Forage and soil mineral concentrations over a three-year period in a warm climate region of central Florida. I. Macrominerals

Forage and soil mineral concentrations over a three-year period in a warm climate region of central Florida. I. Macrominerals

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 macromineral status of tropical forages (bahiagrass) and soils on a ranch in central Florida. Soils and forages were collected twice a year (May and November) for three years. Three composite samples from each of seven experimental pastures were collected each sampling date.

Year differences (P<0.05) were observed in forage Ca, Na, P and protein in both seasons. The month of May showed higher (P<0.05) forage Ca, K, P crude protein and IVOMD for some years. Magnesium was higher (P<0.05) in November. In general, season mean values indicate that forage had higher macromineral concentrations during May. Concentrations below the critical value were observed in Mg for May of year 1 in K for November of all years and in Na and P for both seasons of all years.

In general, higher (P<0.05) concentrations were observed for soil Al, Ca, Mg, K, Na and P in year 3. Season effects (P<0.05) were observed only in Na in year 1; and in pH in years 2 and 3.

The percentages of total forage samples collected with macromineral and crude protein concentrations below values regarded as critical (in parentheses) and suggestive of deficiency were as follow: in forage, Ca (0.30%), 21%; Mg (0.18%), 34%; K (0.60%), 47%; Na (0.06%), 89%; P (0.25%), 85%; and crude protein (7%), 18%. The majority of soils were low to deficient in K and P.

Grazing livestock in warm climates have to depend largely upon forage to fulfil their mineral requirements. Forages rarely satisfy all of the needed mineral requirements of grazing livestock (McDowell 1977). It has been reported that mineral concentrations in both soils and plants affect the mineral status of grazing livestock (Towers and Clark 1983).

Mineral composition of forage plants is affected by soil-plant factors, including pH, drainage, fertilization, forage species, forage maturity and interaction among minerals (Gomide 1978; Reid and Hovarth 1980). Soils of subtropical Florida are dominated by Spodosols and Entisols. With the exception of a few organic soils, the soils are acid, infertile and sandy in texture (Fiskel and Zelazny 1972). Florida pastures containing native forages have been reported to be low in dry matter yield and to be deficient in some plant nutrients.

The historical significance of mineral deficiencies and toxicities in the cattle industry in Florida have been summarized by Cunha et al (1964) and Becker et al (1965).

The purpose of this study was to evaluate forage and soil macromineral concentrations, as well as forage protein and in vitro organic matter digestibility (IVOMD) over a three-year period in central Florida. A companion paper (Espinoza et al 1990) studies trace mineral concentrations of these some forages.

A three-year study was conducted at Deseret Ranches in Osceola County, Florida (central Florida). Three herds of crossbred beef cattle (1/4 to 3/8 Brahman crossed to British breeds) were assigned randomly to three treatments of different concentrations of P supplementation. Soil and forage samples were collected twice a year (May and November) for three years (1986-1988). Three composite soil and forage samples from each of the seven experimental pastures were collected on each sampling date. Approximately 200 ha were assigned to each treatment. Animals grazed year-round at a stocking rate of about one cow per ha. Pastures were fertilized in the spring of years 1 and 2 with 20-10-10 (NPK) at a rate of 100 and 125 kg/ha, respectively, and with 25-18-0 (NPK) in year 3 at a rate of 100 kg/ha. Soil samples were collected using stainless-steel soil sampling tube as described by Bahia (1978). Forage samples were taken with a stainless-steel scissors based on cattle grazing patterns in order to obtain a representative sample.

Forage and soil samples were collected at the same month and site. The principal improved tropical forage species that was collected in all pastures was bahiagrass (Paspalum notatum Flugge). To a lesser extent (less than 5% of total), all pastures were associated with native grasses and legumes. A total of 126 of both soil and forage samples were collected during the three years of the experiment (21 samples per month). Soil samples were analyzed for Ca, P, Mg, Na, K, Al, pH and organic matter according to the procedure used by the IFAS extension soil testing laboratory (Rhue and Kidder 1983). Soil minerals were extracted using Mechlich I extracting solution method (0.05 N HCL + 0.025 N sulfuric acid). Soil mineral concentrations were then determined by inductively coupled argon plasma (ICAP) in a Thermo Jarrel-Ash, Model 9000 (Jarrel-Ash Division 1982).

Forage samples were processed according to methods of Fick et al (1979) and were analyzed for Ca, K, Mg and Na by atomic absorption spectrophotometry (Perkin-Elmer Corp. 1980). Forage crude protein and P were determined using procedures set forth by Gallaher et al (1975) and Technicon Industrial Systems (1978). Forage in vitro organic matter digestibility (IVOMD) was determined by a modification of the two-stage method (Tilley and Terry 1963) by Moore and Mott (1974).

