Effects of Analyzed Calcium to Phosphorus Ratio on Growth Performance, Carcass Characteristics, Bone Mineralization, and Economics of 56- to 279-lb Pigs Fed Diets Containing Phytase

A total of 1,215 barrows and gilts (PIC; 359 × Camborough) with an initial pen average body weight (BW) of 55.7 ± 2.06 lb were used in a 114-d growth trial to determine the effects of different analyzed calcium to analyzed phosphorus ratios on performance of growing-finishing pigs from 57to 279-lb fed diets containing 1,000 phytase units. Pens of pigs were randomly assigned to 1 of 5 dietary treatments in a randomized complete block design with BW as a blocking factor. There were 9 replicate pens per treatment and 27 pigs per pen. The experimental diets were corn-soybean meal-based and were fed in 4 phases. The 5 dietary treatments were formulated to contain 0.75:1, 1.00:1, 1.25:1, 1.50:1, and 2.00:1 analyzed Ca:P ratios. The diets contained 1,000 phytase units (FYT) of Ronozyme Hiphos 2500 (DSM Nutritional Products, Inc., Parsippany, NJ) with assumed releasing values of 0.15% available P (aP) and 0.132% standardized total tract digestible P (STTD P). All the diets were formulated to contain adequate STTD P across the dietary treatments in all phases, which included the expected release of phytase. The treatments were achieved by increasing the amount of calcium carbonate at the expense of corn while maintaining monocalcium phosphate constant. Overall, increasing analyzed Ca:P ratio quadratically increased (P < 0.05) average daily gain (ADG), final BW, and average daily feed intake (ADFI) (P < 0.10). The greatest increase in these criteria was observed as the ratio increased from 0.75:1 to 1.50:1, with no improvement thereafter. Feed efficiency improved linearly (P < 0.05) with increasing analyzed Ca:P ratio up to 2.00:1. For carcass characteristics, hot carcass weight (HCW) increased quadratically (P < 0.05) as the analyzed Ca:P ratio increased from 0.75:1 to 1.50:1, and started to decrease thereafter. Percentage carcass yield decreased (quadratic, P < 0.10) from 0.75:1 analyzed Ca:P ratio to 1.25:1, slightly increasing at higher ratios. Bone mineralization increased quadratically (P < 0.05) with increasing analyzed Ca:P ratio. The greatest improvement in percentage bone ash was observed as analyzed Ca:P ratio increased from 0.75:1 to 1.25:1, with no further increase. Feed cost per pig increased linearly (P < 0.05) with increasing analyzed Ca:P ratio. No evidence of differences (P > 0.10) was observed for feed cost per pound of gain. Gain value per pig increased quadratically (P < 0.05), with the greatest revenue observed for pigs fed diets with 1.50:1 analyzed Ca:P ratio. There was a marginal quadratic improvement (P < 0.10) in income over feed cost (IOFC), with the highest income observed at 1.25:1 analyzed Ca:P ratio. The best fitting models for ADG, ADFI, feed efficiency, HCW, and bone ash were the quadratic polynomial (QP), linear, broken-line linear (BLL), QP, and BLL models, respectively. The maximum responses in ADG, feed efficiency, HCW, and bone ash were estimated at 1.63:1, 1.05:1, 1.11:1, and 1.25:1 analyzed Ca:P ratio, respectively. In conclusion, the estimated analyzed Ca:P ratio requirement for finishing pigs from 56to 279-lb fed diets containing 1,000 phytase units and that were adequate in STTD P ranged from 1.05:1 to 1.63:1 to maximize growth rate, feed efficiency, HCW, and bone mineralization criteria.


Introduction
Among other functions, calcium and phosphorus play an essential role in growth performance and bone mineralization. However, the amount of one mineral can affect the utilization of the other. 5 Moreover, recent studies have demonstrated that feeding excess Ca can result in reduced growth performance and bone mineralization, 6 particularly when pigs are fed diets with low or marginal P. 7,8 Thus, it is important to consider an optimum Ca:P ratio when formulating diets for pigs to ensure an appropriate absorption and utilization of both minerals. However, there is still a need for research to determine the optimum Ca:P ratio to maximize different response criteria. A recent study 9 has shown that the total analyzed Ca:P ratio to maximize growth performance, HCW, and IOFC criteria for growing-finishing pigs from 53 to 287 lb ranged from 1.10:1 to 1.49:1. In addition, a higher analyzed Ca:P ratio, estimated at 1.93:1, was required to maximize bone mineralization. These ratios were achieved with the inclusion of monocalcium P, which is an expensive source of P. In addition, swine diets are formulated with cereal grains that contain a large amount of P in the form of phytic acid. However, this is an unavailable form of P to pigs as they lack endogenous phytase to cleave the phosphates from the phytic acid. Thus, to decrease the inclusion of inorganic sources of P and to increase the P availability to the pig, phytase is typically added to swine diets. However, there is little research that defines the optimum Ca:P ratio in diets that contain phytase. Therefore, the objective of this study was to investigate the effects of different Ca:P ratios on growth performance, carcass characteristics, bone mineralization, and economics of growing-finishing pigs fed diets containing 1,000 phytase units that are adequate in STTD P when considering the P release from the phytase.

