Available phosphorus for 15-to 30-kg pigs kept in hot environment 1

This experiment was conducted to determine the requirement of available phosphorus (aP) for pigs kept in hot environment. Seventy-two pigs (36 castrated males and 36 females) with initial weight of 15.10±0.31 kg were allotted in a completely randomized block design with six treatments (0.107, 0.214, 0.321, 0.428, 0.535, and 0.642% of aP), six replicates and two pigs (1 castrated male and 1 female) per experimental unit. Pigs were kept in a hot environment with temperature of 34.1±0.8 oC. The levels of aP influenced the daily weight gain, which increased quadratically up to the estimated level of 0.477%, and feed conversion, which improved quadratically up to the estimated level of 0.457%. The levels of aP also influenced the content of phosphorus in the bone, which increased quadratically up to the estimated level of 0.529%. The available phosphorus requirement of 15to 30-kg pigs kept in a hot environment, for the best results of daily weight gain, feed conversion and bone parameters are 0.477, 0.457, and 0.529%, corresponding to the estimated daily available phosphorus intakes of 4.75; 4.55 and 5.27 g, respectively.


Introduction
The performance of pigs is influenced, among other factors, by the thermal environment in which they are reared.The poor performance observed in heat-stressed pigs is due to the reduction in feed intake that becomes lower with the rise of the air temperature (Christon, 1988).
From a nutritional point of view, inadequate feed intake results in inadequate intake of nutrients by the pig, having a direct impact on the rate and chemical composition of body gain (Quiniou et al., 2000).In this sense, identifying the need for nutrients in hot condition will enable to formulate diets that provide the adequate amount of nutrients consistent with the feed intake capacity of pigs in this environmental condition.
A nutrient whose requirement was found to be different in pigs with high genetic potential is phosphorus (Saraiva et al., 2009).Phosphorus is an abundant mineral in the animal body, and the bone tissue has the most of it (80.0%).The other 20.0% is found distributed among the soft tissues and body fluids where it performs many essential bodily functions (Underwood & Suttle, 1999).However, few studies have been conducted to determine the available phosphorus requirement of the current genotype of pigs when kept in hot environments.
Thus, an experiment was conducted with the objective to determine the available phosphorus requirement of 15-to 30-kg pigs kept in hot environment.

