*Department of Animal Sciences and Industry y PIC USA, Franklin, USA
A challenge in animal production today is to raise livestock within new environmental rules and regulations. Although most states in the United States regulate swine waste application based on N concentration, more are changing to Pbased regulations. Because of the amounts of N and P in swine waste and their different rate of uptake by most plants, P concentration can be first limiting for waste application if soil P accumulation is not permitted. Therefore, reevaluation of P requirements of swine is an important step in minimizing its excretion. In evaluating available P (aP) requirement estimates, several studies have demonstrated that growth performance will not be negatively affected by feeding 66% of the aP requirement estimate of NRC (1998; Mavromichalis et al., 1999; Shaw et al., 2002). However, pigs raised in typical university research setting (2 to 10 pigs per pen) have been observed to consume approximately 30% more feed than those in commercial facilities (25 to 27 pigs per pen and 1,000 pigs per barn). Therefore the objective of this study was to identify available phosphorus (aP) requirements of pigs reared in commercial facilities.
In Exp. 1, 1,260 gilts (initially 33.8 kg) were allotted randomly to one of five dietary treatments containing 0.18, 0.22, 0.25, 0.29, or 0.32% aP, corresponding to 0.5, 0.6, 0.7, 0.8, or 0.9 g of aP/Mcal of ME. There were 28 pigs per pen and nine pens per treatment. From d 0 to 14, increasing aP increased ADG (linear, P = 0.03; Table 1) and G:F (quadratic, P = 0.07), with the greatest response observed as aP increased from 0.18 to 0.22% (G:F breakpoint = 0.22%). However, from d 0 to 26, no differences (P = 0.12 to 0.81) were observed for any growth traits. Pooled bending moment of the femur, sixth rib, and third and fourth metatarsals increased (linear, P = 0.007) with increasing aP (Table 2). The linear nature of the response from d 0 to 14, followed by no overall response to increasing aP, makes estimating a requirement difficult. The d0 to 14 data may provide a more applicable requirement estimate because of the potential for P mobilization from bone tissue being directed to meet requirements for lean tissue growth.We speculate that d0 to 14 ADG and G:F were decreased immediately by low dietary aP concentrations, but by d 14 to 28, P was mobilized from bone tissue to meet the requirement for growth of pigs fed low aP concentrations. However, this resulted in low bone ash concentration. An alternative hypothesis decreased between the first 14 d of the study to the last 12 d. Because the apparent requirement estimate changed so dramatically over the 26d study and because of the possibility of bone P mobilization meeting the pig’s requirement, we believe the d0 to 14 aP estimate of 0.22% corresponding to 0.60 g of aP/Mcal of ME and 3.30 g of aP/d to be a conservative and more accurate requirement for the entire 26d period.
Table 1. Effects of increasing available phosphorus (aP) on growing pig growth performance, Exp. 1^{a}  
Dietary aP, %

P value


Item  0,18  0,22  0,25  0,29  0,32  Linear Quadratic  SEM  
D 0 to14  
ADG, g  792  840  826  854  837  0,03  0,12  15 
ADFI, kg  1,50  1,46  1,48  1,45  1,49  0,66  0,24  0,26 
G:F  0,53  0,58  0,56  0,59  0,57  0,06  0,07  0,014 
aP intake, g/d^{b}  2,70  3,21  3,69  4,20  4,76  0,01  0,62  0,07 
D 14 to26  
ADG, g  881  886  889  891  883  0,89  0,70  19 
ADFI, kg  1,55  1,55  1,56  1,59  1,57  0,45  0,79  0,35 
G:F  0,57  0,57  0,57  0,56  0,57  0,68  0,93  0,016 
aP intake, g/d^{b}  2,79  3,41  3,91  4,62  5,02  0,01  0,51  0,01 
D 0 to26  
ADG, g  833  861  855  871  858  0,12  0,19  12 
ADFI, kg  1,52  1,50  1,52  1,52  1,52  0,81  0,63  0,26 
G:F  0,55  0,58  0,57  0,58  0,57  0,35  0,22  0,012 
aP intake, g/d^{b}  2,74  3,30  3,79  4,39  4,89  0,01  0,89  0,07 
^{a }A total of 1,260 gilts, initially 33.8 kg BW, was used. Values represent the means of 28 pigs per pen and nine pens per treatment.  
^{b} Calculated dietary aP values multiplied by the ADFI. 
