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Phosphorus requirements of growing-finishing pigs reared in a commercial environment^1,2

C. W. Hastad^*, S. S. Dritz, M. D. Tokach^*, R. D. Goodband^*,3, J. L. Nelssen^*, J. M. DeRouchey^*, R. D. Boyd^,4 and M. E. Johnston^,4

^* Department of Animal Sciences and Industry and and Food Animal Health and Management Center, College of Veterinary Medicine, Kansas State University, Manhattan 66506-0201; and PIC USA, Franklin, KY 42134

	Abstract

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The objective of this study was to identify available phosphorus (aP) requirements of pigs reared in commercial facilities. In a preliminary study, 600 gilts (PIC) were allotted randomly to low (0.30%) or high (0.37%) dietary aP from 43 to 48 kg BW, and later to 0.19 or 0.27% aP from 111 to 121 kg BW. No differences were observed (P = 0.42 to 0.88) in ADG, but G:F from 43 to 48 kg tended to improve (P = 0.07) for pigs fed low aP. Results suggested that the aP requirement was at or below 0.30 and 0.19%. These concentrations were used to titrate aP requirements in Exp. 1 and 2. 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) 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. 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 = 0.008 to 0.02) ADG and G:F; however, from d 0 to 28, increasing aP had no effect (P = 0.17 to 0.74) on growth performance. Increasing aP increased (linear, P < 0.001 to 0.04) metacarpal bone ash percent and bending moment. Results suggest that 33- to 55-kg 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 50-kg 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 120-kg 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.

Key Words: Growth Performance • Pigs • Phosphorus

	Introduction

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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 P-based 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 the partial (66%, Mavromichalis et al., 1999; Shaw et al., 2002) or complete removal of supplemental P from the diet (Lindemann et al., 1995; O’Quinn et al., 1997; McGlone, 2000). However, swine nutrition research trials conducted at universities typically involve relatively few pigs per pen and generous space allowances. As a result of these conditions, finishing pig growth rate and feed intake will often exceed that of pigs housed in commercial facilities. Although direct comparisons should be made with caution, pigs housed in a commercial research facility (1,200-animal barn, with 25 pigs per pen and 0.67 m² per pig) had approximately 33% lower ADFI than those fed similar diets in a university research facility (160-animal barn, with 10 pigs per pen and 0.88 m² per pig, De La Llata et al., 2002).

Because research on the aP requirements of swine primarily has been conducted in university or experiment station settings, and these environments often result in greater feed intake in pigs than is observed in commercial environments, results of these experiments may have underestimated the aP requirements expressed as a percentage of the diet. Therefore, the purpose of these experiments was to estimate the aP requirements in a commercial environment.

	Materials and Methods

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General
Procedures used in these experiments were approved by the Kansas State University Animal Care and Use Committee. All three trials were conducted at a commercial research facility in southwestern Minnesota. The facility is made up of four individual barns, each 12.5 x 76.2 m, with 48 3.05- x 5.49-m pens. Each curtain-sided barn has a deep pit with completely slatted floors and operates on natural ventilation during the summer and mechanically assisted ventilation during the winter. Pens of pigs (Line C22 x 337 gilts, PIC, Franklin, KY) were weighed and allotted randomly to dietary treatments in a randomized complete block design. Each pen contained one four-hole dry self-feeder (Staco, Schaefferstown, PA) and one cup waterer to allow ad libitum access to feed and water. Pen and feeder weights were measured approximately every 14 d to calculate ADG, ADFI, and G:F. Before starting experimental diets, pigs were fed a diet containing 0.40% aP in the preliminary study and Exp. 1, and 0.27% aP in Exp. 2. All diets were formulated using NRC (1998) nutrient composition values for the respective ingredients. Available P values were also calculated using NRC (1998) estimates. Samples of the individual diets were collected and analyzed for CP (AOAC, 1995). Dietary Ca and P were determined using inductively coupled plasma emission spectroscopy with a Fisons ARL model 3410 (Ecublens, Switzerland; AOAC, 1995 Method 985.01).

Before conducting the titration studies, we conducted a preliminary trial to try to narrow the wide range of aP estimates used in commercial production. Results of the preliminary trial would allow us to decrease the interval in aP concentrations between treatments and potentially to improve the accuracy of determining a requirement estimate. A total of 600 gilts with an initial weight of 43.2 kg was blocked by weight and randomly allotted to low (0.30%) or high (0.37%) dietary aP treatments from 43 to 48 kg BW, and later to 0.19 or 0.27% aP from 111 to 121 kg BW. There were 25 pigs per pen and 12 pens per treatment. All diets were corn–soybean meal-based and contained 6% choice white grease. Varying the amounts of monocalcium phosphate and limestone attained the desired levels of Ca and P in the diets. A constant Ca:total P ratio of 1.1:1 was maintained in all diets (NRC, 1998). The differences in aP between the high and low regimen represented the variation in recommendations proposed by swine breeding stock companies and nutritionists for commercial production in the United States.

