ENHANCING YIELD AND PERSISTENCE OF ALFALFA THROUGH IMPROVED PHOSPHORUS AND POTASSIUM MANAGEMENT

Transcription

1 ENHANCING YIELD AND PERSISTENCE OF ALFALFA THROUGH IMPROVED PHOSPHORUS AND POTASSIUM MANAGEMENT W.K. Berg, B.C. Joern, K.D. Johnson, S.M. Brouder, and J.J. Volenec Department of Agronomy, Purdue University, West Lafayette, IN ABSTRACT Fertilization with phosphorus (P) and potassium (K) is essential to maintain alfalfa productivity. Our objective was to examine how P and K fertilizer influenced alfalfa yield and yield components, plant persistence, and root physiology. Replicate plots of P (, 5, 1, 15 lbs P 2 O 5 /acre) and K (, 1, 2, 3, 4 lbs K 2 O/acre) treatments were arranged in a factorial design. Forage harvests occurred four times annually between 1998 and 23 and yield, mass per shoot, and shoots per area were determined. Roots were dug in May and December of each year to estimate plant populations and determine whether plants were dying during "summer" (May to December) or during "winter" (December to May). Roots were analyzed for starch, sugar, amino acid, and protein concentrations. Total annual yield increased with application of P and K, but K application did not increase first harvest yield. Yield increases were due to greater mass per shoot, whereas stems/area were not related to forage yield. Plants died during summer and not during winter. Although P fertilizer increased forage yield significantly, it decreased plant populations by producing fewer but larger individual alfalfa plants. Fertilization with P alone resulted in faster stand thinning than observed in plots provided both P and K, and those left unfertilized. This rapid stand loss was associated with low concentrations of root reserves including starch, amino acids, and protein in taproots. Balanced P and K nutrition is essential to maintain stands and have high forage yield. INTRODUCTION High yield and excellent forage quality make alfalfa (Medicago sativa L.) the forage of choice in many producers livestock systems, but intensive harvest management and poor winter hardiness can undermine yield, persistence, and ultimately profit. Improved fertilizer management represents one approach for increasing alfalfa yield and persistence, but our understanding of how alfalfa responds to P and K application is rudimentary. Our objective was to determine how alfalfa yield components and root physiology are altered by P and K fertilizer application. Our results may lead to new fertilizer management strategies that increase the profitability of growing alfalfa. METHODS In May 1997, a three-acre field of Pioneer Brand 5454 alfalfa was established. Initial soil tests levels were approximately 9 ppm K and 5 ppm P. In September 1997, plots of four P treatments (, 5, 1, 15 lbs P 2 O 5 /acre) and five K treatments (, 1, 2, 3, 4 lbs K 2 O/acre) were arranged in a factorial design (2 treatment combinations) and were replicated 4 times. Beginning in May 1998, forage was harvested four times annually and yield, mass/shoot, and shoots/area determined. Roots were dug in May and December of each year to determine plants/m 2 and to determine