Procedures
The Kansas State University Institutional Animal Care and Use Committee approved the protocol used in this experiment. This experiment was conducted at a commercial research-finishing site in southwestern Minnesota. The barn was naturally ventilated and double-curtain-sided. Pens had completely slatted flooring and deep pits. Each pen was equipped with a 5-hole stainless steel feeder and cup waterer to allow ad libitum access to feed and water. The facility was equipped with a computerized feeding system (FeedPro; Feedlogic Corp., Willmar, MN) capable of measuring and recording daily feed additions to individual pens.
A total of 1,215 barrows and gilts (PIC; 359 × Camborough, initial pen average BW of 55.7 ± 2.06 lb) were used in a 114-d growth trial. At placement (35 lb), pigs were fed a common diet containing 0.66% total Ca and 0.42% STTD P until the initiation of the trial. On d 0, pens of pigs were weighed and ranked by average BW. Pens were then randomly assigned to 1 of 5 dietary treatments in a randomized complete block design, with BW used as a blocking factor. There were 27 pigs per pen and 9 replicate pens per treatment. The 5 dietary treatments were formulated to contain 0.75:1, 1.00:1, 1.25:1, 1.50:1, and 2.00:1 analyzed Ca:analyzed P ratio. The analyzed Ca or P was defined as that calculated based on ingredient analysis. A total of 3 samples of all the ingredients used in the diets that contained Ca and P were analyzed for Ca and P in duplicate (Ward Laboratories, Inc., Kearney, NE; Table  1). The average of the six lab results for each ingredient was used for diet formulation. The experimental diets were corn-soybean meal-based, and fed in 4 different phases (Tables 2, 3, 4, and 5). Phase 1 diets were fed from d 0 to 25 (55.7 to 98.4 lb); phase 2 diets were fed from d 26 to 58 (98.5 to 164.0 lb); phase 3 diets were fed from d 59 to 87 (164.1 to 227.1 lb); and phase 4 diets were fed from d 88 to 114 (227.2 to 279 lb). The diets contained 1,000 phytase units (FYT) of Ronozyme Hiphos 2500 (DSM Nutritional Products, Inc., Parsippany, NJ) with assumed release values of 0.15% available P and 0.132% STTD P. The diets were formulated to contain adequate STTD P across the dietary treatments in all phases based on the estimated requirement previously determined in this facility. 9 Thus, formulated STTD P levels were 0.38, 0.33, 0.29, and 0.25% for phases 1, 2, 3, and 4, respectively. These STTD P concentrations included the expected phytase release of 0.132% STTD P. The analyzed total Ca:P ratios to form the Swine Day 2018 dietary treatments were achieved by increasing the amount of calcium carbonate at the expense of corn while maintaining monocalcium phosphate constant.
Experimental diets were manufactured at the New Horizon Farms Feed Mill (Pipestone, MN) and fed in meal form. Representative samples of treatment diets were taken from 6 feeders per dietary treatment 3 d after the beginning and 3 d before the end of each phase and stored at -4°F. After blending, 6 subsamples were analyzed in duplicates for dry matter (DM), crude protein (CP), crude fiber, ash, ether extract, Ca, and P and average values were reported (Tables 6 and 7; Ward Laboratories, Inc., Kearney, NE, and Midwest Laboratories, Omaha, NE).
Pens of pigs were weighed, and feed disappearance was recorded approximately every 14 d to determine ADG, ADFI, and feed-to-gain ratio (F/G). On d 99, the 2 heaviest pigs in each pen were selected, weighed, and sold according to standard farm procedures. On d 114, final pen weights were taken and one barrow and one gilt with intermediate weights were selected, tattooed with a pen ID, and marketed for bone data collection. These pigs were transported to a commercial abattoir in northwest Iowa (Natural Foods, Sioux Center, IA) for processing and collection of metacarpal bone samples for analysis of their mineral content. The remaining pigs were individually tattooed with the specific pen identity on the shoulder to allow carcass measurements to be recorded on a pen basis. These pigs were transported to a commercial packing plant in southwestern Minnesota (JBS Swift and Company, Worthington, MN) for processing and carcass data collection. Carcass measurements included HCW, loin depth, backfat depth, and percentage lean. Percentage carcass yield was calculated by dividing the average pen HCW by the average final live weight at the farm.
