Effect of dietary phosphorus level on performance and mineral status of grazing cattle in a warm climate region of central Florida

Effect of dietary phosphorus level on performance and mineral status of grazing cattle in a warm climate region of central Florida

J E Espinoza, L R McDowell, N S Wilkinson, J H Conrad, F G Martin and S N Williams

University of Florida, Gainesville, FL 32611-0691
Florida Agricultural Experiment Station Journal Series No. R-01103

Research supported in part by the US Department of Agriculture under CSRC Special Grant No. 86-CRSR-2-2843 managed by the Caribbean Basin Advisory Group (CBAG)

A three year study was conducted to determine the effect of supplemental phosphorus level (in free choice mineral mixture) on performance and mineral status of grazing cattle in a ranch in central Florida. The low phosphorus (LP) group had a lower (P<0.05) pregnancy rate in year 1 when the concentrations of dietary P were 4% (LP), 8% medium phosphorus (MP) and 12% high phosphorus (HP). Pregnancy rates were similar (P>0.05) in years 2 and 3 when dietary P levels were 6% (LP), 8% (MP) and 12% (HP). Treatment effect on body weight was observed only in November with HP group having heavier (P<0.01) weights. November weights were higher than May weights (P<0.01) in all groups.

Treatment effect (P<0.05) on cow serum mineral concentrations of Ca, P, Mg and Cu in both May and November were observed. Cow serum minerals below the critical value were observed for Mg, P and Zn. Treatment effects on tissue mineral concentrations were minimal, suggesting that animals were generally receiving adequate minerals from their diets combined with the mineral supplement. Mean liver mineral concentrations were similar (P>0.05) in all treatments and in all years, except for P and Mn. All mineral elements were above the critical values, except Mn in MP group in year 2. Bone mineral concentrations, percent ash and specific gravity were similar (P>0.10) for all treatments and years. Phosphorus was adequate in all treatments. Ash was low in LP group in all years and MP group of year 2.

Aphosphorosis in grazing cattle is widespread, and P supplementation is a common practice. With the exception of common salt, P is probably the nutrient most frequently given as a supplement to grazing ruminants (Cohen 1980). Phosphorus supplementation has dramatically increased fertility levels and growth in grazing cattle in many parts of the world (NRC 1984; McDowell 1976; Engels 1981; Bauer et al 1982). Other research reports (Call et al 1978; Little 1980; Butcher et al 1979; Pott et al 1987) indicate that there is normal reproduction and production, even when dietary P concentrations are below those commonly recommended. Such reports suggest the need for further research since excess P supplementation can increase unnecessarily the cost of beef cattle production.

Mineral deficiencies, imbalances and toxicities inhibit grazing cattle production in tropical and subtropical areas (McDowell 1976). Most tropical forages have been found to be borderline to deficient in many essential elements (McDowell et al 1983). Reports from these areas indicated that mineral supplementation to grazing cattle have resulted in improved weight gains and dramaticaly increased calving percentages (McDowell 1985).

The purpose of this study was to compare the effect of three levels of P supplements on reproduction, changes in body weight, and mineral status of animal tissue in grazing beef cattle in central Florida during a three-year period.

A three-year study was conducted at a ranch in Osceola County, Fla. (central Florida). Three herds of crossbred beef cattle over 3 years of age were assigned randomly to three treatment groups. Cattle were 1/4 to 3/8 Brahman crossed with British Breeds of Angus, Hereford and Charolais. Each group was fed ad libitum a complete mineral supplement containing different concentrations of phosphorus. The composition of the free-choice mineral mixture is shown in Table 1. For year 1, the three mineral mixtures were low phosphorus (LP) of 4%, medium phosphorus (MP) of 8% and high phosphorus (HP) of 12%. Because of the low calving percentage observed in the LP group following the first year, the composition of the mineral mixture was changed so that LP group received a 6% P supplement for years 2 and 3.