2 when plants died; during "summer" (May to December) or during "winter" (December to May). Root tissues were analyzed for starch, sugar, protein, and amino acid concentrations. RESULTS AND DISCUSSION Yield Forage yield increased markedly with P and K application (Fig. 1). Highest yields were obtained when at least 2 kg K/ha and 5 kg P/ha were provided annually. Addition of K without P did not increase yield, whereas addition of 5 and 75 kg P/ha increased yield of K plots over control plots (K P). However, these yield increases resulted from greater first-cut (May) yields in 1998 to 2, and have recently been offset by severe stand losses the K high-p plots in 22 (see Fig. 1). Yield Components Alfalfa yield is comprised of three yield components: plants/area, shoots/plant, and mass/shoot, that when multiplied together determine forage yield. An increase in forage yield (Fig 1) should be the result of increase(s) in one or more of these yield components. Greater alfalfa yield obtained with P and K fertilization has primarily resulted from increased mass/shoot (Fig. 2). Mass/shoot has consistently been associated with P- and K-induced increases in forage yield each growing season, whereas, the other yield components have either decreased or have not been influenced by P and K addition. Increased mass/shoot can occur as a result of two different mechanisms: rapid initiation of new shoots after defoliation ( bud break ); and faster elongation rate of alfalfa shoots after bud break. Both of these factors increase dramatically with addition of P and K. Increased levels and enhanced mobilization of stored reserves may be physiological factors contributing to the rapid shoot initiation and growth after harvest. Through the first five years of this study, increased stems/area has not been positively associated with forage yield (Fig. 3). We observed nearly a 1-fold range in forage yield at stem densities of 3 to 5 stems/m 2. Traditionally, 4 stems/ft 2 (43 stems/m 2 ) has been used as an indicator of the agronomic viability of an alfalfa stand; stands possessing fewer than 4 stems/ft 2 should be destroyed. While we observed a reduction in yield when fewer that 25 stems/m 2 (about 25 stems/ft 2 ) are present (Fig. 3), high stem counts (>6 shoots/m 2, >6 shoots/ft 2 ) did not automatically result in high forage yield. Intermediate values of 3 to 6 stems/ft 2 were generally associated with the highest forage yields in this study, but because of low mass/shoot in unfertilized plots, these shoot densities were also associated with low yield in many plots. Our data indicate that using a cut-off for stand viability of 4 shoots/ft 2 is not appropriate for assessing the productivity potential of an alfalfa stand. Plants/area declined rapidly after initial establishment of this alfalfa stand (Fig. 4). Since establishment of our stand in 1997, plant population has declined with time and in May 23, there were approximately 6 plants/ft 2 (about 6 plants/m 2 ) averaged over all treatments. Contrary to popular belief, extensive plant losses have occurred during

3 summer and not during winter. Competition for light, water, and nutrients in summer, as well as defoliation every 3 days may have increased the frequency of plant death in summer, and thinned stands prematurely. In addition, injury during the previous winter may have weakened plants that subsequently die during summer. Work has begun in an adjacent set of larger plots that will permit us to determine when alfalfa plants die in summer, and to explore the physiological basis for their demise. Potassium fertilization has not influenced trends in plant population, but to our surprise, addition of P fertilizer has generally decreased plant populations (Fig. 5). Addition of P fertilizer results in much larger crowns and root systems as compared to plants receiving no P. The enhanced growth of the P-sufficient alfalfa may increase competition among adjacent plants leading to the death of physiologically less fit plants early in the life of the stand. The physiological characteristics of these P-sufficient plants that make them competitive are under study, but may include a greater root mass that allows for superior water and nutrient absorption and fast regrowth due to enhanced mobilization of stored root reserves. Balanced P and K Nutrition and Alfalfa Persistence Recent and very extensive stand losses that are associated with specific fertility treatments have prompted us to examine in detail the physiological basis for alfalfa death. Our experimental design placed Replicate 4 on the poorest fertility soils located at this site. During Summer 22, we noticed a severe stand decline in plots where P had been applied without K. To our surprise, these stand losses were even greater than what we observed in plots where no P and K fertilizer had been applied. To understand this stand decline in detail, we grouped the plots in Replicate 4 where P is provided without K (K+25 kg P/ha/yr; K+5 kg P/ha/yr; K+75 kg P/ha/yr) so we could consider them as a single treatment (K Plus P). The responses of these K Plus P plots were compared to those of plots in Replicate 4 that had been provided 2 kg K/ha/yr with these same P rates (2 kg K/ha/yr+25 kg P/ha/yr; 2 kg K/ha/yr+5 kg P/ha/yr; 2 kg K/ha/yr+75 kg P/ha/yr) designated hereafter as 2K Plus P. The 2 kg/ha/yr K rate was selected for comparison because it provided good yields (Fig. 1) without being excessive, and because several of the 2 K plots were immediately adjacent to the K Plus P plots that had suffered extensive plant stand loss. We also included the control plot (K P) from Replicate 4 for comparison. Trends in plant populations for these three "treatments" within Replicate 4 were similar to those observed for the entire study, with plant populations declining from 16 plants/m 2 to about 1 plants/m 2 between May 2 and May 22 (Fig. 6). Extensive stand loss occurred in all plots between May and Dec. of 22, but losses were especially acute in K Plus P plots such that yield data could not be obtained from these plots within Replicate 4 at the July and September forage harvests. Stand counts in December confirmed that these plots contained less than 2 plants/ft 2 ; below the 4 plants/ft 2 minimum generally used to define an "acceptable" alfalfa stand.

