Dormancy Indices, Growth Stages, and Forage Quality of Summer-dormant and Summer-active Tall Fescue

C.P. West1, J.L. Underwood1, D.P. Malinowski2, and C.A. Guerber1, and B.C. Grigg1

1University of Arkansas, Fayetteville, AR, 2Texas AgriLife Research, Vernon, TX USA ABSTRACT Tall fescue [Lolium arundinaceum (Schreb.) Darbysh.] populations can vary in their degree of summer activity vs. summer dormancy, which in turn can impact flowering patterns and forage quality in the spring. An index of summer dormancy was calculated as a ratio of summer dry matter (DM) yield to autumn DM yield (Index A). This index was compared to the index of Norton et al. (2008) (Index B), which relates the summer yield of a population to that of the maximum summer-yielding population. Index A is calculated independently of a maximum-yielding cultivar and is more conducive to statistical analysis than Index B. Populations were compared for spring maturity rating and forage quality. Summer dormant populations matured earlier in the spring than summer-active KY-31 with resulting earlier declines in forage quality. INTRODUCTION There is increasing interest in introducing tall fescue germplasm exhibiting summer dormancy into warm temperate zones to lend improved summer drought survival. Methods are needed for quantifying the degree of summer dormancy since tall fescue populations and cultivars from Mediterranean sources are incompletely summer dormant and are variable in the expression of that trait. The relationship between the degree of summer dormancy and timing of spring flowering can impact forage quality in the spring, a period that is critical for grazing and hay production. In general, summer-dormant populations flower earlier than summer-active ones, which can result in stemmier, lower quality forage. One objective was to compare two indices of summer dormancy, Index A based on a ratio of summer:fall yield, independently of the performance of other populations, and Index B (Norton et al., 2008), based on a ratio of summer yield of the population in question to that of the highest yielding variety in a test. An additional objective was to compare three populations (one summer active and two showing varying degrees of summer dormancy), each with and without endophyte infection, for the timing of spring flowering and trends in forage quality. METHODS Dormancy Indices To assess the degree of summer dormancy expression, two indices, A and B, were calculated using 2007 yield data (irrigated only). Index A consisted of a summer:autumn DM yield ratio, whereby the sum of June, July, and August growth periods constituted the summer yield, and the sum of September, October, and November periods constituted the autumn yield (Equation [a]).

[a]

In this ratio, a lower value signifies greater dormancy, while a higher value signifies greater summer activity. Index B was proposed by Norton et al. (2008) (Equation [b]).

[b]

Σ summer yield (June, July, August) Index A = Σ autumn yield (September, October, November)

summer yield Index B = [1 - mean summer yield of maximum yielding cultivar * 10]

Again, the summer yield constituted the sum of June, July, and August growth periods, but this was divided by the maximum yielding cultivar for the summer, which was KY-31. The maximum-yielding cultivar was determined as the mean of the four replications within each water regime, and index values were calculated for each plot. Index B is designed to produce a scale from zero to 10, with zero signifying least dormancy among the populations compared and 10 signifying greatest dormancy. The summer-dormant populations were ‘Grasslands Flecha’ and an experimental germplasm TX06V-B-FA, referred to herein as TX for brevity. Growth Stage Stage of maturity of plants was determined prior to each biomass harvest from March to June in 2008. Growth stage was determined on 20 random tillers according to Sanderson et al. (1997) as follows: tiller emergence (0.5), leaf development stages (1 to 10), culm development quantified via the number of elongating internodes (11 to 19), boot stage (20), inflorescence emergence (21 to 29), inflorescence complete (30), beginning of anthesis (31), and full anthesis (32). Forage Quality Forage quality was determined in 2008 from each monthly biomass harvest from March to early June. Dried, endophyte-infected biomass samples were ground to pass a 1-mm screen. Plants from the four replications and two irrigation treatments were evaluated for neutral detergent fiber (NDF, essentially total cell wall matter minus pectin.) and acid detergent fiber (ADF, essentially cellulose and lignin) concentrations using near-infrared reflectance spectroscopy. Total nitrogen was determined for crude protein concentration (total N x 6.25) by LECO combustion. Biomass yields in March were bulked across replications within each water regime to provide an adequate sample volume for forage quality analysis, but all replications were sampled in May and June. RESULTS AND DISCUSSION Summer dormancy indices Indices A and B resulted in different scales and reverse numerical values (Table 1); however, the relative rankings of the cultivars’ dormancies were the same. In both cases, the summer-active cultivar, KY-31, exhibited the greatest summer activity, and TX exhibited the greatest dormancy expression. Table 1. Summer dormancy indices A and B for irrigated plots only; means, standard deviations (SD), and coefficients of variation (CV). Index A† Index B‡

Population Mean SD CV Mean SD CV % %

KY-31 1.60a§ 0.20 12 0.57§ 0.80 140

Flecha 0.90b 0.08 9 4.61 0.10 2

TX 0.55c 0.07 13 6.84 0.41 6 † Lower values indicate greater summer dormancy, equation [a]. ‡ Higher values indicate greater summer dormancy, equation [b]. § Dissimilar superscripts indicate means differed at P<0.05. ANOVA not performed for Index B because of non-homogeneous variances.

