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Š 2006 Plant Management Network.
Accepted for publication 6 April 2006. Published 18 July 2006.

Influence of Cutting Time on Alfalfa (Medicago sativa) Sugar Content and Silage Fermentation

Nasser S. Al-Ghumaiz, Richard H. Leep, and Timothy S. Dietz, Department of Crop and Soil Sciences, Michigan State University, East Lansing 48824

Corresponding author: Richard H. Leep. leep@msu.edu

Al-Ghumaiz, N. S., Leep, R. H., and Dietz, T. S. 2006. Influence of cutting time on alfalfa (Medicago sativa) sugar content and silage fermentation. Online. Forage and Grazinglands doi:10.1094/FG-2006-0718-01-RS.

Abstract

Alfalfa (Medicago sativa L.) haylage (silage) is an important component in feed rations for dairy and beef herds in the north-central region. Weather conditions in this region during first cutting don’t always allow adequate curing of forage for dry hay production, so alfalfa may be ensiled. This study was conducted to evaluate the effect of morning (AM) versus afternoon (PM) cutting time upon sugar content of fresh cut alfalfa and the lactic acid concentration of ensiled alfalfa in Michigan. The study was conducted over the 2001 and 2002 growing seasons at the Michigan State University farm in East Lansing, MI. PM cutting normally provided greater sugar content in harvested alfalfa than AM cutting, but there was no improvement in silage fermentation based on lactic acid content. There was also no difference between AM and PM cutting time on alfalfa crude protein, acid detergent fiber, and neutral detergent fiber contents.

Introduction

Silage is preserved forage that is prepared for storage in a silo under anaerobic conditions. Harvest management plays a critical role in determining its forage quality. Making silage includes several important steps, starting in the field and culminating with the animal’s consumption. Naturally occurring Lactobacillus spp. (LAB) ferments plant sugars and produces lactic and acetic acids, which reduces silage pH. Slower wilting times in the field increase the population of LAB and reduce the need for an inoculant. In addition, sugar content is an essential factor for the production of sufficient lactic acid. (6).

At the beginning of the ensiling process, the pH is high (6.0 +) and plant cells remain alive allowing aerobic bacteria to increase while the oxygen supply remains. When the oxygen supply is depleted, the anaerobic bacteria become active and increase in numbers in the oxygen free environment (4). The anaerobic bacteria ferment the sugars into lactic and other short chain volatile fatty acids. This increase in lactic acid reduces the silage pH to between 3 and 5. Low pH stops microbial activity and preserves the silage in a condition that is palatable to animals (2). Under excessively wet silage (> 70% moisture), Clostridial bacteria increases, converting plant sugar to butyric acid and amines causing dry matter loses and raising the pH. Since anaerobic bacteria depend on sugar content of the forage, it is an important factor in the ensiling process.

Alfalfa sugar content is altered during the day. During photosynthesis, plant sugar content reaches a peak at the end of a sunny day and then begins to decline until the next morning (15) leading to a diurnal variation of sugar content in the plant.

The earliest studies of diurnal variation in carbohydrate content of crops were conducted by Miller (15) on corn (Zea mays L.) and sorghum (Sorghum bicolor L). He found sugar content reached a peak between noon and 5:00 pm, and then began to decline again until the next morning. Additional studies have shown the sugar content of wheat (Triticum aestivum L.) seedling-leaves to increase from 9:00 am to 4:00 pm (14). Curtis (7) in New York reported a linear increase in sugar content in alfalfa top dry matter growth from 4.3% to 6.1 % between AM and PM cutting. Melvin (16) examined the effect of alfalfa sugar content on silage quality collected from different cutting times in Melbourne, Australia and reported that PM cut alfalfa was significantly (P < 0.01) higher in sugar than AM resulting in an increase in the level of lactic acid during silage fermentation.

Research was also conducted in Oregon on seventeen pasture grass varieties evaluated for sugar level over six cuttings (8). The sugar content was higher in the PM than in the AM cutting for all grass varieties. Animals were able to preferentially select the PM cut hays. Three ruminant species, sheep (Ovis aries), goats (Capra hircus), and cattle (Bos taurus) preferred alfalfa hay cut at sunset compared to hay cut at sunrise due to higher nutritive value, and consequently, animal production was increased by changing cutting management (10). In the same study, they reported higher digestibility of the carbohydrate fractions in the PM cut hays relative to AM cut hays. In a comparison of AM versus PM cut warm-season grasses, Fisher et al.(9) concluded that ruminant animals did not preferentially select PM cut switchgrass as they had with alfalfa and fescue hays.

Our objective was to determine whether delaying cutting time during the day, to increase sugar content, would affect silage lactic acid content as a result of better fermentation.

