Effect of Exogenous Phytase on the Phosphorus Balance of Lactating Cows Fed A Corn Based Diet

In the past, dairy cows were often fed diets containing P levels markedly higher than the recommendations for P supply. The most common explanation for this oversupply is the perception that high-P diets improve reproductive performance. In addition, the recommendations for adequate P supply differ from nation to nation (CVB, 2005; GfE, 2001; INRA, 2002; NRC, 2001; Schlegel, 2011). Consequential the proportion of excreted P, which is not used to meet the requirements of the cow, increases. Moreover, natural P sources used as mineral feedstuffs become more and more limited in the future (Rodehutscord, 2008).

Therefore, it remains a challenge for animal nutrition to reduce the dietary P supply while meeting the requirement at the same time. One way to increase the P-absorption and to reduce faecal P is the supply of exogenous phytase to the diets (Knowlton et al., 2007). The enzyme phytase has a relevant impact in the reduction of P-excretion by excrements from monogastrics. Due to its ability to cleave phosphate from its binding to the inositol ring, phytase supply more P for absorption in the small intestine (Guyton et al., 2003). In ruminants, phytase is secreted intracellular by ruminal bacteria (Yanke et al., 1998) and phytate hydrolysis also occurs in the lower gastrointestinal tract (small intestine with duodenum, jejunum, ileum) of ruminants. Thus the total tract hydrolysis of phytate is nearly complete (Brask-Pedersen et al., 2013). However, for P to be absorbed from the small intestine, the phytate hydrolysis must occur in the rumen. Using in vitro ruminal techniques Morse et al. (1992) and Brask-Pedersen et al. (2011) found out that the effect of exogenous phytase is closely related to the composition of the feedstuff, the pH-value level, the kind of phytase and the time of incubation. Additionally, Brask-Pedersen et al. (2011) observed that the supply of exogenous phytase in vitro can influence the P-utilization positively. These results are sustained by Garikipati and Kincaid (2004), who figured out a positive effect of the influence of exogenous phytase in dairy cows. However, data regarding the intake of P and the use of exogenous phytase are inadequate. Kincaid et al. (2005) tested the effects of grain source and exogenous phytase supplementation on the digestibility of P and concluded that exogenous phytase could have an influence on the faecal P-excretion of dairy cows.

Although the main part of phytase activity in the rumen is of bacterial origin (Yanke et al., 1998), phytin hydrolysis might also be caused by intrinsic phytase contained in the diet, whereby however only some cereals and their by-products show phytase activities of more than 100 units/kg (Eeckhout & De Paepe, 1994). The phytase activity in corn is below the detection limit (Zimmermann et al., 2002). According to current knowledge, there is no certain way to identify the quantitative influence of plant phytase on ruminal phytate hydrolysis (Kincaid & Rodehutscord, 2005). Experiments in a semi-continuous culture system by Godoy and Meschy (1999) with P of inorganic and organic origin suggest that in some situations the ruminal phytase activity does not hydrolyze all dietary phytate. Time of incubation of feed in the gastro intestinal tract is getting shorter in high lactating cows, because the feed intake increases with the increasing performance. Based on this, the passage rate increases and with this the time of P-hydrolyses gets even lower (Garikipati & Kincaid, 2004).

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(Author: Laura Winter, Ulrich Meyer, Markus Spolders, Liane Hüther, Peter Lebzien, Sven Dänicke

Published by Macrothink Institute)