Forage and soil mineral concentrations over a three-year period in a warm climate region of central Florida. II. Trace minerals

Forage and soil mineral concentrations over a three-year period in a warm climate region of central Florida. II. Trace minerals

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 trace mineral status of forage (bahiagrass) and soils on a ranch in central Florida. Forage and soil samples were collected twice a year (May and November) for three years. Three composite samples from each of seven pastures were collected each sampling date.

Higher (P<0.01) forage values of Cu, Mn, Mo and Se were found in November. Iron and Zn were higher (P<0.01) in May. In general, higher (P<0.05) trace elements values were found in year 2. Among forage trace minerals, only Mn and Mo were adequate. Concentrations below the critical level were found for forage Co, Cu, Fe, Se and Zn in both seasons in all years.

All soil trace elements were higher (P<0.01) in year 3. Only Cu was higher (P<0.05) in November. Concentrations below critical levels were found for soil Cu in both seasons of year 2 and in May of year 1 and for Mn in both seasons in all years.

Mineral status of soils and forage influence mineral status of grazing livestock, but many other animal factors and mineral interactions also play an important role (Towers and Clark 1983). Grazing livestock have to depend largely upon forages to fulfil their mineral requirements. Forage can rarely satisfy completely the needs for each mineral (McDowell 1977). 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. Most soils in Florida are acid, infertile and sandy in texture (Fiskell and Zelazny 1972). Native Florida pastures have been reported to be low in dry matter yield and to be deficient in some plant nutrients. From Florida, Becker et al (1965) reported that nutritional anaemia or "salt sick" disease in cattle was caused by a Cu, Co and Fe deficiency.

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

The purpose of this study was to evaluate forage and soil trace mineral concentrations over a three-year period in central Florida.

An experiment was conducted for three years at Deseret Ranches in Florida. Three herds of crossbred beef cattle (1/4 to 3/8 Brahman crossed to British breeds) were assigned to three treatments of different levels of P supplementation. Pasture fertilization and management and collection procedures for soils and forages are described in the companion paper (Espinoza et al 1990).

Forage and soil samples were collected at the same month and at the same site. The main improved tropical forage species that were collected in all pastures in which the experimental animals grazed was bahiagrass (Paspalum notatum Flugge). To a lesser (less than 5% of total) extent, all pastures were associated with native grasses and legumes. A total of 126 of both forage and soil samples were collected during the three-year period (21 samples per month).

Soil samples were analyzed for Cu, Fe, Mn and Zn according to standardized procedures (Rhue and Kidder 1983). Minerals were extracted from soil using Mechlich I extracting solution method (.05 N HCl + .025 N sulphuric acid). Soil mineral concentrations were then determined by Inductively Couple 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 Cu, Fe, Mn and Zn by atomic absorption spectrophotometry (Perkin-Elmer Corp. 1980). Cobalt and Mo were determined by flameless atomic absorption spectrophotometry (Perkin-Elmer Corp. 1984) and Se by a modified fluorometric technique (Whetter and Ullrey 1878). Sulfur concentration was determined using LECO model S-132 S analyzer, Warrendale, Penn. Reference material (e.g., tomato leaves) from the National Bureau of Standards (NBS) was included as an internal standard with all forage samples analyzed for trace mineral content.

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).

Year differences (P<0.01) were observed for all soil trace mineral concentrations in both May and November (Table 1). Month differences (P<0.05) were observed only for Cu in year 1, with November highest. Month X year interaction (P<0.05) was observed only for Cu.

Among years, year 3 showed highest (P<0.01) Cu concentrations in May. Mean soil Cu values below critical level of 0.3 ppm (Rhue and Kidder 1983) were observed in May of year 1 and in May and November of year 2. Higher values (0.3 ppm to 0.7 ppm) of soil Cu were reported (Mooso 1982) from the same area. Soils with less than 0.6 ppm of extractable Cu are considered deficient for pastures and crops (Horowitz and Dantas 1973).

Among years, soil Fe concentration was higher (P<0.01) in year 3 during May. No month differences (P>0.05) were observed in soil Fe for any of the years. Mean Fe values were generally high compared to the critical level of 2.5 ppm (Viets and Lindsay 1973) for Florida soils. Higher soil Fe values (6 ppm) were reported by Mooso (1982). Merkel et al (1990) from north central Florida reported 14.3 ppm as mean Fe content in soils.

Table 1: Soil trace mineral concentration as related to season and year

Variable	Month	Year 1	Year 2	Year 3	SE

Cu, ppm	May	0.089 **	0.095**	0.337*	0.053
	Nov	0.350* ***	0.128**	0.354*	0.053

Fe, ppm	May	6.09**	5.58**	10.15*	1.34
	Nov	5.77* **	3.52**	8.70*	1.34

Mn, ppm	May	0.88**	1.82**	3.22*	0.47
	Nov	1.02**	2.67*	3.40*	0.47

Zn, ppm	May	0.70**	1.24**	4.37*	0.6
	Nov	0.77**	2.01**	4.42*	0.6

Least square means are based on 21 samples per month with two months per year for three years;
SE = standard error of the least square mean;
*, ** Means among years for the same month with different superscripts differ P<0.01;
***, **** Means between seasons for the same year with different superscript differ P<0.05;

Among years, higher (P<0.01) soil manganese was found in year 3 in May. In November, year 3 was higher (P<0.01) than year 1 but similar (P>0.05) to year 2. Mean soil Mn values observed were below the critical level of 5 ppm (Rhue and Kidder 1983). Mooso (1982) also reported low values ranging from 0.9 ppm to 2.2 ppm Mn in soils of the same area. Even though soil Mn was low, it apparently was adequate since forage concentrations of the mineral were adequate (i.e., 39 ppm to 66 ppm). Manganese availability is affected by soil acidity, with Mn more available for pH values around 4.0 (Leeper 1947). Also, higher concentrations of organic matter result in increased Mn solubility.