Material and Methods
The experiment was conducted from January to March of 2008.Seventy-two pigs (36 castrated males and 36 females) with initial weight of 15.10±0.31kg were allotted in a completely randomized block design with six treatments (0.107, 0.214, 0.321, 0.428, 0.535, and 0.642% of aP), six replicates and two pigs (1 castrated male and 1 female) per experimental unit.The experimental unit was represented by the pen.Body weight and kinship of pigs were used as criteria in the blocks formation.
Pigs were housed in suspended metal cages, with wired mash floor and sides, equipped with semi-automatic feeders and nipple drinkers, located in rooms with controlled air temperature.The room temperature was kept constant at 34 ºC.
The thermal environment in the room was monitored three times a day (7:00 a.m., 12:00 a.m. and 5:00 p.m.) using a set of thermometers (maximum and minimum, dry and wet bulb and black globe) placed in an empty cage in the middle of the room, and at half the height of the body of the pigs.These data were then converted into the black globe humidity index (BGHI), according to Buffington et al. (1981).
Experimental diets (Table 1) were corn/soybean-based meal supplemented with minerals, vitamins and industrial amino acids to meet the requirements of piglets in the initial growth phase (15 to 30 kg), as defined by Rostagno et al. (2005) for all nutrients, except phosphorus.The aP levels were obtained through the inclusion of dicalcium phosphate in place of kaolin, resulting in diets with 0.107, 0.214, 0.321, 0.428, 0.535 and 0.642% aP.Industrial amino acids were added to the diets to keep the relationship between digestible lysine and the other digestible amino acids according to the ideal protein concept for this animal category recommended by Rostagno et al. (2005).
Feed and water were provided ad libitum throughout the experimental period.Experimental diets, daily feed waste and leftovers were weighted daily, and pigs were weighed at the beginning and end of the experimental period (25 days) for calculation of the average daily feed and phosphorus intakes, average daily gain and feed conversion.
On the first day of the experimental period, each piglet received 0.5 mL of a commercial antihelmintic ivermectin (1%) injected subcutaneously.
Pigs remained on trial until they reached an average of 29.09±1.93kg and then fasted for a period of 24 hours.After fasting, one pig from each experimental unit with a final body weight closest to 30 kg was stunned and immediately exsanguinated.Carcasses were then opened and eviscerated.
An additional group of five pigs weighing 15.55±0.75kg was euthanized, following the same procedure as the experimental animals to determine the carcass composition at the beginning of the experiment.
Eviscerated carcasses with no blood were divided longitudinally and the left side of each carcass (including head and feet) was crushed for 15 minutes by a 30HP, 1775 rpm commercial cutter.Afterwards, crushed and homogenized samples were taken and stored at -12 °C.
Samples were thawed at room temperature for a period of 24 hours and then oven-dried at 65 °C for 72 hours.Because of the high fat concentration, samples were defatted by a petroleum ether procedure using Soxhlet equipment for 4 1 Provided per kg of product: vitamin A -8,000,000 IU; vitamin D3 -2,000,000 IU; vitamin E -10,000 mg; vitamin K3 -1,500 mg; vitamin B12 -20,000 mg; vitamin B2 -5,000 mg; biotin -50 mg; calcium pantothenate -12,000 mg; niacin -25,000 mg; antioxidant -30,000 mg; vitamin B1-1,500 g; vitamin B6 -2,000 mg; folic acid -800 mg; selenium -320 mg; vehicle q.s.p. -1000 g. 2 Provided per kg of product: iron -100,000 mg; copper -30,000 mg; manganese -70,000 mg; zinc -160,000 mg; iodine -1,900 mg; and vehicle q.s.p. -1000 g. 3 Provided per kg of product: colistin -80,000 mg; and tylosin -400,000 mg. 4 Butylhydroxitoluene. 5 Values estimated based on the digestibility coefficients of amino acids of ingredients, according to Rostagno et al. (2005).hours.Dried and defatted samples were ground and packed into identified glass jars for further laboratory analysis.The water and fat removed during sample preparation were considered for correction of the values for the subsequent analysis, according to the technique described by Donzele et al. (1992).
The chemical analyses of protein and lipid from the carcass samples were performed at the Laboratório de Nutrição Animal of the Departamento de Zootecnia in the Universidade Federal de Viçosa, according to the techniques described by Silva (1990).
The deposition of protein and fat in the carcass was measured by comparative criteria between the carcasses of the first group of animals with body weight of 15.55±0.75kg and the second group of animals, according to the methodology proposed by Donzele et al. (1992).
The front foot from the right side of each carcass was collected, placed in an aluminum container with water and boiled to soften the skin and muscle around the bones for removal of the third metacarpal.
Metacarpals were dissected and oven-dried at 65 °C for 72 hours.Dried bones were then crushed and defatted following a petroleum ether procedure using Soxhlet equipment for 3 hours.After, they were taken to the ventilated oven at 65 °C for a period of 24 hours and then ground in a ball mill.
The diet levels of phosphorus and concentrations of calcium, phosphorus and ash in the bones were analyzed by Rodes Química Cajati LTDA (Cajati, SP).
The statistical analysis of the performance data, bone parameters and daily carcass protein and fat deposition was performed using the procedures for analysis of variance and regression contained in the System for Statistical and Genetics Analysis (UFV, 2000), version 8.0.
The requirement of aP was obtained based on the results of daily weight gain, feed conversion, bone parameters and daily deposition of protein and fat in the carcass using linear, quadratic or linear response plateau models, according to the best model adjustment for each variable.

