Yield and Quality of Winter Wheat Forage As Affected by Lime

Hailin Zhang, Jackie L. Schroder, Eugene G. Krenzer, Olga M. Kachurina, Department of Plant and Soil Sciences, and Mark E. Payton, Department of Statistics, Oklahoma State University, Stillwater 74078

Abstract

The majority of winter wheat (Triticum aestivum L.) planted in the southern Great Plains is intended for forage or dual-purpose (grazing and grain) production. Up to 28% of Oklahoma wheat fields have a soil pH less than 5.5 (0- to 6-inch depth) and should be limed to sustain forage and grain wheat yields. This study investigated the effects of seven lime rates on fall forage yields and quality of a winter wheat cultivar planted in a field with the initial pH of 4.5. Soil pH was increased as lime rate increased. Forage yields nearly doubled in the first year when lime rates of 1.25 tons/acre and higher were applied. An application of 1.25 tons/acre, half of the rate recommended to raise soil pH to 6.8, was found to be the most economical, as the value from increased forage production is more than double the cost of liming at this rate. The cost of liming at other tested rates can also be recouped if increased forage yields occurred for two or more years. Our results suggest it is economical to lime low pH soils used for forage or dual-purpose winter wheat production.

Introduction

Soil acidity in the central and southern Great Plains has become a serious problem for wheat production, as both forage and grain yields have been reduced due to low soil pH (Fig. 1). A recent study showed that 28% of the wheat fields had a pH less than 5.5 in Oklahoma (11). Soil acidity is harmful for plant growth due to nutritional disorders (e.g., deficiency of Ca and Mg, decreased availability of P and Mo) as well as the immediate toxicity of soluble Al, Mn, and H⁺ (1).

Fig. 1. Impact of soil acidity on different wheat varieties (left front is an Al-susceptible variety).

The use of wheat as feed for cattle has increased in recent years in the Great Plains due to low wheat grain prices and higher profit margins in cattle production (Fig. 2). Winter wheat may be grazed full season, harvested for hay or silage, or grazed during the vegetative growth stage and then harvested for grain (7). The total of fully grazed and dual-purpose (grazing/grain) wheat production occurs on about 66% of the wheat acreage in Oklahoma (3). Although wheat forage is of great economic importance, the impact of soil acidity and liming on wheat forage production has not been well documented. Higher use of N fertilizer and higher removal of cations when using the wheat for both forage and grain compared to grain only may accelerate acidification, especially when soils are not limed. Although numerous studies have investigated the effects of lime on grain yields of various crops, only a few studies have reported how the biomass production of wheat is affected by soil pH and liming rates (2,9). The objectives of the present work were to: (i) determine the effect of lime applications on soil pH; (ii) examine the effect of liming on the yield and quality of winter wheat forage; and (iii) examine the economics of liming winter wheat for forage.

Fig. 2. About 2/3 of winter wheat fields planted in Oklahoma are grazed either partially or completely.

Experimental Approach

The field study was conducted at a producer’s field in Garfield County, Oklahoma where Tonkawa winter wheat, an acid-susceptible cultivar, was grown. Soil pH was measured in a 1:1 soil-to-deionized water suspension (10). Soil texture was determined using the hydrometer method (5), while organic carbon was determined by combustion using a LECO CN 2000 (LECO Corporation, St. Joseph, MI). The soil was a Tabler silt loam with an initial soil pH of 4.5 and contained approximately 1.25% organic carbon. Liming requirement was determined using the SMP buffer method (8). The calibrated lime rate to raise the pH to 6.8 for this soil was 2.5 tons of effective calcium carbonate equivalent (ECCE) per acre. Typically 25% of the value required for raising the pH to 6.8 or 0.5 tons/acre, whichever is greater, is recommended for continuous wheat production to avoid root disease in Oklahoma. Test plots were established on 10 July 1997 under conventional tillage. A randomized complete block design with four replicates was used. Plots were 10 by 18 ft and were fertilized according to soil test results and yield goal for wheat. Commercially available calcium carbonate limestone was used for the treatments, which consisted of seven lime rates: 0 (no lime), 0.16, 0.32, 0.63, 1.25, 2.5, and 3.75 tons of ECCE per acre. Wheat forage was harvested by clipping to the soil surface in December 1997 and 1998, and in January 2000. Forage samples were oven-dried at 65ºC for 24 h, weighed for yields, and analyzed for protein for several mineral nutrient elements. Forage protein was determined using automated dry combustion (LECO CN 2000) while forage mineral element contents were determined by dry-ashing followed by dissolution in HCl and subsequent analyses by inductively coupled plasma atomic emission spectroscopy (ICP-AES) (4).