Animals from each group received the mineral supplement from covered mineral feeders installed in their respective pasture. Mineral supplement consumed per animal per day was estimated from total supplement fed and the number of animals per pasture per year. All animals grazed year round on pastures predominantly of bahiagrass (Paspalum notatum) and were provided free-access to water. Energy and protein supplements were provided to all animals for 90 days during the winter (Dec 15 to March 15) of each year. The supplement consisted of 0.454 kg of cottonseed cubes (33% protein) per animal per day and 1.82 kg of molasses (blackstrap, no urea) daily.

Table 1: Composition of free-choice mineral mixture (%)*

	Treatments
	LP Yr 1	LP Yrs 2 and 3	MP	HP

Ca, not less than	12.00	12.00	12.00	12.00
P, not less than	4.00	6.00	8.00	12.00
Salt, not more than	15.00	15.00	15.00	15.00
Cu, not less than	0.20	0.20	0.20	0.20
Co, not less than	0.003	0.003	0.003	0.003
Se, not more than	0.002	0.002	0.002	0.002
Fe, not less than	0.70	0.60	0.60	0.60
Mn, not less than	0.20	0.20	0.20	0.20
I, not less than	0.01	0.015	0.015	0.015
Zn, not less than	0.50	0.50	0.50	0.50
Mg, not less than	0.67	0.50	0.50	0.50
K, not less than	0.85	0.70	0.70	0.70
F, not more than	0.03	0.07	0.11	0.11

* Supplement was manufactured by Lakeland Cash Feed Company Inc., Lakeland, Fla, from dicalcium phosphate, monocalcium phosphate, cane molasses, Ca carbonate, cottonseed meal, salt, Cu sulfate, Co sulfate, Na selenite, Fe sulfate

During the first year of the experiment, cows were weighed once a year (November), and for the second and third year weights were recorded twice a year (May and November). Pregnancy percentage was determined once a year in late gestation (November) by rectal palpation. Liver and bone samples were collected from six to seven culled cows per treatment each November. Blood samples also were collected from 50 cows, randomly selected from each treatment group twice a year (May and November).

Liver samples were taken in vivo using a liver biopsy technique (Fick et al 1979). Bone biopsy samples were taken using a modified surgical procedure (Little 1972). Blood samples were taken by jugular vein puncture using California bleeding needles (Fick et al 1979).

Samples were prepared and chemically analyzed in the Nutrition Laboratory at the University of Florida. Serum Ca, Cu, Mg and Zn; liver Cu, Fe, Mn and Zn; and bone Ca and Mg concentrations were determined by atomic absorption spectrophotometry using a Perkin- Elmer 5000 (Perkin-Elmer 1980). Serum and bone P were determined colorimetricaly (Harris and Popat 1954). Selenium concentrations in serum and liver were determined fluorometricaly (Whetter and Ullrey 1978). Liver Co and Mo concentrations were determined by flameless atomic absorption spectrophotometry using a Perkin-Elmer 3030 graphite furnace (Perkin-Elmer 1984).

Data were statistically analyzed using a completely randomized design (for liver and bone) and factorial design (for body weight and cows serum) (Snedecor and Cochran 1980), with the General Linear Models (GLM) procedure of the SAS System (SAS Institute Inc. 1987). Treatment effects on pregnancy rate were analyzed by a Log Linear Model (Grizzle et al 1969) using PROC CATMOD option in SAS (SAS Institute Inc. 1987). Correlation coefficients were estimated for minerals in liver, serum and bone.

During year 1 the MP treatment group showed highest (P<0.01) pregnancy rates (88%) followed by HP (78%) which was higher (P<0.01) than LP (60%) (Table 2). In years 2 and 3, no differences (P>0.05) were observed among the three treatment groups.