4 This fertility-specific loss in plant stand provided us an excellent opportunity to examine the negative impact of nutrient imbalance on the root physiology of alfalfa, and relate these findings to alfalfa persistence. Root starch represents the largest pool of nonstructural carbohydrate available for shoot regrowth and respiration of crowns and roots after harvest. Root starch concentrations (Fig. 7) were unaffected by fertility treatment in May 2, but thereafter root starch concentrations of the K Plus P plants were consistently lower than those observed in the K P and 2K Plus P plants. At Harvest 1 in May 22 root starch levels of the K Plus P plants averaged 6 mg/g dry wt. (6% of dry wt.), which was about onehalf the concentration observed in roots of the 2K Plus plants and the K P control plants. This low root starch concentration may be below the minimum necessary for rapid shoot regrowth and plant survival. Sugars represent another pool of carbohydrate available for alfalfa regrowth and respiration after harvest. To our surprise, root sugar concentrations of the K Plus P plants with poor persistence were higher in May 22 when compared to the other fertility treatments (Fig. 8). This suggests that the low root starch concentrations observed in the K Plus P plants was not due to low sugar concentrations limiting root starch synthesis prior to harvest in May 22. It also indicates that high root sugar concentrations alone are not sufficient to ensure good alfalfa persistence under the conditions of this study. Recently we have examined the role of root proteins, and specific polypeptides called vegetative storage proteins (VSPs) in alfalfa shoot regrowth after harvest. Our findings to date indicate that about 5% of the N found in regrowing alfalfa shoots 1 days after hay harvest was remobilized from these taproot VSPs. Root protein concentrations did not vary in a consistent manner among the three fertility treatments (Fig. 9). Remaining to be resolved is the impact of P and K imbalance on protein composition of these alfalfa roots, and specifically VSP abundance. This is important because reduced abundance or loss of a single protein can have devastating, and often lethal effects. These analyses are currently being conducted. Amino acids represent an additional pool of reserve N we examined in roots of these alfalfa plants. Like root starch, amino acid concentrations were lower in roots of the K Plus P plants when sampled in May and Dec. 21 and May 22 (Fig. 1). The very low amino acid levels in the K Plus P plants in May 22, coupled with the very low root starch concentrations described earlier (Fig. 7) together may have slowed regrowth after harvest in May 22, and resulted in poor competitive ability of plants and ultimately extensive stand losses we observed. We have recently initiated a second study at this site where large plots are sampled frequently after harvest in June and again in September to determine root reserve levels at harvest and to follow their use during alfalfa shoot regrowth. In addition, it should be kept in mind that we are only able to obtain root composition data on plants that survive, not those that die because of

5 nutrient imbalance. As a result, the reserve levels measured in roots of these surviving plants may be different (higher) than those of plants that died as a result of harvest in May 22. Nevertheless, these data provide clues as to the root reserves that differ between unfertilized alfalfa, those provided balanced P and K, and those receiving imbalanced mineral nutrition (only P). Clearly, nutrient imbalance (adding P without K) has much graver consequences for alfalfa survival than we had anticipated. From an alfalfa persistence point of view, producers are better off not fertilizing than applying P alone if they suspect soil K levels are adequate. Obviously, the best choice is to soil test and apply P and K as required to meet yield goals of the alfalfa stand.

