Effects of standardised total tract digestible phosphorus on performance, carcass characteristics, and economics of 24 to 130 kg pigs

The 2012 National Research Council (NRC) adopted the concept of standardised total tract digestibility (STTD), which was based on a factorial approach to report the phosphorus (P) requirements of pigs. There is a need for more data to validate the model-derived digestible P requirement since there was only one empirical estimate for pigs greater than 65 kg bodyweight (BW) included in these recommendations. The objective of this study was to determine the effects of STTD P on growth performance, bone mineralisation, carcass characteristics, and economics of 24 to 130 kg pigs housed under commercial conditions. A total of 1130 barrows and gilts (PIC; 359 × Camborough, initially 24.1 ± 0.73 kg BW) were used in a 111 day growth trial. Pens of pigs were randomly assigned to one of six dietary treatments in a randomised complete block design. Treatments were formulated to contain 80, 90, 100, 115, 130, and 150% of the NRC (2012) STTD P requirement for growing-finishing pigs within each phase. There were seven replicate pens per treatment and 26 to 27 pigs per pen (at least 13 barrows and gilts per pen). The experimental diets were corn-soybean-meal–based and fed in four phases. Treatments were achieved by increasing the inclusion of limestone and monocalcium phosphate at the expense of corn. A similar 1.14 : 1 to 1.17 : 1 total Ca : P ratio was maintained, with no phytase added to the diets. Data were analysed using generalised linear and non-linear mixed models, and polynomial contrasts were implemented with pen as the experimental unit. Competing models, including a linear model, quadratic polynomial (QP), broken-line linear, and broken-line quadratic were fit using GLIMMIX and NLMIXED procedure of SAS (v9.4, SAS Institute Inc., Cary, NC, USA) according to Gonçalves et al. (2016). For the overall period, increasing STTD P increased average daily gain (ADG), feed efficiency (G : F), final BW, and hot carcass weight (quadratic, P < 0.05). Average daily feed intake, grams of STTD P intake per day, ashed bone weight and bone percentage ash increased linearly as the inclusion of STTD P increased in the diets (P < 0.05). Carcass yield decreased with increasing STTD P (linear, P < 0.05), while there was a decrease in backfat and increase in fat-free lean (P < 0.10). No evidence for differences were observed for loin depth measurements (P > 0.10). Feed cost per pig increased linearly (P < 0.05) with increasing STTD P levels while gain value per pig increased quadratically (P < 0.05). Similarly, income over feed cost increased in a quadratic manner (P < 0.05). For ADG and G : F, the QP model demonstrated best fit (Fig. 1). For ADG, the maximum response was estimated with STTD P at 122% of current NRC estimates, with 99% of maximum ADG achieved at 102% STTD P. For G : F, the maximum response was estimated with STTD P at 116% of current NRC estimates, with 99% of maximum ADG achieved at 82% STTD P.

Fig. 1.  Fitted QP regression model for ADG and G : F as a function of increasing STTD P in 24–130 kg pigs.

In conclusion, the estimated STTD P requirement for pigs from 24–130 kg to maximise growth performance ranged from 116% to 122% of the NRC (2012) recommendations for each phase, depending on the response criteria and statistical model.