Abstract: The objective of this study was to determine effect of dietary phosphorus (P) on P utilization and excretion in Thai-indigenous heifers. The experimental design was a 4x4 Latin square design with 21 day period. Four Thai-indigenous heifers, average body weight of 107±4.53 kg, were randomly received 1 of 4 diets containing 0.10, 0.20, 0.30 and 0.40% P at 2% of BW. The results found that feed intake and nutrient digestibility of heifers were not significantly different (p>0.05) among treatments. Intake of P, P excretion, P absorption, P retention and plasma P concentration significantly increased (p < 0.05) with increasing dietary P. Additionally, P absorption and P retention of heifers fed 0.30 % dietary P were not significantly different (p>0.05) with those of heifers fed 0.40% dietary P. Based on the results, it could be concluded that dietary P levels affected significantly (p < 0.05) P utilization and excretion in Thai-indigenous heifers. The P requirement of heifers was lower than 0.4% of diet.
INTRODUCTION
Phosphorus is an essential element for body structure and plays a vital role in energy metabolism (Pfeffer et al., 2005). Therefore, osteomalacia, rickets, reduce feed intake and low fertility can occur when animal received inadequate P (Underwood and Suttle, 1999). Extensive areas of P-deficient soils occur throughout the world, especially in tropical area and a deficiency of this element can be regarded as the most widespread and economically important of all the mineral disabilities affecting grazing livestock (McDonald et al., 1995). On the other hand, excess P intake is liked to increase P loss in manure. The excretion of P is associated with environmental concern or eutrophication of water lakes (Valk et al., 2000; Ekelund et al., 2006). Reducing P intake by cattle and thus also that excreted in the manure will contribute to reducing environmental pollution (Valk and Beynen, 2003). Consequently, the optimal way to decrease P excretion must be to develop diets that closely match the requirement of the animal (Ekelund et al., 2006).
The tropical crossbred beef heifers of 100 to 300 kg of BW required 0.17 to 0.35% of dietary P for maintenance and growth (Kearl, 1982). National Research Council (1996) estimated the P requirement for finishing cattle as 0.20% of diet DM. However, Erickson et al. (1999) found that P requirement for finishing yearlings was 0.14% of diet DM or less. Erickson et al. (2002) consistently observed P requirement for finishing feedlot steers was lower than 0.16% of diet DM. The P requirement is dependent on breed, body weight, weigh gain, pregnant stage, milk production and age of the animal (McDonald et al., 1995). Thai-indigenous cattle is a small frame ( < 250 kg of BW) and lower growth rate than the temperate beef cattle (Ubunratchatani Livestock Breeding Station, 1999).
They probably have the different P requirement from other cattle. Furthermore, their P requirement has not been investigated. Therefore, the objective of this study was to determine effect of dietary P on P utilization and excretion in Thai-indigenous heifers.
MATERIALS AND METHODS
Four-Thai-indigenous yearling heifers an initial body weight of 107 ±4.53 kg were used. The animals were dewormed by Ivomectin and injected with AD3E vitamin-complex prior to the beginning of the experiment. They were housed in individual stall and daily fed at 2.0% of BW at 0800 and 16:00 h. Drinking water was available at all time.
Animals were randomly allocated to one of four dietary treatments in a 4x4 Latin Square Design with 21 day periods. Each period consisted of 14 day of adaptation and 7 day test. Dietary treatments (Table 1) contained 0.1, 0.2, 0.3 and 0.4% of dietary P. Monosodium phosphate (Sigma-Aldrich, Germany) was used as P source in the diets. The diets were prepared in total mixed ration with rice straw as a roughage source and formulated to be isonitrogenous and isocaloric diets according to National Research Council (1996).