6 1 Yield, Mg/ha Phosphorus, kg P/ha/yr Potassium, kg K/ha/yr Figure 1. Influence of P and K application on cumulative forage yield of alfalfa between 1998 and 23. Yield, kg/ha Mass/Shoot, g 1998 y= x-579x 2 R 2 = y= x-2258x 2 R 2 =.97 2 y= x-17x 2 R 2 =.9 21 y= x-3149x 2 R 2 = y= x-491x 2 R 2 = y= x-222x 2 R 2 =.83 Figure 2. Influence of mass per shoot on forage yield of alfalfa over the years 1998 to 23. All regressions were highly significant except for the 21 growing season.

7 Yield kg/ha "Critical" Shoot Density of 4 shoots/ft Stems/m 2 Figure 3. Influence of stems/m 2 on forage yield of alfalfa in 2 to 22. In our study forage yield is independent of the "critical shoot density" of 4 stems/ft 2 (43 stems/m 2 ) generally thought of as being required for high forage yield. Plant Population, plants/m Dec 97 May 98 Dec 98 May 99 Dec 99 May Mar 1 May 1 Dec 1 May 2 Dec 2 May 3 LSD.5 =14

8 Figure 4. Changes in alfalfa stand during the six years of the study. Data are averaged over all fertility treatments. Plants could not be dug in December 2 because of frozen ground, so instead plants were dug and counted in March 21. The "critical plant density of 4 plants/ft 2 is equal to 43 plants/m 2. The largest changes in plant stand are occurring between May and December of each year. Plants/ft 2 =(plants/m 2 )/1.7. Plant Population, plants/m kg P/ha/yr 25 kg P/ha/yr 5 kg P/ha/yr 75 kg P/ha/yr LSD.5 =12 LSD.5 =1 LSD.5 =13 LSD.5 =11 LSD.5 =11 May 21 Dec 21 May 22 Dec 22 May 23 Figure 5. Changes in alfalfa stand between May 21 and May 23 as influenced by P fertilization. Data are averaged over the K fertility treatments. The "critical plant density of 4 plants/ft 2 is equal to 43 plants/m 2. Plants per m K P K Plus P 2K Plus P Extensive Stand Loss 4 May- Mar-1 May-1 Dec-1 May-2 Dec-2

9 Figure 6. Changes in alfalfa stand between May 2 and December 22 as influenced by P and K fertilization. Data are from Replicate 4 where extensive plant loss occurred in plots provided P fertilizer without addition of K fertilizer. Plant populations are presented for the control plot (K P), plots provided 25, 5, and 75 kg P/ha/yr without additional K (K Plus P; mean of these three P treatments in Replicate 4 ± standard error), and plots provided 2 kg K/ha/yr receiving 25, 5, and 75 kg P/ha/yr (2K Plus P; mean of these three treatments in Replicate 4 ± standard error). Plants could not be dug in December 2 because of frozen ground, and instead were dug in March 21. The "critical plant density of 4 plants/ft 2 is indicated by the horizontal line. Plants/ft 2 =(plants/m 2 )/1.7. Root Starch, mg/g dry wt K P K Plus P 2K Plus P May- Mar-1 May-1 Dec-1 May-2 Figure 7. Root starch concentrations of alfalfa between May 2 and May 22 as influenced by P and K fertilization. See the heading of Fig. 6 for treatment details. Root Sugar, mg/g dry wt K P K Plus P 2K Plus P May- Mar-1 May-1 Dec-1 May-2

10 Figure 8. Root sugar concentrations of alfalfa between May 2 and May 22 as influenced by P and K fertilization. See the heading of Fig. 6 for treatment details. Root Protein, mg/g dry wt K P K Plus P 2K Plus P May- May-1 Dec-1 May-2 Figure 9. Root protein concentrations of alfalfa between May 2 and May 22 as influenced by P and K fertilization. See the heading of Fig. 6 for treatment details. Root Amino Acids, mg/g dry wt K P K Plus P 2K Plus P May- May-1 Dec-1 May-2 Figure 1. Amino acid concentrations of alfalfa roots between May 2 and May 22 as influenced by P and K fertilization. See the heading of Fig. 6 for treatment details.