Table 2: Effect of dietary phosphorus concentration on percent of cows pregnant by year

	LP		MP		HP
	No. Cows	%	No. Cows	%	No. Cows	%

Year 1	(225)	60***	(224)	88*	(273)	78**
Year 2	(181)	83	(173)	84	(232)	86
Year 3	(194)	88	(189)	87	(198)	82

Pregnancy was tested via rectal palpation in late pregnancy (November); Figures in parenthesis indicate number of cows tested;
*, **, *** For each year, means in rows having different superscripts differ P<0.05

Many scientists have associated reduced reproductive performance with P deficient diets. In some studies, fertility in cattle appeared to be very sensitive to P intake (Theiler et al 1928; Short and Bellows 1971; Preston 1976). On the contrary, other scientists reported that P supplementation has failed to show a diminished reproductive performance (Palmer et al 1941; Call et al 1978; Butcher et al 1982; Call et al 1987). Results observed during the first year would suggest agreement with the first proposition that low dietary P (in this case, 4% P in the mineral supplement) negatively affected reproductive performance. Results of the second and third years would suggest that 6% P in the mineral supplement is adequate for normal reproductive performance. In both conditions, all animals were also receiving P from forage (14.7 g/day) and from cottonseed and molasses (4.7 g/day during winter only).

There were no treatment effects (P>0.05) on body weight in spring (Table 3). However, P supplementation influenced body weights in November, with the HP group having the highest (P<0.05) weight when compared to the MP and LP groups. As expected, as a result of going through the winter, May weights were less (P<0.01) than November weights for all treatments. Treatment by season interaction effects (P<0.01) were also found. Results of the present experiment are in disagreement with those of Call et al (1978) who reported no difference in weight gain, feed intake or feed efficiency of Hereford heifers fed either 0.14% or 0.36% P (as fed) over a two- year period. Similar results were observed by Little (1980), who reported that beef cattle receiving only Stylosanthes humilis (.12% P) had similar dry matter intake and liveweight gain compared to those animals supplemented at the rate of 5 g of P per day. No differences were observed in either body mass or reproductive performance between supplemented (lick consisting of 44% salt, 44% dicalcium phosphate and 12% molasses powder) and unsupplemented (salt) cattle in a region where P was not deficient in soil and grass (Marion and Engels 1985). Butcher et al (1979) reported that appetite and growth were reduced only when dietary P concentration was reduced to 0.09%.

Table 3: Influence of dietary P concentration and season on body weight (kg) of cows (years 2 and 3)

	LP		MP		HP
Month	Mean	SE	Mean	SE	Mean	SE

May	394****	5.6	389****	5.9	387****	5.6
November	422 *	5.9	431 *	6.0	448* ***	5.0

Least square means are based on 43, 38 and 43 means for LP, MP, and HP treatment groups, respectively, in May, and on 38, 37 and 54 means for LP, MP and HP groups, respectively, in November (Animals were weighed in groups of five to 12.);
SE = standard error of the least square mean;
* ** Means within a row having different superscripts differ P<0.01;
***, **** Means within a column having different superscripts differ P<0.01;

Higher body weights observed in November for all three treatment groups are probably associated with factors like compensatory gain, higher forage quality and the advanced stage of pregnancy of cows during that month.

Treatment differences (P<0.01) were observed for all mineral elements studied for each year (Table 4), except for zinc in year 2. Blood serum Ca concentration was higher (P<0.01) in LP group in years 1 and 2. The average Ca concentration for all treatments was greater than the critical level of 8 mg/100 ml suggested by Cunha (1964). Mean serum Ca concentrations for all treatments for all years varied from 8.46 mg/100 ml to 10.00 mg/100 ml. These values are similar to the values (8.8 mg/100 ml to 9.6 mg/100 ml) reported by Kiatoko et al (1982) in four regions of Florida.

Among treatments within years, highest (P<0.05) serum Mg concentrations were found in LP and HP groups for year 1 and the MP group for years 2 and 3. Mean Mg concentrations below the critical level of 1.8 mg/100 ml (Underwood 1966) were observed in LP (1.75 mg/100 ml) and HP (1.77 mg/100 ml) groups in year 2. Adequate (> 1.8 mg/100 ml) Mg concentrations were found for the same general region by Kiatoko et al (1982) who reported mean plasma Mg content of 2.3 mg/100 ml. Merkel (1989) reported Charolais cows in north central Florida having 2.0 mg/100 ml Mg.