Feed intake was monitored daily throughout the experiment, however only the intake during the 7 day test period was used. During each collection period, feed and orts were collected daily and composite by animal for chemical analysis. Simultaneously, feces of each animal was weighed, mixed and a 10% subsample was taken, stored at -4 °C and later pool individually the end of each period. Fecal samples were dried at 65 °C for 72 h and ground through 1 mm screen for chemical analysis. The urinary sample was collected via modified urinary cap and kept into plastic container pre-added with 250 mL of 10% H2SO4 (v/v). Acidified urine was kept at -20 °C for P determination. Rumen fluid and blood samples were taken at the end of collection period. Rumen fluid was sampled using esophageal tube. The sample was filtered through a muslin cloth and rumen fluid was immediately acidified 10%H2SO4 (v/v) and frozen at -20 °C for ruminal P determination. Blood was drawn from jugular vein. Plasma was harvested by centrifugation of the whole blood for 15 min at 3000 g and kept at -20 °C for P analysis.
Feed offered, orts and fecal samples were analyzed for DM, OM, CP (AOAC, 1996), ADF and NDF according to Robertson and Van Soest (1981). Nitrogen in feed, orts and urine was determined by macro-Kjeldahl method (Association of Official Analysis Chemists, 1996). Nutrient digestibility and N utilization were estimated using the procedure of Schnieder and Flalt (1975) and McDonald et al. (1995), respectively.
Statistical Analyses
The data were analyzed using the general linear models procedure of
Statistical Analysis System Institute SAS (1996) according to the following
statistical model: Yijk = μ + Ai + Pj
+ Dk + eijk, where A, P and D are animal, period
and diet effects, respectively. The differences among means were compared
by Least Significant Different (Steel and Torries, 1989). Significance
was declared at p < 0.05.
RESULTS AND DICUSSION
The analyzed P concentrations in the dietary treatments were 0.15, 0.22, 0.32 and 0.4% (Table 1). The obtained values were similar to the calculated value of 0.1, 0.2, 0.3 and 0.4% of dietary P, respectively. Dietary P did not significantly alter (p < 0.05) feed intake and digestibilies of DM, OM, CP, NDF and ADF of the indigenous heifers (Table 2). The outcomes are consistent with the foregoing reports by Erickson et al. (1999, 2002) and Hurley et al. (2003). Those reports found that feed intake, daily gain and feed efficiency of feedlot and finishing steers were not affected by dietary P. Furthermore, Valk et al. (2002) and Wu et al. (2003) were similarly reported that no effect of dietary P on nutrient digestibility was observed. The ruminal P concentration of heifers fed diets containing 0.1 to 0.4% of P ranged from 137.33 to 431.50 mg L-1 (Table 3). Generally, rumen fluid contained 150 to 600 mg L-1 of P (Cohen, 1980). Ruminal microorganisms require P for their activities especially in fiber digestion. The microbial digestion of cellulose and hemicelluloses, therefore, decreased significantly when P content in the rumen was lower than 50 mg L-1 (Komisarczuk et al., 1987). Additionally, it has been reported that dietary P level of less than 0.25% can reduce rumen microbial growth resulting in less microbial protein and lower ration digestibility (Durand and Kawashima, 1980). The present findings demonstrated that ruminal P content of heifers was adequate for fiber digestion of microorganism. Therefore, the levels of dietary P (0.1 to 0.4 %) did not negatively influent nutrient digestibility in the indigenous heifers.
P absorption (g/day), P retention (g/day) and P excretion (g/day) of
heifers increased significantly (p < 0.05) with increasing dietary P
concentration. However, P digestibility (%) decreased dramatically (p < 0.05)
in heifers fed diet containing 0.4% of P (Table 3).