Year 3 had higher (P<0.01) soil Zn content in both months. Mean soil Zn values below the critical level of 1.5 ppm (Sanchez 1976) for normal plant growth were found in May and November during year 1 and in May in year 2. Previous studies on the same ranch (Mooso 1982) showed soil values ranging from 0.7 to 2.2 ppm of Zn.

Mean forage trace mineral concentrations for May and November of the three years are presented in Table 2. No year effect (P>0.05) was observed on forage trace mineral concentrations, except for Cu (P<0.05), iron (P<0.05) and molybdenum (P<01). Month differences (P>.05) were observed for iron, molybdenum and zinc. Year X month interactions (P<0.05) was observed in all forage trace mineral concentrations observed.

Table 2: Forage trace minerals related to season and year (dry basis)

Variable	Month	Year 1	Year 2	Year 3	SE

Co, ppm	May	0.05	0.07	0.03	0.01
	Nov	0.05	0.04	0.07	0.01
Cu, ppm	May	2.8**	3.6*	4.2*	0.4
	Nov	4.4*	2.9**	4.2*	0.4
Fe, ppm	May	42.6 **	49.1* ****	42.0 ***	1.7
	Nov	37.3 ***	36.6 ***	47.6* ****	1.7
Mn, ppm	May	48.4	60.2	38.6	6.2
	Nov	54.6	55.8	66.0	6.2
Mo, ppm	May	0.42	0.57	0.48*****	0.2
	Nov	0.29***	0.84**	3.63* ****	0.2
Se, ppm	May	0.07	0.05*****	0.06	0.007
	Nov	0.07	0.09****	0.07	0.007
Zn, ppm	May	17.7	20.6****	22.2****	1.8
	Nov	16.6	13.8*****	11.0*****	1.8

Least square means are based on 21 samples per month;
*, **, *** Means among years for the same month (P<0.05);
****, ***** Means between months (P<0.01)

Mean forage Co concentrations were all below the NRC (1984) requirement of 0.1 ppm. Cobalt concentrations ranging from 0.09 ppm to 0.11 ppm in four regions of Florida were reported by McDowell et al (1982). Low concentrations of Co also were reported by Merkel et al (1990) for the northern central region of Florida. There are factors that interfere with Co absorption by ruminants; Grace (1983), cited by Mayland et al (1987), indicated that Mn and Fe reduce Co absorption. McDowell et al (1984) reported that, with the exception of P and Cu, Co deficiency is most often the limiting element for grazing livestock in tropical areas.

Mean forage Cu concentrations were lower (P<0.05) during May in year 1 and during November in year 2. Forage Cu values were all below the critical level of 8 ppm (Jones 1972). Similar values (3.4 ppm to 5.2 ppm) were reported by Merkel et al (1990) from the north central part of Florida. McDowell et al (1982) reported higher values varying from 22.3 to 51.5 for the dry and wet season, respectively. High concentrations of Mo and S interfere with Cu absorption. A Cu-Mo ratio in herbage of 2.0 or greater is desirable to avoid molybdenosis (Ward 1978).

Forage Mo concentrations were different (P<0.05) among years in November but not in May. Month difference was observed only in year 3, with the November collection highest (P<0.01). Mean Mo values were all below the toxic level of 6 ppm suggested by McDowell (1985). Similar values were reported from Florida by McDowell et al (1982) and Merkel et al (1990). The critical Cu-Mo ratio of 2 or less (Ward 1978) was reached during November in year 3.

Forage Fe concentrations varied (P<0.05) among years in both months. All mean forage Fe concentrations observed were below the critical level of 50 ppm (Jones (1972). McDowell et al (1982) reported mean values ranging from 127 ppm to 130 ppm for the winter and fall seasons, respectively. Similar normal values also were reported by Merkel et al (1990). Iron deficiency is rare in grazing cattle due to a generally adequate content in forages (McDowell et al 1984). However, under Florida conditions, Fe deficiency in cattle grazing on sandy soils has been reported (Becker et al 1965).

Forage Mn levels were similar (P>0.05) among years and between months (P>0.05). All Mn values, except that of May in year 3, were adequate according to the critical value of 40 ppm (McDowell 1985). Under Florida conditions, higher concentrations of Mn in forage were reported by McDowell et al (1982) and Merkel et al (1990).

Forage Se values among years were similar (P>0.05) in both months. Month differences were observed only in year 2, with higher (P<0.01) concentrations in November. All mean forage Se values were below the critical level of 0.2 ppm (NRC 1984). Similar low values also were reported for the same general area by McDowell et al (1982) and Merkel et al (1990). In some areas, cattle can grow normally on pastures containing 0.03 ppm of Se (Mayland et al 1987).

Forage Zn concentrations were similar (P>0.05) among years in both months. Month differences were found only in years 2 and 3 with higher (P<0.01) Zn concentration in May. Mean forage Zn concentrations found were below 25 ppm suggested as a critical level (Mayland et al 1980). Previous studies in the same area (Mooso 1982) indicate that bahiagrass Zn concentrations vary from 17.3 ppm to 27.6 ppm. Zinc concentrations may be as high as 30 ppm and occasionally higher, but this concentration declines rapidly as plants mature and levels can decrease to less than 15 ppm (Mayland et al 1987).

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