Table 4: Serum minerals for cows by year

	Ca	Mg	P	Zn	Cu
	mg/100 ml			ppm

Critical level*	8.00	1.80	4.50	0.80	0.65
Year 1
LP	10.00**	2.16**	4.12***	0.90**	0.76***
MP	9.54***	1.94***	5.37**	0.97**	0.89**
HP	8.68****	2.12**	3.90***	0.73***	0.97**
Year 2
LP	9.55**	1.75***	4.88***	0.86	0.93***
MP	8.46****	2.27**	3.42****	0.76	1.12**
HP	9.04***	1.77***	4.27***	0.80	0.83***
Year 3
LP	9.41**	2.07***	4.58**	0.63**	0.89***
MP	9.16***	2.26**	3.21****	0.44****	0.83***
HP	9.47**	1.97***	3.75***	0.54***	1.15****

Least square means are based on 92 samples per treatment per year.
* McDowell and Conrad 1977; NCMN 1973; Underwood 1966, 1981; Cunha 1964
**, ***, **** Means having different superscripts differ P<0.05

Mean serum P concentration in MP group (5.37 mg/100 ml) were higher (P<0.01) than LP (4.12 mg/100 ml) and HP (3.90 mg/100 ml) groups during the first year of the experiment. For years 2 and 3, LP group showed highest (P<0.01) serum P concentrations (4.88 and 4.58 mg/100 ml, respectively). Mean serum P concentration below the critical level of 4.5 mg/100 ml (Underwood 1966; McDowell 1985) were found for LP (4.12 mg/100 ml) and HP (3.90 mg/100 ml) groups in year 1, for MP (3.42 mg/100 ml) and HP (4.27 mg/100 ml) groups in year 2, and for MP (3.21 mg/100 ml) and HP (3.75 mg/100 ml) groups in year 3. For the same general region, Kiatoko et al (1982) reported mean plasma P values of 6.1 mg/100 ml in the fall and 5.2 mg/100 ml in the winter.

The NCMN (1973) does not recommend the use of serum P concentration as a practical criterion for assessing P status of grazing ruminants due to its great variation and the poor understanding of the factors that cause this variation. The factors that may increase blood inorganic P concentration include water restriction (Rollison and Bredon 1960), increased storage time or temperature post sampling (Burdin and Howard 1963), and time of sampling (Perge et al 1983). Underwood (1981) reported the adequacy of serum P as a satisfactory criterion in assessing P status in cattle.

In year 1, Zn concentrations were lower (P<0.05) for the HP treatment. No effect (P>0.05) of dietary P content was observed on blood serum Zn concentration in year 2, but differences (P<0.05) among treatments were found for year 3. Serum Zn concentration values were all in the normal range of 0.50 ppm to 1.20 ppm (Underwood 1981), except the value (0.44 ppm) of MP group in year 3. Merkel (1989) reported serum Zn values of 0.63 and 0.91 ppm for March-April and October-November, respectively.

In year 1, serum Cu values in HP were higher (P<0.01) than LP group (0.97 ppm vs 0.76 ppm) and MP and HP had similar (P>0.05) Cu values. In year 2, MP group exhibited the highest (P<0.05) serum Cu value, while for year 3 the HP group was the highest (P<0.05). Mean serum Cu values observed were all above the critical level of 0.65 ppm (McDowell and Conrad 1977). Approximately similar values for the summer-fall (1.08 ppm) and for the winter-spring (0.98 ppm) seasons were reported by McDowell et al (1989).

Table 5 shows the effect of dietary P content and sampling date on serum mineral concentrations for years 2 and 3. All serum minerals tested exhibited treatment by sampling date interaction effects (P<0.01), except Zn (P>0.05).