Data obtained in the present study were statistically analyzed using a 3 (years) by 2 (seasons) factorial design (Snedecor and Cochran 1980) using the General Linear Models (GLM) procedure of the SAS System (SAS Institute Inc. 1987).

Soil macrominerals and pH analyses as related to month and year are presented in Table 1. Year differences (P<.05) were observed in both months for all macromineral concentrations except for sodium. No year differences (P>0.05) were observed for pH values. Seasonal differences (P<0.05) were observed only for sodium and pH. No year X season interaction (P>0.05) was observed.

Table 1: Soil macromineral and pH concentrations as related to season and year

Variable		Year 1	Year 2	Year 3	SE

Al, ppm	May	19.5**	25.2**	70.5*	14.8
	Nov	57.8* **	41.9**	84.0*	14.8
Ca, ppm	May	718**	860**	1141*	77
	Nov	679**	1140*	1244*	77
Mg, ppm	May	40.6*	91.3**	127.7**	13.9
	Nov	54.5***	101.3**	160.7*	13.9
K, ppm	May	10.3**	25.4**	52.5*	7.2
	Nov	11.6**	30.6**	65.1*	7.2
Na, ppm	May	8.8*****	15.1	27.7	4.9
	Nov	40.2****	14.2	21.4	4.9
P, ppm	May	5.43**	9.45**	20.64*	2.19
	Nov	5.77**	7.48**	13.03*	2.19
pH	May	5.4	5.4*****	5.2*****	0.1
	Nov	5.5	5.8****	5.7****	0.1

Least square means are based on the following number of samples: 21 samples per month with two months per year for three years;
*, **, *** Means among years for the same month (P<0.05);
****, ***** Means between seasons (P<0.05)

Higher (P<0.05) aluminum concentrations were found in year 3 for both months. No monthly differences (P>0.05) were found for soil Al in any year. Since the pH values were around 5.5, soil Al did not appear to have an effect on P uptake by plants (Sanchez 1981). Year 3 showed higher (P<0.05) Ca concentrations in May among years, while in November, years 2 and 3 had higher (P<0.05) Ca concentrations than year 1. Mean soil Ca values were higher than those of 437 ppm (summer-fall) and 405 ppm (winter-spring) reported by Salih (1988) from north central Florida.

Among years, mean soil Mg during May was higher (P<0.05) in years 2 and 3, while in November, year 3 had higher (P<0.05) soil Mg. Mean soil Mg values were all above adequate levels of 9.2 ppm to 21.1 ppm (Breland 1976). Mean soil Mg concentrations for four soil orders in Florida varied from 29.6 ppm to 116.9 ppm (Kiatoko et al 1982). Previous studies in the same ranch (Mooso 1982) showed soil Mg values ranging from 36.3 ppm to 79.5 ppm.

Soil P in year 3 was higher (P<0.05) in both May and November. Potassium values were similar (P>0.05) in years 1 and 2 for both months. Mean K concentrations were all below the normal level of 80 ppm (Warncke and Robertson 1976). These results agree closely with previous studies in Florida. Very low soil K values ranging from 18.2 to 46.8 ppm also were reported for the same ranch (Mooso 1982). Low K values were reported from north central Florida, as Merkel (1989) found a mean of 40.5 ppm K content in soils. Low soil K values may be due to the high K leaching from Florida soils (Gammon 1957).

Month differences (P<0.05) were observed in soil Na concentrations in year 1. Average Na content for the three years in both months was below the critical value of 62 ppm (Rhue and Kidder 1983). Soil Na values varied from 10.2 ppm to 22.1 ppm for four regions in Florida (Kiatoko et al 1982).

Soil P content was highest (P<0.01) in both months for year 3. No season effect (P>0.05) on mean soil P was observed in any of the years. Mean soil P concentrations were all below the critical level of 17 ppm (Rhue and Kidder 1983), except in May of year 3. Soil P concentrations varying from 1.9 ppm to 50.1 ppm were reported by Mooso (1982) from unfertilized pastures for the same ranch. Higher P values ranging from 14.9 ppm to 78.8 ppm were reported by Kiatoko et al (1982) in the same general region. Phosphorus deficiency may be caused by fixation of phosphates by Fe and Al (Dudal 1977).

No differences (P>0.05) among years were found for soil pH. Soil pH was highest (P<0.05) in November for years 2 and 3. Similar values were reported by Kiatoko et al (1982) and by Mooso (1982). The pH values were in the range of what is found for soils of subtropical Florida.

Forage macrominerals, crude protein and IVOMD concentrations for May and November for the three years studied are presented in Table 2. Year X season interactions (P<0.05) were observed for Mg and K. Calcium concentrations were highest (P<0.05) for year 1 for both the May and November collections. November forages contained higher (P<0.05) Ca in May in years 1 and 2. Mean forage Ca during May and November were 0.44% and 0.37%, respectively, both adequate compared to the NRC (1976) requirement (0.30%) for growing heifers and mature cows. These values are in agreement with soil Ca analyses.