Bones were collected following processing for mineralization analysis. The left front feet were separated at the junction of carpals and radius and ulna, and individually placed in a plastic bag with a permanent identification tag within the bag, then stored at -20°C. Feet were thawed overnight, and then were autoclaved for 1 h at 121°C. The third metacarcapal of each foot was removed. These bones were cleaned of extraneous soft tissue, and refrozen. The third metacarpal was dried at ambient temperature for 24 h, cut in half, and weighed. They were wrapped in cheesecloth to keep their tag ID, and defatted by petroleum ether using a Soxhlet apparatus for 7 d. Defatted metacarpals were placed in a 105°C drying oven for another 7 d to determine the dry fat-free weight. Bones were then ashed in a muffle furnace at 600°C for 24 h to determine percentage ash. Ash is expressed as a percentage of dried fat-free bone weight.
For the economic analysis, total feed cost per pig, cost per lb of gain, gain value, and income over feed cost (IOFC) were calculated. The total feed cost per pig was calculated by multiplying ADFI by feed cost per pound and number of days the diet was fed in each respective period, then taking the sum of these values for each period. Cost per lb of gain was calculated by dividing total feed cost per pig by total gain per pig. Gain value per pig was calculated by multiplying carcass gain by an assumed carcass value of $58.35 per cwt. To calculate IOFC, total feed cost was subtracted from gain value. For all economic evaluations, prices of ingredients during spring of 2018 were used with corn valued at $3.55/bu ($127/ton), soybean meal at $373/ton, L-lysine HCL at $0.74/lb, DL-methionine at $1.40/lb, L-threonine at $1.05/lb, L-tryptophan at $9.00/lb, Rono-Swine Day 2018 zyme Hiphos 2500 at $0.77/lb, monocalcium phosphate at $0.29/lb, and calcium carbonate at $0.02/lb.
Growth performance data were analyzed as a randomized complete block design, with pen considered the experimental unit and BW the blocking factor. The study was structured as a split-plot design in a randomized complete block design for the bone data. The whole-plot treatments included the different total Ca:P ratios. Within each of the dietary treatments, there was a one-way treatment structure with gender as the factor level. A random effect of block by treatment was used to identify the pair of pigs (one barrow and one gilt) within each pen as the experimental unit for gender. The two-way interaction between dietary treatments and gender was tested, and no significant interactions were observed. Response variables were analyzed using generalized linear and non-linear mixed models. Polynomial contrasts were implemented to evaluate the functional form of the dose response to increasing analyzed Ca:P on growth performance, carcass characteristics, and economic response variables. Statistical models were fitted using GLIMMIX procedure of SAS (Version 9.3, SAS Institute Inc., Cary, NC). Results were considered significant at P ≤ 0.05 and marginally significant at 0.05 ≤ P ≤ 0.10.
In addition, the effects of analyzed Ca:analyzed P dose response on ADG, ADFI, feed efficiency (modeled as gain to feed, G:F), HCW, and percentage bone ash were fit using GLIMMIX and NLMIXED procedure of SAS according to Gonçalves et al. 10 Models were expanded to account for heterogeneous residual variances when needed. Competing statistical models included a linear (LM), quadratic polynomial (QP), broken-line linear (BLL), and broken-line quadratic (BLQ). Dose response models were compared based on the Bayesian information criterion (BIC), where the smaller the value, the better. 11 A decrease in BIC greater than 2 was considered a significant improvement in fit. The 95% confidence interval of the estimated requirement to reach maximum performance or to reach plateau performance was computed. Results reported correspond to inferences yielded by the best fitting models.

Results and Discussion
Analysis of DM, CP, crude fiber, fat, and ash contents of experimental diets (Tables 6  and 7) showed that values were reasonably consistent with formulated estimates. Average values of analyzed Ca and P concentrations slightly differed from the formulated values; however, they followed similar patterns as the designed treatment structure. The average Ca:P ratio across the four phases were 0.78:1, 0.97:1, 1.32:1, 1.47:1, and 2.06:1 for the 0.75:1, 1.00:1, 1.25, 1.50, and 2.00:1 treatments, respectively.
