Factors affecting the faecal P status of lactating beef cattle in Northern Australia

Abstract

Phosphorus (P) deficiency can be challenging to diagnose and its impact on beef production difficult to quantify. However, an indicator of wet season P has been identified as a major factor affecting the reproductive performance of beef breeding females across northern Australia (McGowan et al., 2014). Faecal near infrared reflectance spectroscopy (FNIRS, to estimate dietary metabolisable energy (ME)) and wet chemistry (to estimate faecal P (FP)) are methods used to create a newly developed ratio of faecal phosphorus to metabolisable energy (FP:ME) to assess the risk of P deficiency adversely affecting the reproductive performance of beef breeding herds. This study investigated property-level environmental, management and animal factors affecting wet season FP:ME under extensive grazing conditions. Pooled faecal samples were collected over the wet season (November, January, February, April) for three years from commercial beef breeding mobs managed on 78 properties located across northern Australia. Animal, management and environmental information was subjected to univariate and multivariate analysis to identify those factors that significantly influenced wet season FP:ME. The three major factors affecting the likelihood of FP:ME being less than 500mgP/MJME, i.e.where the risk of wet season P deficiency was considered to adversely affect the reproductive performance of heifers and cows, were the soil P classification of the property according to McCosker and Winks (1994), an early onset of the wet season, and the duration of wet season P supplementation. In locations where soil P had been historically categorised as "acute and deficient" the prevalence with which the wet season FP:ME was considered to adversely affect reproductive performance was 42% greater than for locations categorised as "adequate" (Odds ratio (OR)=17.8; p=0.002). When the onset of the wet season was "normal to late" the prevalence of an adverse effect on reproductive performance was 27% greater than when the wet season onset was "early" (i.e. before 15th September) (OR=5.4; p=0.002). Where properties provided a P supplement for greater than one month during the wet season the prevalence of an adverse effect was 25% greater than when wet season P supplement was provided for one month or less (OR=4.7; p=0.048). As only 22% of producers provided wet season P supplement for more than one month and of them, 68% were located on soil types acutely deficient in P, it is suggested there has been a confounding of the "duration of wet season P 3 | P a g e M . P h i l . -W h i t n e y D o l l e m o r e -4 1 0 1 4 8 8 6supplementation" result by soil P classification and producer awareness of breeder requirements for wet season P supplementation. Further research is required to define the direct and modifying effects of each of these factors on the FP:ME ratio.This study has served to identify the main factors affecting the FP:ME and may assist in the decision-making process for management strategies to reduce the risk of P deficiency adversely ...