Serum Ca concentrations in LP and HP treatments were higher (P<0.05) in May than in November. Serum Mg content in LP and MP groups were higher (P<0.05) in May than in November, while HP group exhibited higher (P<0.05) serum Mg in November than May. No sampling date difference (P<0.05) was found in LP group for serum P concentration. Serum P from MP group collected in November was higher (P<0.05) than that collected in May (4.34 vs 2.29 mg per 100 ml). The HP group was just the reverse, serum P content collected in May was higher (P<0.05) than that collected in November (4.37 vs 3.65 mg per 100 ml).

From Florida, Merkel et al (1990) reported no serum Ca, Mg and P concentration differences (P>0.05) when comparing sampling dates of March-April and October-November. Kiatoko et al (1982) reported no season (winter vs fall) differences (P>0.05) in Ca and P concentrations, but serum Mg was higher (P<0.01) during the fall than winter (2.6 vs 2.1 mg/100 ml). Shirley et al (1968) found plasma P seasonal variation, with concentrations higher during fall than in winter.

Serum Zn content collected in May were higher (P<0.05) than those collected in November for all treatments. Merkel et al (1990) reported that serum Zn concentrations were higher (P<0.05) in October-November than in March-April. Kiatoko et al (1982) reported no differences on serum Zn content between summer and winter seasons. All treatment groups had mean serum Zn levels between the range of 0.50 ppm and 1.20 ppm (Underwood 1981), except MP group in November.

Table 5: Influence of dietary P concentration and season on serum mineral concentrations for cows

Macrominerals/		Treatments
Trace Minerals		LP	MP	HP	SE

Ca, mg/100 ml	May	9.74* ***	8.84****	9.69* ***	0.09
	Nov	9.21 *	8.78****	8.82 **	0.09
Mg, mg/100 ml	May	1.99* ****	2.45* ***	1.70 ***	0.03
	Nov	1.83 **	2.08 *	2.04* ***	0.03
P, mg/100 ml	May	4.60***	2.29 **	4.37* ***	0.12
	Nov	4.87***	4.34* ****	3.65 ***	0.12
Zn, ppm	May	0.97*	0.72*	0.82*	0.05
	Nov	0.51**	0.49**	0.53**	0.05
Cu, ppm	May	0.86****	1.12***	0.96****	0.06
	Nov	0.96** *	0.83****	1.02***	0.06

Least square means are based on 46 samples per season, per treatment group per year (two year);
SE = standard error of the least square mean;
*, ** Means within a column having different superscripts differ P<0.05;
***, ****, ***** Means within a row with different superscripts differ P<0.05

No difference (P<0.05) was found in serum Cu concentrations between the two sampling dates for any treatments. Copper concentrations on both sampling dates appeared to be normal compared to critical levels of 0.65 ppm (NCMN 1973). In Florida, McDowell et al (1982) also found no differences (P<0.01) in plasma Cu concentrations between fall and winter seasons.

Table 6 shows the influence of dietary P content on liver mineral concentration in cows by year. Dietary P content had little effect (P>0.05) on liver mineral concentrations. According to McDowell et al (1984) liver mineral concentrations are valuable for determining mineral status of Co, Cu, Mn and Se. All mean liver mineral concentrations observed in the present study were above the suggested critical levels, except Mn, which was found to be deficient (<6 ppm) in MP group during year 2 (Eagan 1975). Individual evaluation of liver samples based on their respective critical levels (Eagan 1975; McDowell 1985; and Powell et al 1964) indicated that 0% of Co, 5% of Cu, 8% of Fe, 13% of Mn, 0% of Se and 20% of Zn were deficient. The generally favorable liver mineral status of these animals could be accounted for by the consumption of minerals in the supplement.

