Effects of Dietary Total Calcium to Total Phosphorus Ratio on Growth Performance, Carcass Characteristics, Bone Mineralization, and Economics in 58- to 281-lb Pigs

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Introduction
Calcium is the most abundant mineral in the body followed by phosphorus . These minerals are involved in many physiological functions such as protein synthesis, maintenance of osmotic and acid-base balances, components in membranes, and bone development and mineralization. Additional roles of Ca and P within the body include muscle contraction, transmission of nerve impulses, enzyme activation, and metabolic reactions. Moreover, an excess or deficiency of one mineral may affect the utilization of the other. 5 To have an adequate absorption and utilization of both Ca and P, it is necessary to consider an appropriate Ca:P ratio when formulating diets for pigs. Research has shown that feeding excess Ca can result in reduced growth performance and bone mineralization, 6 especially when diets are marginally deficient in P. 7,8 A recent study has determined the requirement of standardized total tract digestible (STTD) P of growing-finishing pigs. 9 Therefore, the objective of the present study was to investigate the effects of feeding different Ca:P ratios in diets adequate in STTD P on growth performance, carcass characteristics, bone mineralization, and economics of growingfinishing pigs housed in a commercial environment.

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 1,134 barrows and gilts (PIC; 359 × Camborough, initial pen average BW of 58.0 ± 1.57 lb) were used in a 110-d growth trial. Upon arrival (38 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, pigs were weighed in pens, and pens were ranked by average pig BW. Pens were then randomly allotted to 1 of 6 dietary treatments in a randomized complete block design, with BW as a blocking factor. There were 7 replicate pens per treatment and 27 pigs per pen.
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). 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. Phase 1 diets were fed from d 0 to 28 (58.0 to 110.6 lb); phase 2 diets were fed from d 29 to 56 (110.6 to 172.4 lb); phase 3 diets were fed from d 57 to 85 (172.4 to 237.7 lb); and phase 4 diets were fed from d 86 to 110 (237.7 to 281.4 lb). The analyzed total Ca:P ratios 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 on d 0, 28, 56, 85, and 110 to determine ADG, ADFI, and F/G. On d 86, the 2 heaviest pigs in each pen were selected, weighed, and sold according to standard farm procedures. On d 110, 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 for 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. Carcass ADG was calculated by multiplying the overall ADG by percentage carcass yield. Carcass F/G was calculated by dividing the overall ADFI by carcass ADG.
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 zip-lock 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 were removed. These bones were cleaned of extraneous soft tissue, and refrozen. When it was time to process, bones were 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. De-fatted 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, 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 analyzed 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 dietary total 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.

Swine Day 2018
In addition, the effects of analyzed total Ca:P dose response on ADG, feed efficiency (modeled as gain to feed, G:F), 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 were slightly greater than the formulated values; however, they followed similar patterns as the designed treatment structure. The average Ca:P ratio across the four phases were 77, 102, 125, 155, 177, and 207% for the 75, 100, 125, 150, 175, and 200% treatments, respectively.
During the grower period, which corresponds to phases 1 and 2 (d 0 to 56), there was no evidence of differences (P > 0.05) in ADG as analyzed total Ca:P ratio increased (Table 8). On the other hand, increasing the analyzed total Ca:P ratio resulted in a marginal linear increase (P < 0.10) in ADFI, and in a quadratic change (P < 0.05) in F/G. The greatest improvement in feed efficiency occurred as the analyzed total Ca:P ratio increased from 0.75:1 to 1.00:1, and then it worsens at the highest ratio of 2.00:1 analyzed total Ca:P.
During the finisher period, which corresponds to phases 3 and 4 (d 56 to 110), increasing analyzed total Ca:P ratio resulted in an increase (P < 0.05) in ADG driven by an increase (P < 0.05) in ADFI, both in a quadratic manner. The greatest improvement occurred as the analyzed total Ca:P ratio increased from 0.75:1 to 1.25:1 for ADG and from 0.75:1 to 1.00:1 for ADFI, with no improvement thereafter. In contrast, F/G was not affected (P > 0.05) by the dietary treatment. Overall, increasing the analyzed total Ca:P ratio increased (quadratic, P < 0.05) ADG, ADFI, and final BW. The greatest increase was observed as the ratio increased from 0.75:1 to approximately 1.25:1, with no further improvement. Similarly, feed efficiency was relatively flat from an analyzed total Ca:P ratio of 0.75:1 to 1.75:1, starting to worsen (quadratic, P < 0.05) at the highest ratio of 2.00:1.
For carcass characteristics, HCW increased (quadratic, P < 0.05) as the analyzed total Ca:P ratio increased up to 1.25:1, and started to decrease thereafter. Similarly, carcass ADG increased quadratically (P < 0.05), with the greatest response observed at 1.25:1 analyzed total Ca:P ratio. Percentage carcass yield decreased (quadratic, P < 0.05) from 0.75:1 analyzed total Ca:P ratio to 1.50:1, with no further changes. There was also a marginally significant response (quadratic, P < 0.10) in loin depth, with the greatest improvement occurring up to 1.50:1 analyzed total Ca:P ratio. No statistically significant differences (P > 0.10) were observed for carcass F/G, backfat, and fat-free lean measurements.
Feed cost per pig increased (quadratic, P < 0.05) with increasing analyzed total Ca:P ratio, with the highest feed cost observed at 2.00:1. Feed cost per pound of gain increased linearly (P < 0.05) up to the highest analyzed total Ca:P ratio. On the contrary, gain value per pig increased (quadratic, P < 0.05) with the greatest revenue observed for pigs fed diets with 1.25:1 analyzed total 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 cost, resulting in a quadratic improvement (P < 0.05) in IOFC, with the highest income observed at 1.00:1 analyzed total 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 (Table 8). There was also no evidence (P > 0.10) for a significant gender effect on percentage bone ash (62.09 and 61.98% for barrows and gilts, respectively). Bone mineralization increased quadratically (P < 0.05) with increasing analyzed total Ca:P ratio. The greatest improvement in percentage bone ash was observed as analyzed total Ca:P ratio increased from 0.75:1 to 1.25:1, with little increase thereafter.
Homogeneous variance was used for ADG models and heterogeneous variance was used for ADFI, feed efficiency, HCW, IOFC, and bone ash models. The best fitting model for all these response criteria was the QP (Figures 1, 2, 3, 4, 5, and 6, respectively). The analyzed total Ca:P ratio for maximum ADG was estimated at 1. Increasing the analyzed total Ca:P ratio quadratically improved ADG, ADFI, final BW, HCW, as well as carcass ADG, yield, loin depth, and bone ash. Although feed Swine Day 2018 cost increased with increasing analyzed total Ca:P ratios, the improved value in gain was sufficient to overcome the increase in diet cost. Thus, the return over the feed cost increased quadratically with increasing analyzed total Ca:P ratio. In conclusion, the estimated total Ca:P ratio requirement for finishing pigs from 58 to 281 lb when fed diets adequate in STTD P ranged from 1.10:1 to 1.49:1 to maximize growth performance, HCW, and IOFC criteria. To maximize bone mineralization, a higher analyzed total Ca:P ratio was required, which was estimated at 1.93:1.