The standard deviation (SD) of KY-31 for Index B was relatively high because the four-replicate data points varied substantially around a mean of nearly zero, yielding a high coefficient of variation (CV). Index B, as proposed by Norton et al. (2008), involves a ratio of summer yield:summer yield of the maximum-yielding (“control”) population for that season, always forcing the index value for the latter population to be zero or close to zero. In large trials with multiple entries and locations, the population used as the maximum-yielding control may vary among years or among locations, which may alter the ranking of the populations’ dormancy index values, irrespective of a particular population’s actual dormancy. Index A was created so that the index relies on a ratio strictly within a given population, thus avoiding the possibility of varying the control population. Growth stage Mean growth stage was more advanced in Flecha and TX than in KY-31 on 4 March and 1 May. TX had a less advanced growth stage than KY and Flecha on 2 June (Fig. 1) because its earlier-flowering shoot apexes were mostly removed at the May harvest, thereby shifting the plant sooner into the vegetative regrowth phase. These results are consistent with those of Norton et al. (2006) in which the relatively summer dormant Flecha reached anthesis earlier than the more summer active Demeter at Mauguio, France (43°37’N, 4°01’E).

Figure 1. Growth stages for spring 2008 sampling dates. Means were of 20 plants per plot averaged across irrigation treatments, endophyte treatments, and four replications. Means with the same letters were not significantly different at α=0.05. FL=Flecha, TX= TX06V-B-FA. The role of anthesis in the induction of summer dormancy in tall fescue, and additionally on forage quality, has practical implications for forage utilization and management. The early-flowering, Mediterranean types shift from high quality, leafy stages to lower-yielding, stemmy stages, and slow down initiation of new leafy growth somewhat early in the spring. Therefore, the time period for producing grazeable forage during the spring is reduced when compared with continental types, which exhibit later flowering and lush leafy regrowth. Spring haymaking is

also adversely affected because of earlier reproductive development in Mediterranean types, which presents challenges to field curing and harvesting of hay without rain damage. Forage quality Forage quality analyses were conducted to determine the quality of each cultivar at a specific harvest date, rather than comparing the qualities at the same stage of maturity. Generally, forage quality decreases as maturity increases because of cell wall lignification and a decreased proportion of leaves to stems. Thus, the effect of the cultivars’ varying maturity dates on forage quality was of importance. On 1 May, when the dormant populations were at inflorescence emergence (heading) and KY-31 was in boot stage (pre-heading), KY-31 exhibited a lower NDF and ADF than Flecha and TX (Table 2). On 3 June, TX was vegetative (post-flowering), while KY-31 and Flecha were still in a reproductive stage, and accordingly, TX had the lowest NDF and ADF and highest crude protein levels. The latter was expected as TX was in a vegetative (post-flowering) stage, whereas Flecha was still reproductive (stemmy) (Fig. 1). Surprisingly, Flecha exhibited a greater crude protein concentration than KY-31, even though Flecha was at a more advanced growth stage. Table 2. Fiber and crude protein analyses for three populations with means of four replications and two irrigation levels.

Measurement

Population Sampling date

1 May 3 June ----------- g kg-1 DM ------------ Neutral detergent fiber KY-31 547b 624ab

Flecha 583a 637a TX 586a 590b Acid detergent fiber KY-31 276b 347a

Flecha 292a 334b TX 302a 296c Crude protein KY-31 130 107c Flecha 124 134b TX 129 162a Means with different superscripts differ at P<0.05. TX= TX06V-B-FA CONCLUSIONS A negative trait of tall fescue germplasm exhibiting summer dormancy is earlier spring maturity and consequent earlier declines in forage quality, relative to summer-active KY-31. Acknowledgements. Research was supported by USDA-ARS Agreement 6227-21310-008-38S. REFERENCES Norton, M.R., F. Volaire, and F. Lelievre. 2008. Measuring summer dormancy in temperate

perennial pasture grasses. Aust. J. Agric. Res. 59:498-509. Norton, M.R., Volaire, F., Lelievre, F. 2006. Summer dormancy in Festuca arundinacea Schreb.,

the influence of season of sowing and a simulated mid-summer storm on two contrasting cultivars. Aust. J. Agric. Res. 57:1267-1277.