Table 2. Effects of increasing available phosphorus (aP) on growing pig bone properties, Exp. 1^{a}  
Dietary aP, %  P value  
Item  0,18  0,22  0,25  0,29  0,32  Linear Quadratic  SEM  
Metatarsal 3  
Bending moment, kg/cm  36  28  24  28  33  0,77  0,18  7,6 
Ash, %  49,1  52,1  50,1  50,3  49,8  0,97  0,51  1,9 
Metatarsal 4  
Bending moment, kg/cm  37  32  37  37  32  0,82  0,76  4,7 
Ash, %  46,3  49,5  48,1  48,4  49,8  0,01  0,40  0,6 
Rib  
Bending moment, kg/cm  19  26  25  28  28  0,001  0,03  1,2 
Ash, %  47,1  48,1  48,3  48,8  48,3  0,16  0,64  0,9 
Femur ^{b}  
Bending moment, kg/cm  289  338  319  339  338  0,01  0,17  11,8 
^{a }One pig from each pen was randomly selected for harvest of bones. Values represent the mean of nine observations per treatment.  
^{b} Percentage of bone ash was not conducted on femurs. 
In Exp. 2, 1,239 gilts (initially 88.5 kg BW) were randomly allotted to one of five dietary treatments containing 0.05,0.10, 0.14, 0.19, or 0.23% aP, equivalent to 0.14, 0.28, 0.39, 0.53, or 0.64 g of aP/Mcal of ME. The diet with 0.05% aP contained no added inorganic P. From d 0 to 14, increasing aP increased (linear, P = .008 to 0.02; Table 2) ADG and G:F; however like Exp. 1, from d 0 to 28, increasing aP had no effect (P = 0.17 to 0.74; Table 3) on growth performance. Increasing aP increased (linear, P < 0.001 to 0.04) metacarpal bone ash percent and bending moment (Table 4). Results suggest that 33 to 55kg pigs require approximately 0.22% aP, which corresponds to 0.60 g of aP/Mcal of ME or 3.30 g of aP/d to maximize ADG and G:F compared with NRC (1998) estimates of 0.23%, 0.70 g of aP/Mcal of ME, and 4.27 g of aP/d for 20 to 50kg pigs. Finishing pigs (88 to 109 kg) require at least 0.19% aP, corresponding to 0.53 g of aP/Mcal of ME or 4.07 g aP/d compared with NRC (1998) estimates of 0.15%, 0.46 g of aP/Mcal of ME and 4.61 g of aP/d for 80 to 120kg pigs. However, the percentage of bone ash and bending moment continued to increase with increasing aP. These data also suggest that complete removal of supplemental P in diets for finishing pigs (>88 kg) will decrease ADG and G:F.
Table 3. Effects of increasing available phosphorus (aP) on finishing pig growth performance, Exp. 2^{a}  
Dietary aP, %  P value  
Item  0,05  0,10  0,14  0,19  0,23  Linear Quadratic  SEM  
D 0 to14  
ADG, g  621  683  691  734  707  0,008  0,14  26 
ADFI, kg  1,92  2,01  2,01  2,03  1,96  0,43  0,10  0,45 
G:F  0,33  0,34  0,34  0,36  0,36  0,02  0,66  0,012 
aP intake g/d^{b}  0,96  2,00  2,81  3,86  4,51  0,01  0,01  0,05 
D 14 to28  
ADG, g  763  738  760  756  763  0,89  0,82  43 
ADFI, kg  2,25  2,19  2,28  2,24  2,28  0,49  0,73  0,56 
G:F  0,34  0,34  0,33  0,34  0,33  0,92  0,93  0,014 
aP intake g/d^{b}  1,12  2,19  3,19  4,26  5,25  0,01  0,78  0,08 
D 0to 28  
ADG, g  696  713  728  746  737  0,17  0,63  26 
ADFI, kg  2,10  2,10  2,15  2,14  2,13  0,34  0,52  0,39 
G:F  0,33  0,34  0,34  0,35  0,35  0,19  0,74  0,009 
aP intake g/d^{b}  1,05  2,10  3,01  4,07  4,91  0,01  0,11  0,05 
^{a }1,236 gilts, initially 88.5 kg BW, were used. Values represent the means of 27 or 28 pigs per pen and nine pens per treatment.  
^{b} Calculated dietary aP values multiplied by the ADFI 
Table 4. Effects of increasing available phosphorus (aP) on finishing pig bone properties, Exp. 2^{a}  
Dietary aP, %  P value  
Item  0,05  0,10  0,14  0,19  0,23  Linear Quadratic  SEM  
Metacarpal 3  
Bending moment, kg/cm  100  111  118  113  120  0,003  0,24  4,36 
Ash, %  50,1  50,7  51,9  52,0  52,1  0,001  0,14  0,36 
Metacarpal 4  
Bending moment, kg/cm  93  96  93  97  96  0,59  0,93  4,34 
Ash, %  51,2  51,6  51,8  52,7  53,3  0,001  0,48  0,52 
^{a} Two pigs were randomly selected from each pen for harvest of bones. Values represent the mean of nine observations per treatment. 