The other works (Wu et al., 2001, 2003)
| Table 1: | Ingredients and chemical composition of dietary treatments |
| 1The premix provided per kilogram of diet: 10,000 IU vitamin A; 2,000 IU vitamin D3; 20 IU vitamin E; 0.01 g Cu; 0.08 g Mn; 0.04 g Zn; 0.05 g Fe; 0.0008 g I; 0.0003 g Co; 0.0003 g Se; 0.005 g Ethoxiquin and 0.05 g SiO2. 2ME: Metabolisable Energy. 3Calculated values | |
| Table 2: | Effect of dietary P on feed intake and nutrient digestibility in Thai-indigenous heifers |
| 1SEM: Standard Error of the Means. 2NDF: Neutral Detergent Fiber, ADF: Acid Detergent Fiber | |
| Table 3: | Effect of dietary P on P utilization, plasma P and ruminal P in Thai-indigenous heifers |
| a-c: Means on the same row with different superscripts are significantly different (p < 0.05). 1SEM: Standard error of the means. 3 NS: Not Significant | |
previously indicated that fecal P excretion increased (p < 0.05), however, P digestibility declined significantly (p < 0.05) when dietary P was increased. Normally, cattle have the ability to absorb the amount of P needed, when P is provided in sufficient amount or in excess (Morse et al., 1992). Passive absorption prevails when sufficient or excessive amount of P are consumed (Wasserman and Taylor, 1976). The decreased P digestibility was also due to depressing 1, 25 Dihydroxy vitamin D synthesis in the kidney that inhibit intestinal mucosa to absorb excessive dietary P (Reinhardt et al., 1988). Subsequently, the surplus or indigested P was mainly excreted in the feces. (Valk and Beynen, 2003). The highest levels of apparent digestibility were obtained when P is not supplied in excess (Ekelund et al., 2006). It is probably that the animal adapted to the low dietary P by increasing the transport capacity in the small intestine (Huber et al., 2002). Furthermore, P absorption and retention of heifers fed diet containing 0.3 and 0.4% of P were not significantly different (p>0.05). The results in the present study obviously indicated that 0.4% of dietary P was an excessive level of P requirement for the indigenous heifers. The P digestibility of heifers varied between 56.90 to 75.33% (Table 3). The obtained values were higher than those previously reported (24 to 52.5%) in lactating dairy cows (Wu et al., 2001; Ekelund et al., 2003, 2006). However, Bravo et al. (2003) mentioned that the efficiency of dietary P digestibility in lactating cows (0.69) was generally lower than that of growing cattle (0.76). Unfortunately, there was no comparable information of P digestibility of Thai-indigenous cattle to clarify the current results. Grazing livestock in the tropics, including in Thailand (Vilchulata et al., 1983; Ngampongsai et al., 1999) were usually deficient in P due to insufficient of P in forage and soil (McDowell, 1992). Thai-indigenous cattle are familiar with this harsh condition. They have maybe adjusted themselves by enhancing P digestibility as mentioned above (Huber et at., 2002). The further research is needed to verify this issue.
Plasma P concentrations of the indigenous heifers increased dramatically (p < 0.05) with increasing dietary P (Table 3). Wu et al. (2001), Lopez et al. (2004) and Peterson et al. (2005) consistently reported the concentration of P in blood serum was higher (p < 0.01) for cows fed the highest P diet than for those fed the lower P diets. The normal level of plasma P in cattle was 31 to 46.5 mg L-1 (Underwood and Suttle, 1999). Hence, plasma P (32.66 to 51.37 mg L-1) of indigenous heifers was in the normal range. Plasma P levels is directly affected by P intake. It is therefore may have more value as an indicator of dietary P, than as a P status indicator. Because it is evident that the physiological stage of production and the length of time on a P-deficient all affect an animal`s P status and thus have a modulating effect on blood P levels (Karn, 2001).
CONCLUSIONS
Dietary P levels influenced significantly (p < 0.05) P utilization and excretion in Thai-indigenous heifers by increasing P absorption, P excretion and P retention. The 0.4% of dietary P was excessive level of requirement for Thai-indigenous heifers.
ACKNOWLEDGMENT
This research project No. 5003026 was funded by budget fiscal year 2007 of Mahasarakham University.