made between the different procedures. Using this nutritional model to make this comparison in 10 rations that were supplied to us, it was evident that NUE could be improved in addition to income over feed costs (IOFC). When implementing the uN fraction, as opposed to the traditional detergent system, the rations resulted in a reduction of IOFC from R0.10 to R39.50 per cow/day. In addition, the nutritional model showed a range of milk loss from 0.31 l to 7.90 l per cow/day as a result of the true protein unavailable to the animal. Moreover, by optimizing the rations for both IOFC and productive N, an improved composition was noted which resulted in an improved IOFC ranging from R0.16 to R7.16 per cow/day. However, owing to the new assay being a novel and an in vitro procedure, we would recommend further studies on both in vitro and in vivo trials to confirm our findings.

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