Phosphorus Fertilization Increased Macronutrient Concentrations in Leaves of Stockpiled Tall Fescue

William E. McClain II and Dale G. Blevins, Division of Plant Sciences, University of Missouri, Columbia 65211

Abstract

Beef producers utilize stockpiled tall fescue (Festuca arundinacea Schreb. ‘Kentucky 31’) in the Midwest to extend the grazing season and reduce winter feeding costs. Previous studies indicated concentrations of some macronutrients in stockpiled tall fescue leaves were below those required by lactating beef cows in late winter and early spring. Our previous studies found P fertilization in late winter increased leaf Mg and Ca concentrations in early spring. Therefore, the objective of this study was to determine the effects of 2 years of P fertilization on leaf macronutrient concentrations of stockpiled tall fescue during late fall, winter, and early spring. Leaf concentrations of P, Mg, and Ca were higher with P fertilization than those of the untreated controls. During the second year, the highest P treatment maintained leaf P and Mg concentrations above the critical 0.2% required by lactating beef cows during peak lactation. The leaf concentrations of phloem mobile macronutrients declined from October to February with the exception of N, which reached its lowest concentration in January. Decreases in leaf concentration of mobile elements like P, Mg, N, and K may be the result of nutrient remobilization from leaves to roots during late fall and early winter as a strategy to provide support for next spring’s growth. Producers should account for the possibility of low leaf macronutrient concentrations in tall fescue during late winter, especially when beef cattle are grazing pastures on soils with low plant-available P.

Introduction

Tall fescue grows on approximately 17 million acres in Missouri making it the most important forage for the state’s beef industry (19), which ranks second in the nation in cow-calf operations (12). Cattle producers stockpile tall fescue pasture to extend the grazing season (1,2,4,24) and reduce winter-feeding costs (5). The hardy nature of tall fescue and its ability to maintain growth with the onset of cooler temperatures in late fall make it an excellent forage for stockpiling (18,23,24). This resilient cool season grass often persists with minimal management, but forage yields of tall fescue can be enhanced by fertilization. Reinbott and Blevins (17) found P fertilization improved forage yields by 1000 lb/acre compared to unfertilized control plots in southwest Missouri. Gerrish et al. (8) obtained a 35% increase in forage yields of stockpiled tall fescue with N fertilization. Although Archer and Decker (1,2) stimulated growth and crude protein (CP) of stockpiled tall fescue with N fertilization, overall forage quality was not affected. More recently, Singer et al. (22) found that a late-summer application of N increased the quality and quantity of stockpiled tall fescue.

Efficient utilization of pasture in grazing systems is not only dependent on the quantity of forage produced, but also on its nutrient quality. Studies in Tennessee and West Virginia found macronutrient concentrations in tall fescue leaves declined through winter months (4,6,7,20,21). In Missouri, the majority of soils used for tall fescue pastures are low in plant-available P (Bray I P) (14). Reinbott and Blevins (16,17) found P fertilization of soils in southwest Missouri with Bray I P soil test less than 16 lb/acre increased macronutrient concentrations in tall fescue leaves in early spring. Therefore, forage production on such soils may impact the macronutrient concentrations of stockpiled tall fescue leaves; however, there are no reports of P fertilization improving the macronutrient quality of stockpiled tall fescue.

Currently, some beef producers compensate for macronutrient deficiencies in forages by supplementing the diets of livestock grazing in winter and early spring. Reducing the need for supplementation by managing pastures with low soil P in a practical and cost effective manner that improves the macronutrient concentrations in tall fescue leaves would benefit beef producers. The objective of this research was to identify management strategies that improve the macronutrient concentrations of stockpiled tall fescue during winter and early spring.

A Stockpiled Tall Fescue Field Study

This two-year study was conducted on an established tall fescue (Festuca arundinacea Schreb. ‘Kentucky 31’) pasture (67% infection of the endophyte Neotyphodium coenophialum), at the University of Missouri Southwest Research Center near Mt. Vernon, Missouri (37°04’N; 93°53’W; elevation 1150 ft) on a Creldon silty clay loam (fine, mixed, active, mesic, Oxyaquic Fragiudalf). The site was low in plant-available P (8 lb/acre Bray I), and had the following soil test results for the 0 to 6 inch depth: pH, 5.4; soil organic matter, 3.0 %; Ca, 2687 lb/acre; Mg, 247 lb/acre; and K, 667 lb/acre.

Treatments. During the third week of August 2001, forage was removed from the pasture with a forage harvester and 10- × 25-ft plots with 5-ft alleys were flagged in six replicate blocks. Treatments consisted of P at 0, 12.5, and 25 lb/acre in the form of triple super phosphate (0-46-0). Each treatment was randomly applied to three subplots in six replicate blocks for a total of 18 plots of each treatment. The entire plot area received N at 100 lb/acre in the form of ammonium nitrate (34-0-0). In August of the following year, the forage was again removed and identical treatments were applied to the same plots. In August of each year, soil samples were collected from each plot at the 0- to 6-inch depth and analyzed by the University of Missouri Soil Testing Laboratory. For the first year, temperatures and rainfall were consistent with the 30-year averages for the area. Overall, the second year was slightly cooler and received approximately 6 inches less precipitation than the first year. The lack of rainfall provides the best explanation for the differences seen in the data between years.