Experiment 1
A total of 1,260 gilts with an initial weight of 33.8 kg was blocked by weight and randomly allotted to one of five dietary treatments in a 26-d experiment. The corn–soybean meal-based diets contained 6% choice white grease and were formulated to 1.25% total lysine. Treatments consisted of five levels of aP; 0.18, 0.22, 0.25, 0.29, or 0.32%, which corresponded to 0.5, 0.6, 0.7, 0.8, or 0.9 g of aP/Mcal of ME (Table 1). These concentrations were selected based on results of the preliminary study. There were 28 pigs per pen and nine pens per treatment. A constant Ca:total P ratio of 1.1:1 was maintained in all diets (NRC, 1998). Varying the amounts of monocalcium phosphate and limestone attained the desired levels of Ca and P in the diets.

Ribs were removed from the freezer and immediately cleaned of all connective tissue while still frozen. The three ribs were separated, labeled, and stored in individual plastic bags at –12°C. The sixth rib was used to obtain all bone data response variables.

The right rear legs were removed from the freezer and allowed to partially thaw for 12 h at 7°C. Legs were dissected to obtain the right femur and third and fourth metatarsals, which were then manually cleaned of connective tissue. The bones were labeled, placed in plastic bags, and stored in a freezer at –12°C.

Experiment 2
A total of 1,239 gilts with an initial weight of 88.5 kg was blocked by weight and randomly allotted to one of five dietary treatments in a 28-d experiment. Pigs were fed diets with 0.05, 0.10, 0.14, 0.19, or 0.23% aP, which corresponds to 0.14, 0.28, 0.39, 0.53 or 0.64 g of aP/Mcal (Table 2). These concentrations were again selected based on results of the preliminary trial. There were 27 or 28 pigs per pen and nine pens per treatment. A constant Ca:total P ratio of 1.1:1 was maintained in all diets. Varying the amounts of monocalcium phosphate and limestone attained the desired levels of Ca and P in the diets.

Bone Analyses
Bones were removed from the freezer and placed in a cooler at 7°C for 12 h, and then removed and allowed to thaw at room temperature (24°C) in plastic bags for 24 h. Bones were then measured for bending moment using a three-point flexure test (Crenshaw et al., 1981) with force applied by an Instron Universal Testing Machine (model 4201, Instron Corp., Canton MA). Cross-head speed was 100 mm/min. Ribs (Exp. 1), metatarsals (Exp. 1), and metacarpals (Exp. 2) were oriented to the crosshead such that the force applied was medial-lateral, whereas the femurs (Exp. 1) were oriented such that force was applied dorsal-ventral. The distance between the two fulcra points for metatarsals, metacarpals, and ribs was 2 cm, whereas the bridge or fulcra for femurs was 4 cm.

After analysis, bones were cut in half with a model 5215 Hobart meat saw (Hobart Corp., Troy, OH) with a blade that was 0.32 cm thick. Bones were then placed in petroleum ether for 7 d, and then dried for 12 h at 105°C three times to determine the absolute dry, fat-free weight. Bones were then ashed at 600°C for 24 h to determine percentage of ash. Ash is expressed as a percentage of dried, fat-free bone weight.

Statistical Analyses
Treatments were arranged in a randomized complete block design. Analysis of variance was conducted on all data using the PROC MIXED procedure of SAS (Version 8.01, SAS Inst., Inc., Cary, NC), with a Kenward and Roger error correction for degrees of freedom. Pen was used as the experimental unit of analysis for all treatment effects. For growth performance, the statistical model included treatment as a fixed effect and block as a random effect. A repeated-measures analysis was used to analyze the bone criteria (Littell et al., 1996). In Exp. 1, the model included the fixed effects of treatment x bone interaction, with the random effects of block and repeated measures of bone within pig (pen). Because one pig per pen was sampled in Exp. 1, the individual pig represented the pen mean. A similar statistical model was used for Exp. 2, with the exception that the repeated measure was bone within pig within pen. Linear and quadratic orthogonal polynomial contrasts (Peterson, 1985) were used to further characterize treatment effects. Breakpoint analysis was conducted according to Robbins (1986).

	Results and Discussion

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In the preliminary study (data not reported), there were no differences observed (P = 0.10 to 0.81) in ADG or ADFI. Pigs fed the low-aP diets had improved (P = 0.07) G:F compared with those fed the high-aP diets from 43 to 48 kg, but there were no differences (P = 0.46) in G:F from 111 to 121 kg BW. These results suggest that the aP requirement necessary for maximum growth for these pigs was at or above the concentrations in the low-aP diets. These values (with minor extrapolation for differences in initial BW) were used to establish our highest aP concentrations in the two subsequent titration studies with 33- to 55- and 88- to 109-kg pigs. We also expanded our response criteria to include bone mechanical properties because, typically, aP requirements to maximize bone strength are greater than those required to maximize growth (Crenshaw et al., 1981).