Results and Discussion
During the experimental period, the temperature in the room was kept at 34.1±0.8°C, the relative humidity at 70.1±8.1% and the black globe temperature at 35.2±0.7 °C with calculated black globe humidity index (BGHI) value of 86.7±1.14.It can be inferred, based on the values of mean air temperature (34.1 ºC) and BGHI (86.7), that pigs were exposed to heat stress considering that the thermoneutral range for this animal category is 18 to 28 ºC (Coffey et al. 2000) and that a BGHI value above 80.0 (Campos et al. 2008) characterizes an environment of heat stress.
The levels of aP did not influence (P>0.05) the average daily feed intake (ADFI) of pigs (Table 2).Taking as reference the results obtained by Saraiva et al. (2009), who verified significant variation in the ADFI by increasing aP in diets for pigs from the same genetic and weight range kept on thermoneutral environment (19.5 and 28.3 °C), one can deduce that the thermal environment to which pigs are exposed influences the pattern of feed consumption concerning the levels of aP in the diet.Probably, this altered response to feed intake between the thermal environments is related to the fact that pigs exposed to high ambient temperatures have a significant reduction in voluntary feed intake, regardless of their nutritional levels (Collin et al., 2001).
As there was no significant variation in voluntary feed intake, one can assume that the increase in the daily aP intake occurred due to the concentration of this mineral in the diet.Assessing aP levels in diets of castrated males and females pigs from 9 to 37 kg and 15 to 30 kg, Stahly et al. (2000) and Saraiva et al. (2009) also found a linear increase in the daily available phosphorus intake by increasing dietary aP.
There was effect (P<0.01) of aP on the average daily gain (ADG) of pigs (Table 2), which increased quadratically up to the estimated level of 0.477% (Figure 1).This result is consistent with those reported by Stahly et al. (2000) who verified positive effect of the aP levels in the diet on the growth rate of castrated male and females pigs from 9 to 37 kg up to the estimated level of 0.480%.In studies conducted with barrows and barrows and gilts up to 30 kg Stahly & Cook (1997) and Saraiva et al. (2009), respectively, also observed improvement in pigs weight gain by increasing the level of aP in the diet, although the greatest results between the studies were obtained at 0.500 and 0.509% of aP, respectively.Positive responses of ADG of pigs as a consequence of increasing dietary aP may be related to increases in the synthesis and formation of bone and muscle tissues, since low phosphorus availability in the diet can limit the formation of these tissues (Stahly, 2007) which have priority for development in the early stage of pigs growth (Shields Junior et al., 1983).
The inconsistency of results between studies regarding the values of aP that provided the greatest results ADG may be related to factors such as genetic potential of pigs for growth and the thermal environment.Assessing available phosphorus levels in diets for pigs with different growth potential from 28 to 42 days old, Hittmeier et al. (2006) found that the pigs' genetic potential interact with the aP content of the diet resulting in weight gains that are genotype specific.As for the effect of the thermal environment, Manno et al. (2005) found that pigs genetically similar and fed the same diet from 15 to 30 kg showed a decrease of 28.2% in the ADG when kept in a high temperature environment (34.2 °C), compared with the thermoneutral environment (22.7 °C).
Feed conversion (FC) of pigs was influenced (P<0.01) by the levels of aP in the diet (Table 2) increasing up to 0.457% aP (Figure 2).In a study with pigs in the initial phase of growth, Saraiva et al. (2009) found effect of the aP in the diet on the FC that improved quadratically up to the estimated level of 0.477% aP.However, the result of FC obtained in this study differs from that found by Kegley et al. (2001), who observed no variation in the FC of 6-to 17-kg piglets by increasing aP in the diet.As improvement in FC is usually related to the possible change in the composition of weight gain of pigs with increase in the proportion of protein (Marinho et al., 2007) and considering that quantitatively the muscle constitutes the second largest reserves of organic phosphorus (Stahly, 2007), one can assume that the variation in the genetic of pigs for meat deposition capacity justifies the difference seen in the results between studies.
Based on aP daily intakes of 4.75 and 4.55 g, corresponding to the estimated levels of 0.477 and 0.457% aP in the diet that provided the greatest results of ADG and FC in this study, respectively, it can be verified that the values of 3.74 and 4.40 g/day of aP recommended, respectively, by the NRC (1998) and Rostagno et al. (2005), may not be sufficient to meet the requirement of the current pig genotypes in the initial phase of growth (15 to 30 kg), kept in hot environment.