During the grower period, which corresponds to phases 1 and 2 (d 0 to 58), there was a marginal quadratic increase (P < 0.10) in ADG, with the greatest gain observed at 1.50:1 analyzed Ca:P ratio and no improvements thereafter (Table 8) there was no evidence (P > 0.10) of differences in ADFI and F/G due to increasing the analyzed Ca:P ratio from 0.75:1 to 2.00:1. During the finisher period, which corresponds to phases 3 and 4 (d 59 to 114), increasing analyzed Ca:P ratio resulted in an increase (quadratic, P < 0.05) in ADG driven by a quadratic increase (P < 0.05) in ADFI. The greatest improvement in ADG occurred as the analyzed Ca:P ratio increased from 0.75:1 to 1.25:1, with little increase thereafter. The greatest improvement in ADFI was observed as the analyzed Ca:P ratio increased from 0.75:1 to 1.50:1, with no further improvement. Feed efficiency improved linearly (P < 0.05) as the analyzed Ca:P ratio increased up to the highest ratio of 2.00:1.
Overall, increasing analyzed Ca:P ratio quadratically increased (P < 0.05) ADG and final BW. The greatest increase in both criteria was observed as the ratio increased from 0.75:1 to 1.50:1, with no improvement thereafter. Similarly, there was a marginal quadratic increase (P < 0.10) in ADFI, with the greatest intake observed at an analyzed Ca:P ratio of 1.50:1. Feed efficiency improved linearly (P < 0.05) with increasing the analyzed Ca:P ratio from 0.75:1 to 2.00:1.
For carcass characteristics, HCW increased quadratically (P < 0.05) as the analyzed Ca:P ratio increased up to 1.50:1, and started to decrease thereafter. Percentage carcass yield decreased (quadratic, P < 0.10) from 0.75:1 analyzed Ca:P ratio to 1.25:1, slightly increasing at higher ratios. No evidence of differences (P > 0.10) was observed for backfat depth, fat-free lean, and loin depth measurements.
Feed cost per pig increased linearly (P < 0.05) with increasing analyzed Ca:P ratio up to the highest ratio at 2.00:1. No evidence of difference (P > 0.10) was observed for feed cost per pound of gain. Gain value per pig increased (quadratic, P < 0.05) with increasing analyzed Ca:P ratio. The greatest revenue was observed for pigs fed diets with 1.50:1 analyzed Ca:P ratio, which is a result of the quadratic improvement in ADG and final BW. The increased gain value was sufficient to overcome the increased feed cost, resulting in a marginal quadratic improvement (P < 0.10) in IOFC, with the greatest income observed at 1.25:1 analyzed Ca:P ratio.
For bone mineralization, the two-way interaction between dietary treatment and gender was tested and no evidence (P > 0.10) for significant interaction was observed. There was a marginal significant gender effect (P < 0.10) on percentage bone ash, with barrows having greater bone mineralization than gilts (61.55 and 61.30% for barrows and gilts, respectively). Bone mineralization increased quadratically (P < 0.05) with increasing analyzed Ca:P ratio (Table 8). The greatest improvement in percentage bone ash was observed as analyzed Ca:P ratio increased from 0.75:1 to 1.25:1, with no further increase.
Increasing analyzed Ca:P ratio in the diets quadratically improved ADG, ADFI, final BW, HCW, carcass yield, and bone ash. Although feed cost increased with increasing analyzed Ca:P ratios, the improved value in gain was sufficient to overcome the increase in feed cost. Thus, the return over the feed cost increased quadratically with increasing analyzed Ca:P ratio. In conclusion, the estimated analyzed Ca:P ratio requirement for finishing pigs from 56-to 279-lb fed diets containing 1,000 FYT of phytase and that were adequate in STTD P ranged from 1.05:1 to 1.63:1 to maximize growth rate, feed efficiency, HCW, and bone mineralization criteria.           Income over feed cost = gain value -feed cost. 11 90 pigs (2 pigs/pen, 1 barrow/1 gilt) visually assumed to represent the mean live weight of the pen were subsampled and shipped to a separate processing facility for bone collection (Natural Foods Holdings, Inc., Sioux Center, IA).

12
A total of 90 third metacarpals were autoclaved for 1 h. After cleaning, bones were placed in Soxhlets containing petroleum ether for 7 d as a means of removing water and fat. They were then dried at 105°C for 7 d, and then ashed at 600°C for 24 h. 13 Adjusted for HCW. 14 The two-way interaction was tested and no evidence for significant interaction was observed. There was a marginal significant gender effect (P < 0.10) on percentage bone ash, with barrows having greater bone mineralization than gilts (61.55 and 61.30% for barrows and gilts, respectively).