Table 6: Mean liver minerals as affected by year and treatment

		P	Fe	Mn	Cu	Co	Zn	Se
		%	ppm

Critical level*			<190	<6	<75	<.05	<84	<.25
Year 1
LP	Mean	0.72***	324	7.56	313	0.63	130	0.60
MP	Mean	0.80**	235	7.94	259	0.77	86	0.62
HP	Mean	0.77**	235	8.32	250	0.65	102	0.47
Year 2
LP	Mean	0.70	386	6.62	222	0.71	136	0.47
MP	Mean	0.69	341	5.49	211	0.64	117	0.40
HP	Mean	0.65	418	7.00	186	0.77	98	0.47
Year 3
LP	Mean	0.78	308	8.87**	209	0.79	124	0.47
MP	Mean	0.78	265	8.34**	190	0.57	125	0.38
HP	Mean	0.81	299	7.29***	241	0.82	175	0.50

Least square means are based on the following number of samples: 7, 6 and 6 for LP, MP and HP, respectively, in year 1, and 7 samples for each treatment group for years 2 and 3;
* McDowell 1976, McDowell and Conrad 1977; McDowell 1985;
**, *** Means within a column having different superscripts differ P<0.01

No treatment differences (P>0.10) were observed for bone macromineral concentrations, percent ash or specific gravity in any of the three years (Table 7).

Langlands (1987) considers that the vertebrae and ribs are more sensitive than long bones to changes in Ca and P status. Mean bone Ca concentrations below the critical level of 24% (Little 1972) was found in LP and HP groups in year 1 and in all treatments in year 2. Mean bone Mg values found during the present study were all below the suggested normal values of 0.67% to 0.70% (Blaxter and Sharman 1955). Bone P values were found to be all normal (> 11.5%) based on the suggested critical level (Little 1972). Ash values were found to be borderline to deficient with respect to the critical level (Little 1972). Specific gravity (g/cubic centimeter) values were all below the critical level (1.68%). Individual evaluation of bone samples based on the critical level (Little 1972) indicated that Ca, P, ash and specific gravity were deficient in 48%, 0%, 50% and 100% of samples, respectively. In general, normal bone P concentrations indicated that intake of P by cattle on the three treatments during the three years of the experiment were adequate.

Table 7: Influence of dietary P concentration on bone mineral content

		Ca	Mg	P	Ash	SG**
		%

Critical level*		24.5	--	11.5	66.8	1.68
Year 1
LP	Mean	22.1	0.20	17.6	66.4	1.40
MP	Mean	27.7	0.30	16.7	67.0	1.40
HP	Mean	22.5	0.21	17.7	66.9	1.46
Year 2
LP	Mean	23.9	0.29	17.2	65.7	1.52
MP	Mean	20.5	0.22	17.1	66.0	1.53
HP	Mean	19.8	0.22	17.1	66.5	1.54
Year 3
LP	Mean	27.6	0.32	17.2	66.1	1.54
MP	Mean	28.8	0.30	17.5	66.7	1.54
HP	Mean	27.7	0.30	17.0	66.4	1.50

Least square means are based on the following number of samples: 7, 6 and 6 for LP, MP and HP, respectively, in year 1, and 7 for each treatment group in years 2 and 3;
* Little 1972;
** Specific gravity in grammes per cubic centimeter

Considering values of correlation coefficients (r > |0.5| at P<0.05), no relationship was observed between blood serum and bone mineral concentrations, while in bone, only Ca and Mg were correlated (r = 0.859). Correlation coefficients between serum Ca and Mg (r = 0.512) and serum Mg and Zn (r = 0.521) were observed. The few or nonexisting correlations (r > |0.5|, P<0.05) of minerals between or within animal tissues demonstrated the problem of finding significant correlations between soil, forage, and animal tissues (Conrad et al 1980).

Special appreciation is due to the owners of Deseret Ranches of Florida, who offered their land and animals and through their personnel assisted in the conduct of the experiment. A special thanks goes to Paul Genho, Gene Crosby and Leonard Story for making this experiment possible. Deep appreciation also goes to Osvaldo Balbuena, Vanessa Carbia, Pablo Cuesta, Larry Lawrence, Roger Merkel, Libardo Ochoa, Alfonso Ortega, Diana Pastrana, Rodrigo Pastrana and Scot Williams.

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