Harvests and macronutrient analyses. Beginning in mid-October and monthly through April of each year, 20 of the most recently collared leaves were randomly harvested from each plot. Other than leaf harvests, forage was only removed from plots in mid-May and mid-August before each experimental year. While stockpiled tall fescue produces higher fall yields than other cool season grasses, new leaf growth usually ends by November with herbage mass declining or showing no change until spring (4,9,18). All samples were oven dried, ground, and digested in nitric acid with a microwave accelerated digestion system. Digested samples were filtered, diluted, and analyzed for macronutrient concentrations. Potassium, Ca, and Mg were determined by ICP-OES (Varian Inc., Palo Alto, CA), N by thermal conductivity of nitrogenous gases with a Leco Model FP-428 nitrogen analyzer (Leco Corp., Saint Joseph, MI), and P by colorimetric analysis (13).

Statistical analysis. The experiment was a randomized complete block design analyzed as a split-split plot in time model with repeated measures. This model was used to test for statistical significance of P treatment effects as well as interactions with month and year using PROC MIXED in SAS version 9.1 (SAS Institute Inc., Cary, NC). The main plot consisted of P treatment, harvest date (month) was considered the split plot and year was the split-split plot. Fertilization treatment and harvest date (month) were considered fixed effect factors and year and block were treated as random factors. All effects and interactions were considered significant when P < 0.05. When F test showed significance (P < 0.05), means were separated using Fisher’s protected LSD (α = 0.05). Experimental years are presented separately due to significant interactions for all macronutrients.

Macronutrients in Stockpiled Tall Fescue Leaves

Phosphorus. Phosphorus fertilization increased leaf P concentrations compared to untreated control plots throughout the stockpile period (Fig.1). Leaf P concentrations in plants from all treatments decreased during late fall and winter months, reaching their lowest levels by mid-February of both years. By mid-January of the first year, leaf P concentrations in plants from all treatments were below those required by lactating beef cows (15), and these P concentrations in all treatments remained below 0.20% through mid-April. During the second year, leaves from untreated plots remained between 0.10 and 0.15% P during late fall and winter (Fig. 1). While these forage P concentrations are adequate for dry cows, they are much lower than levels required for lactating beef cows (15). Leaf P concentrations remained around 0.20% throughout the winter with the 25-lb/acre P treatment during the second season. With the 12.5-lb/acre P treatment, leaves harvested during December, January, and February of the second year remained below the target level of 0.20% P. Stockpiled forage deficient in P could impact milk production and lead to lower calf-weaning weights (11). In stockpiled tall fescue pastures where soil P levels are low, producers should consider raising soil P through fertilization or add another source of supplemental P in order to provide proper nutritional levels for the lactating beef cow. The decline in P concentrations of the most recently collared leaves during late fall and early winter may reflect mobilization of P through phloem tissue to the root system for winter storage and subsequent utilization for spring growth.

Fig. 1. Leaf phosphorus concentrations in stockpiled tall fescue treated with P at 0, 12.5, and 25 lb/acre. The red line indicates the requirement for a lactating beef cow and the blue line for a dry cow. Treatment means (n = 18) for each month. LSD (P < 0.05) = 0.019 for comparing P treatments within a month and LSD (P < 0.05) = 0.009 for comparing months within the same treatment.

Magnesium. As with P concentrations, leaf Mg concentrations from P-treated plots were higher compared to control plots (Fig. 2). Leaf Mg concentrations in plants from all treatments declined during late fall and winter of both years, reaching the lowest levels in mid-March. The decline in leaf Mg concentrations was very similar to the reduction in leaf P concentrations, except P levels were lowest in mid-February. In March of the first year, leaves from all treatments fell below the 0.20% Mg concentration required in the diets of lactating beef cows (15). However, during the second year, the 25-lb/acre P treatment maintained leaf Mg concentrations at or above the critical 0.20% concentration. Magnesium concentrations in leaves of untreated tall fescue dropped below the 0.20% target in January, February, and March of both years, indicating a possible nutritional problem for cows in early spring/late winter calving beef herds. This is an important consideration for producers in areas where low soil P might influence the uptake of Mg by tall fescue. Our hypothesis is that this phloem mobile divalent cation, Mg, is mobilized and translocated during late fall and winter months from leaves to roots.

Fig. 2. Leaf magnesium concentrations in stockpiled tall fescue treated with P at 0, 12.5, and 25 lb/acre. The red line indicates the requirement for a lactating beef cow and the blue line for a dry cow. Treatment means (n = 18) for each month. LSD (P < 0.05) = 0.023 for comparing P treatments within a month and LSD (P < 0.05) = 0.013 for comparing months within the same treatment.