The amount of phosphorus deposited in the bones (BP) was influenced (P<0.01) by the aP levels (Table 2), which increased quadratically up to the estimated level of 0.529%  aP in the diet (Figure 3).Studying the influence of dietary aP on the BP content of pigs from the same genetic and weight range of this study, but kept in a thermoneutral environment (24.5 ºC), Alebrante et al. (2011) also observed that the amount of BP increased up to the level of 0.525% aP.
Based on these results, we can infer that regardless of the thermal environment to which pigs are exposed, bone mineralization constitutes a priority of the metabolic use of phosphorus.
There was effect (P<0.01) of aP on the amount of calcium in the bones (BCa) (Table 2), which increased linearly according to the equation: Y ˆ= 164.66 + 34.44Pd (r² = 0.73).Significant increase in the concentration of BCa in castrated male and female piglets from 15 to 30 kg by increasing aP in the diet was also observed by Saraiva et al. (2009), up to the level of 0.619%.
Analyzing the metabolism of calcium and phosphorus in growing pigs, Fernández (1995) found that the calcium:phosphorus ratio (Ca:P) in the bone was maintained close to 2:1, irrespective to the amount of phosphorus consumed by the pigs.Consistent with this finding, it was observed in this study that although the Ca:P varied from 7.47 to 1.24, the calculated ratio between these minerals in the bones ranged only between 2.00 and 2.12.Thus, one can conclude that there is a proportionality of calcium and phosphorus deposited in the bone which does not seem to change with the level of these minerals in the diet.
It was verified in this study that the estimated aP levels of 0.529 to 0.642% which provided the highest values of phosphorus and calcium, respectively, in the bone were consistently above those that resulted in greatest response of weight gain (0.477%) and feed conversion (0.457%).Similarly, Kornegay et al. (1981), Mahan (1982) and Combs et al. (1991) also found that the greater bone mineralization occurred at dietary phosphorus levels above those required to maximize performance of pigs.These results are consistent with the NRC (1998) recommendations, which state that phosphorus requirement for maximum bone development is approximately 0.1% higher than the requirement for maximum weight gain.
The percentage of bone ash (BA) increased (P<0.01)linearly by increasing aP in the diets (Table 2).However, the Linear Response Plateau model provided the best fit to the data, estimating at 0.328% the level of aP from which BA remained on a plateau (Figure 4).This result is consistent with those obtained by O'Quinn et al. (1997), Spencer et al. (2000) and Hastad et al. (2004), who also found increase in the percentage of BA due to the increasing levels of aP in the diets of pigs at different stages of production.
Despite the similarity of results observed between studies concerning the positive effects of dietary available phosphorus in the concentration of bone ash in pigs, studies conducted by Ketaren et al. (1993) suggested that the bone analyzed is considered a factor that can lead to different responses to the phosphorus levels studied.These authors observed that levels of phosphorus in the diet that did not alter the percentage of ash in the metatarsal bone resulted, however, in significant variation in ash content of radius and femur bones.
The levels of aP did not influence (P>0.05)daily protein (DPD) or fat (DFD) deposition in the carcass (Table 2).These results differed from those obtained by Frederick & Stahly (1998), who studying levels of aP for pigs in the early stages of growth observed increase in the DPD and reduction in DFD as a result of the increase in the dietary aP.Although no significant change occurred, there was a 14.1% increase in the DPD between the levels of 0.107 and 0.428% aP.This result is consistent with the improvements observed in feed conversion between these levels of aP, since the deposition of protein in the carcass of pigs is a major factor that may explain the result of increased weight gain associated with improvement in feed conversion (Marinho et al., 2007).

Conclusions
The available phosphorus requirement of 15-to 30-kg pigs kept in hot environment for greatest results of daily weight gain, feed conversion and bone parameters are 0.477, 0.457, and 0.529%, respectively, corresponding to the estimated daily consumptions of 4.75, 4.55 and 5.27 g available phosphorus and to the calcium:available phosphorus ratios of 1.68:1, 1.75:1, and 1.51:1.

Figure 1 -
Figure 1 -Effect of available phosphorus levels on the weight gain of 15-to 30-kg pigs kept in hot environment.

Figure 2 -
Figure 2 -Effect of available phosphorus levels on feed conversion of 15-to 30-kg pigs kept in hot environment.

Figure 3 -
Figure 3 -Effect of available phosphorus levels on the amount of phosphorus in the bones of 15-to 30-kg pigs kept in hot environment.

Figure 4 -
Figure 4 -Effect of available phosphorus levels on the bone ash content of 15-to 30-kg pigs kept in hot environment.

Table 2 -
Performance, bone parameters and daily carcass protein and fat deposition of pigs fed different levels of available phosphorus (aP)