No differences (P>0.05) in forage Mg concentrations among years were found. However, years 1 and 3 had higher (P<0.05) Mg in November. Mean Mg concentrations were generally adequate. In agreement, soil Mg concentrations were adequate. Kiatoko et al (1982) reported low mean forage Mg values (0.14%) for the winter season. Acid and highly leached soils resulted in reduced availability and absorption of Mg to the plants (Reid and Horvath 1980).

Forage K was higher in May than November (P<0.05) for all years with no year differences (P>0.05). For all years, mean forage K was above the requirement of 0.60% (NRC 1984) for May collections but deficient in November. Similar K seasonal variation has been reported previously (Kiatoko et al 1982). In agreement, with soil K analyses, the forage K concentrations were low during May.

Table 2: Forage nutrients as related to season and year (dry basis)

Variable		Year 1	Year 2	Year 3	SE

Ca, %	May	0.48*	0.44 **	0.41**	0.01
	Nov	0.42* *****	0.33 ***	0.38*	0.01
Mg, %	May	0.17*****	0.22	0.21*****	0.01
	Nov	0.28****	0.20	0.27****	0.01
K, %	May	0.86****	1.05****	1.09****	0.05
	Nov	0.58*****	0.54*****	0.50*****	0.05
Na, %	May	0.037**	0.032**	0.049*	0.004
	Nov	0.025**	0.032**	0.053*	0.004
P, %	May	0.13**	0.23* ****	0.21*	0.01
	Nov	0.12**	0.16* *****	0.17*	0.01
CP, %	May	8.3***	11.9* ****	9.7 **	0.4
	Nov	8.0	7.8*****	7.7*****	0.4
IVOMD, %	May	50.0****	52.5****	52.8****	1.1
	Nov	39.7* *****	41.5* *****	35.6* ***	1.1

Least square means are based on 21 samples per month (May, November)
*, **, *** Means among years for the same month (P<0.05);
****, ***** Means between months for the same year (P<0.05).

Forage Na concentrations for year 3 were higher (P<0.05) for both May and November, with no seasonal differences observed (P>0.05). Average Na levels of 0.04% for both months in the three years were below the critical value of 0.06% (NRC 1976; McDowell 1985). Similar low values of 0.05% for summer-fall and 0.02% for winter- spring was reported (Salih et al 1988). Normal values of 0.07% to 0.18% for fall and 0.07% to 0.1% for winter season were reported by Kiatoko et al (1982) for Florida.

Forage P was higher (P<0.05) in years 2 and 3 during the May collection. Seasonal difference was observed only in year 2, when forage from May (0.23%) had higher (P<0.05) P than November (0.16%). Although values were higher during May, borderline to deficient mean forage P concentrations were observed at all collection times (0.15-0.19%) below the suggested critical level of 0.25% (McDowell 1976). Similar low mean forage P values of 0.16% (fall season) and 0.10% (winter season) were observed in Florida (Kiatoko et al 1982). Salih et al (1988) found values varying from 0.20% to 0.23% during the winter-spring and the summer-fall seasons, respectively. Low forage P observed is in agreement with low soil P.

Among years, mean forage protein was higher (P<0.05) in year 2 during May, and no differences (P>0.05) among years was observed during November. Seasonal differences were observed in years 2 and 3 with higher (P<0.05) crude protein values in May than in November. Average forage protein values of 10.0% (May) and 7.8% (November) were above the critical value of 7.0% (Minson 1971). Kiatoko et al (1982) also reported normal mean forage protein values of 9.4% and 8.45% for the fall and winter seasons, respectively. On the other hand, Salih (1988) reported values of 6.3% for the summer-fall season and 6.0% for the winter-spring season.

Lower mineral and protein concentrations found in the more mature forages grazed during November agrees with Gomide (1978), who found decreased forage nitrogen, P and K with increasing forage maturity.

In vitro organic matter digestibility percentages varied (P<0.05) among years only during November. However, seasonal differences (P<0.05) were observed in all years. Forage IVOMD content was higher (P<0.05) during May in all years, averaging 51.8% for May and 38.9% for November. From northern Florida, Merkel et al (1990) reported average forage IVOMD values of 44.6%, 43.5%, 29.2%, 33.1% and 62.0% for October, November, December, January and February, respectively. These values suggest that IVOMD values during the spring season were not limiting the production of the animals (Duble et al 1971).

A limited number of forage samples (24) were analyzed for S. Average S concentration was 0.23% (±0.06) and values ranged from 0.16% to 0.40% (DMB). All forage S concentrations were above the suggested requirement of 0.10% (NRC 1984).

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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