Calcium. Leaf Ca concentrations showed no effect of P treatments during the first year (Fig. 3). All treatments were at or above the dietary requirements of lactating beef cows (15) throughout both years. During the second year, leaves from the 25-lb/acre P treatment were higher in Ca than those from the 12.5-lb/acre P and control treatments. Unlike other macronutrients, Ca is not mobile in the phloem tissue of plants (10), and therefore, not remobilized from leaves to roots during winter. Calcium concentrations in leaves remained relatively constant during late fall and winter.

Fig. 3. Leaf calcium concentrations in stockpiled tall fescue treated with P at 0, 12.5, and 25 lb/acre. The red line indicates the requirement for a lactating beef cow and the blue line for a dry cow. Treatment means (n = 18) for each month. LSD (P < 0.05) = 0.016 for comparing P treatments within a month and LSD (P < 0.05) = 0.021 for comparing months within the same treatment.

Potassium. As with P and Mg concentrations, leaf K concentrations declined from fall through winter, reaching the lowest levels in February of both years (Fig. 4). In the fall of both years, P fertilization increased leaf K concentrations; however, for winter and spring there was no treatment response. During both years, leaf K concentrations were well above the requirements of lactating beef cows (15). Again, the decline in leaf K concentrations might indicate translocation of this mobile nutrient for storage in roots during winter.

Fig. 4. Leaf potassium concentrations in stockpiled tall fescue treated with P at 0, 12.5, and 25 lb/acre. The red line indicates the requirement for a lactating beef cow and the blue line for a dry cow. Treatment means (n = 18) for each month. LSD (P < 0.05) = 0.67 for comparing P treatments within a month and LSD (P < 0.05) = 0.11 for comparing months within the same treatment.

Nitrogen. P fertilization had no affect on N concentrations in the leaves of stockpiled tall fescue during the first year (Fig. 5). However, in the fall of the second year, P treatments increased N concentrations of leaves from November through February. Like the other mobile macronutrients, leaf N concentrations decreased from fall through winter of both years. Leaf N concentrations reached their lowest levels in January of both years. These results indicate that N may be remobilized from leaves earlier than the other mobile elements.

Fig. 5. Leaf nitrogen concentrations in stockpiled tall fescue treated with P at 0, 12.5, and 25 lb/acre. Treatment means (n = 18) for each month. LSD (P < 0.05) = 0.39 for comparing P treatments within a month and LSD (P < 0.05) = 0.09 for comparing months within the same treatment.

Soil Tests after Phosphorus Fertilization

The initial soil test Bray I P concentration was 8 lb of P per acre, and the University of Missouri recommends 30 to 40 lb of P per acre for pastures. Upon completion of the study, Bray I P concentration in the 0- to 6-inch depth dropped to 5 lb of P per acre in the control plots and increased to 8 and 12 with the 12.5- and 25-lb/acre P treatments, respectively (Fig. 6). Given the high acidity of the soil in the test area, Bray II analysis was used to illustrate the amount of P fertilizer adsorbed by the soil and thereby made unavailable to the plant. The Bray II analysis showed that the bulk of the P fertilizer added in the study was sorbed by the soil, with an increase of 18 and 30 lb of P per acre after 2 years for the 12.5 and 25 lb/acre P treatments, respectively.

Fig. 6. Soil test results after completion of the stockpiled tall fescue study where a total of 0, 25, and 50 lb of P per acre were applied over two years. Means ± SE (n = 18) for all treatments.

Conclusions

The physiological nature of the perennial grass tall fescue may have an impact on the macronutrient concentrations of the stockpiled forage. Producers should take this into account when utilizing stockpiled tall fescue for late-winter grazing, especially when beef cattle are on pastures with low P soils. Leaf P and Mg concentrations from unfertilized plots dropped well below the requirements for lactating beef cows in late winter and early spring, which could increase the risk for nutritional disorders. Phosphorus fertilization increased P, Mg, Ca, and to some extent K and N concentrations of tall fescue leaves during the stockpiling period.

The results show a decline in all macronutrient concentrations, except Ca, in leaves during late fall and winter, and an increase as spring growth began. The drop in leaf N, P, Mg, and K concentrations between October and February may result from remobilization of these nutrients from leaves to rhizomes and roots during late fall and winter as a storage mechanism. Leaching of nutrients from leaves is another possibility. In alfalfa, remobilized N is stored as amino acids and soluble proteins in taproots and used to support early spring growth (3,25). Remobilizing phloem mobile macronutrients to underground structures is a strategy that tall fescue may employ to have macronutrient pools available for spring growth when nutrient acquisition is slow in cool soil. Producers must pay special attention to the fertility of stockpiled tall fescue in late winter and early spring or provide livestock with mineral supplementation to avoid nutritional disorders during this time. Research is needed to determine if higher rates of P fertilization would maintain leaf macronutrient concentrations of tall fescue throughout the stockpiling season, or if liming tall fescue pastures would free up sorbed P in the acid soils found in southwest Missouri.

Acknowledgments

Contributions of Elizabeth Hamilton, Maru Kering, Matt Massie, and Melissa Remley are gratefully acknowledged. The authors would also like to thank Dr. Mark Ellersieck for statistical consultation. This research was supported by the Missouri Fertilizer and Lime Council.

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