In Exp. 1, from d 0 to 14, increasing aP increased (linear, P = 0.03) ADG and tended to increase (quadratic, P = 0.07) G:F (Table 3). The greatest improvement in both ADG and G:F was observed as aP increased from 0.18 to 0.22% of the diet (G:F breakpoint of 0.22%), corresponding with intakes of 2.70 and 3.21 g of aP/d. However, from d 14 to 26 and for the overall study, no differences were observed (P = 0.12 to 0.81) in ADG, ADFI, or G:F. Although not different, numerical trends similar to those observed from d 0 to 14 were observed for overall ADG and G:F as aP increased from 0.18 to 0.22% corresponding to 2.74 and 3.30 g of aP/d.

There were no individual bone x treatment interactions for bone criteria. Rib and femur bending moment increased (quadratic P = 0.03 and linear P = 0.01, respectively) with increasing aP (Table 4). However, increasing aP had no effect (P = 0.18 to 0.82) on metatarsal bending moment. The percentage of bone ash increased (linear, P = 0.01) with increasing aP in the fourth metatarsal, but not in the third metatarsal or rib. Femurs were only evaluated for bending moment. Based on the repeated-measures analysis, the main effect of dietary aP was significant, with increasing aP increasing (linear, P = 0.007) bending moment, but the percentage of bone ash was not affected.

In Exp. 2, from d 0 to 14, increasing aP increased (linear, P = 0.008 to 0.02) ADG and G:F (Table 5). Although the response in ADG and G:F to increasing aP was linear, the greatest ADG and G:F was observed in pigs fed 0.19% aP, corresponding to 3.86 g aP/d intake. Average daily feed intake tended to increase (quadratic, P = 0.10), with the greatest increase observed as aP increased from 0.05 to 0.10% aP. From d 14 to 28 and from d 0 to 28, no differences (P = 0.17 to 0.93) were observed for ADG, ADFI, or G:F. As in Exp. 1, we believe the d-0 to -14 data to be a more accurate estimate of the pig’s aP for the 28-d study because of the potential for P mobilization from bone tissue to meet requirements for growth.

In previous studies (Lindemann et al., 1995; O’Quinn et al., 1997; Mavromichalis et al., 1999; McGlone, 2000; Shaw et al., 2002) where a complete or up to 66% removal of inorganic P had no affect on pig growth performance, feed intake was much higher than that observed in our study. Therefore, those pigs were able to meet or exceed their aP requirements on a grams-per-day basis despite the low percentage of aP in the diet. Factors influencing feed intake, such as housing, environment, stocking density, and diet, need to be evaluated before adapting a requirement estimate to a particular production system.

Cera and Mahan (1988) suggest that previous levels of dietary Ca and P fortification are likely to affect the requirements of pigs in late finishing. Pigs in our studies were fed adequate levels of Ca and P before being fed their respective experimental diets. Because of this, responses in these current studies may be different than responses in pigs fed Ca and P deficient diets for the entire growing-finishing period.

Another consideration in interpreting our results is the Ca:P ratio used. A wide range of total Ca to total P has been shown to decrease P absorption (Qian et al., 1996; Liu et al., 2000). Thus, pigs fed diets with wide Ca:P ratios may show signs of P deficiency even though adequate P is provided. Estimates by NRC (1998) suggest that corn-soybean meal-based diets should be between 1:1 and 1:1.25 Ca:P.

Several factors can influence ADFI, and because pigs studied in university research conditions generally consume more feed than those raised in commercial environments, there may be differences in nutrient requirement estimates when expressed as a percentage or on a g/d basis. If percentage requirements for pigs are based on research from university environments, they may underestimate requirements because of the low feed intake in commercial facilities. Differences in the energy density of the diets between studies can also occur (i.e., use of added dietary fat), which may also influence ADFI (De La Llata et al., 2001). Therefore, expressing aP as a ratio of grams of aP required per unit of dietary energy will more accurately match the grams of aP per day requirement of pigs raised in commercial facilities.

	Implications

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Phosphorus requirements of commercially reared pigs in these studies are similar on a percentage basis when compared with the current requirements suggested by the National Research Council. However, these estimates are lower than National Research Council estimates on a grams-per-day basis. From 33 to 55 kg of body weight, pigs raised in commercial facilities require 0.22% available phosphorus corresponding to 3.30 g of available phosphorus per day. From 88 to 109 kg of body weight, pigs raised in commercial facilities require at least 0.19% available phosphorus, corresponding to 4.07 g of available phosphorus per day. These data also suggest that complete removal of supplemental phosphorus in late finishing diets will result in decreased growth performance.

	Footnotes

 
¹ Contribution No. 03-22-J from the Kansas Agric. Exp. Stn., Manhattan.

² Appreciation is expressed to PIC USA, Franklin, KY, for partial financial support as well as New Horizons Farm, Inc. Pipestone, MN, for use of their facilities and animals.

⁴ Present address: The Hanor Co., Spring Green, WI 53588.

³ Correspondence: 242 Weber Hall (phone: 785-532-1228; fax: 785-532-7059; e-mail: Goodband{at}ksu.edu).

Received for publication February 5, 2003. Accepted for publication June 2, 2004.

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