The effect of lime application on yearly wheat forage yield and on soil pH was analyzed using analysis of variance with PC SAS Version 8.2 (SAS Institute, Cary, NC) and PROC MIXED. The simple effects of treatment for a given year were examined using a SLICE option in an LSMEANS statement. If the slice test for a given year was significant, the treatment means were compared with a DIFF option. A 0.05 level of significance was used for all comparisons. The effect of lime application on wheat forage protein and element content and on relative yield was determined by analysis of variance using PC SAS Version 8.2 (SAS Institute, Cary, NC) and PROC GLM. If the analysis of variance was significant, the treatment means were compared with an LSMEANS statement and a PDIFF option. A 0.05 level of significance was used for all comparisons.

Effects of Liming on Soil pH

For each time sampled, with the exception of Day 1, soil pH increased as lime application rate increased (Table 1). Soil pH also increased with time, with the increase of soil pH being the fastest during the first 2 months, with a slight further increase by the end of the study (Table 2). The initial change in soil pH was probably more rapid than farmers might expect, due to the thorough incorporation and heavy rain that occurring immediately after lime application. One quarter of the normal lime rate required to raise pH to 6.8 (0.63 tons/acre) increased soil pH to almost 5.0 in the first two weeks and then slowly increased it to a stable level (5.5) in the following three and half months (Fig. 3). Wheat should grow normally at this soil pH (6). A slight pH increase in the control plot was also observed over time (Table 2), probably due to lime contamination from the treated plots. Soil pH remained elevated during the entire study period (850 days) without signs of decrease even for the lowest lime rate. This confirms that a single lime application can have a long-term effect on soil pH.

Fig. 3. Soil pH changes with time after lime was applied at 7 lime rates (tons of effective calcium carbonate equivalent per acre) for 850 days. The calibrated full lime rate to raise the pH to 6.8 for this soil was 2.5 tons of ECCE per acre.

Effects of Liming on Winter Wheat Forage Yields

Yield response to lime application varied slightly among the three years (Table 3). In 1997, wheat forage yields increased with lime rate up to 1.25 tons/acre. During 1998, differences were not observed (P > 0.05) between the low rates (i.e., 0.16 to 0.63 tons/acre) of lime application, and a lime application rate of 1.25 tons/acre was needed to produce an increase in wheat forage yields. Similarly, in 1999 low application rates (0.16 and 0.32 tons/acre) did not result in increased wheat forage yields but a lime application rate of 0.63 tons/acre and greater resulted in increased yields (P < 0.05). In 1997, 1998, and 1999, lime application rates greater than 1.25 tons/acre did not result in increased wheat forage yields (P > 0.05) (Table 3).

Relative wheat forage yields (three-year average) increased with lime rates up to 1.25 tons/acre (Fig. 4). Similar to individual years, lime application rates greater than 1.25 tons/acre did not result in increased relative wheat forage yields (P > 0.05) (Fig. 4). This increase of forage is significant for wheat farmers engaged in cattle production since most farmers in the central Great Plains use winter wheat for grazing during the winter and spring or for grazing and grain dual production.

Fig. 4. Three-year average relative wheat forage yields with different lime application rates (100% yield equaled 2290 lbs/acre, the highest of all treatments and replicates). Bars with the same letter are not statistically different at an alpha level of 0.05.

The estimated return to investment for liming acid soil indicates that all lime rates had a positive net revenue except for the lowest and the highest rates (Table 3). The half rate (1.25 tons/acre) had an impressive 269% rate of return in three years (Table 3). In fact, the first year’s forage yield increase was sufficient to offset the liming costs, since the cost of liming was about $25/acre and the value of the increased forage production was $27.6/acre (Fig. 5). The effect of liming would normally last more than five years; therefore, the potential economic return would be much higher than the cost of liming at the end of the third year (Fig. 5). This indicates that it is economical to lime acid soils for winter wheat forage or for dual purpose (grazing and grain) wheat production. Lime application rates greater than 0.63 tons/acre significantly reduced (P < 0.05) the protein content of wheat forage, probably due to increased biomass diluting protein content (Table 4). Liming did not significantly affect (P > 0.05) mineral element concentrations in wheat forage (Table 4).

Fig. 5. The revenue from wheat forage for the liming rate of 1.25 tons/acre in the first 3 years after lime was applied. Bars with different letters are statistically different at an alpha level of 0.05.

Conclusion

Liming can increase soil pH and improve wheat forage yields. It is economically beneficial to lime acidic soils for winter wheat forage production, even at rates below the estimated full lime rate. When forage was harvested in the early vegetative stage, protein and mineral nutrient contents were not significantly affected by liming but protein content was reduced slightly due to increased biomass.

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