Procedure for Collecting and Fermenting Diurnal Alfalfa Forage Samples

The study was conducted on an established alfalfa field during the 2001 and 2002 growing seasons at the Michigan State University Farm in East Lansing, MI (42°47´N, 84°36´W, elevation of 258 m) on a Capac loam soil (fine-loamy, mixed, mesic Aeric Ochra qualf). Alfalfa fields were divided into sections for AM (9:00-10:30 a.m. EDT) and PM (4:00-5:00 p.m. EDT) cutting. Alfalfa was harvested three times per season (2001-2002) at approximately 10% bloom. In 2001, AM and PM cutting times took place in the same day where in 2002, the PM cutting time occurred the afternoon before the morning of the AM cutting time. The field was cut on 13 June, 16 July, and 21 August and on 19/20 June, 17/18 July, and 21/22 August in 2001 and 2002, respectively.

Harvesting and sampling. Alfalfa was harvested using a Carter Flail Harvester (Carter Manufacturing Co. Inc., Brookston, IN). Fresh samples were collected (500 g of each) from the freshly cut alfalfa windrow immediately after each cutting time and frozen at -15°C (5°F) to limit sugar loss from respiration until samples were freeze-dried.

Five mini-silos (18-inch height by 4-inch diameter PVC pipe) were packed with alfalfa collected from each of the AM and PM cutting times at moisture levels of 65 to 70%. In 2001, proper ensiling moisture content was estimated by visual and physical characteristics, and in 2002, a more accurate dry matter determination was obtained using a microwave oven and scale as described by Brusewitz et al. (3). The packing was done by placing the chopped alfalfa into one end of the mini-silo and rapidly pressing all the air out of the mini-silo as it was filled (Fig. 1). The mini-silos were then sealed and kept at room temperature 21 to 26°C (70 to 78°F) for more than 60 days to ensure complete fermentation as described by Burns (J. C. Burns, personal communication).

Fig. 1. Packing the PVC mini-silos.

The experimental design was a spilt-plot with five replications. The treatments were cutting time (AM or PM) as a whole plot factor and the harvest number in the growing season (1st, 2nd, or 3rd) as a subplot factor. Analysis of variance (ANOVA) was obtained using PROC GLM, and treatment means were compared using Tukey’s procedures from SAS software v. 8.2 (SAS Institute Inc., Cary, NC).

Sugar analysis of fresh alfalfa samples. Fresh samples were freeze-dried (Tri-Philizer MP [FTS, Kinetics, Stone ridge, NW]) for 3 to 4 days to bring the moisture level below 10%. Dried samples were ground to pass through a 2-mm screen using a Wiley Grinding Mill (Arthur H. Thomas Co., Philadelphia, PA) and then passed through a 1-mm screen using a UDY Cyclone Mill (Udy Mill Corp., Fort Collins, CO).

The sugar analysis was done in the Rumen Fermentation Profiling Lab-West Virginia University based on the method of partitioning of neutral detergent soluble carbohydrates using 80:20 (v/v) ethanol/water as described by Hall et al. (12).

Forage quality analyses. Forage quality was determined in 2001 samples using Goering and Van Soest (11) method to measure acid detergent fiber (ADF) and neutral detergent fiber (NDF) with the addition of 1 ml of alpha-amylase to the neutral detergent solution for the breakdown of starch. Total nitrogen was determined on the subset by the Hach modified Kjeldahl procedure (19) and crude protein (CP) was estimated by multiplying total N by 6.25. 

In 2002, fresh alfalfa forage samples were scanned with a FOSS 6500 near-infrared spectrophotometer (FOSS NIRSystems, Inc., Eden Prairie, MN) with wavelengths between 800 and 2500 nm. Reflected wavelengths were recorded.  Crude protein, ADF, and NDF were predicted from equations developed by the NIRS Consortium (Madison, WI) and the MSU Forage Lab based on wet chemical analysis. 

Lactic acid analysis of ensiled alfalfa samples. Alfalfa silage samples were also analyzed following completion of fermentation. Lactic acid content and silage pH values were obtained using the procedure of Canale et al. (5) modified by Rodrėguez- Carėas (17). The analyses were conducted at the Michigan State University Animal Science Nutrition Lab using HPLC (High Performance Liquid Chromatography) Turbochrom 3 Waters 501 pump, 712 WISP and 410 refractometer (Waters Chromatography Division, Milford, MA), and an Aminex HPX-87H column (Bio-Rad, Hercules, CA).

Cutting Timing Effect on Ensiled Alfalfa

First year (2001). Alfalfa sugar content was higher in the PM compared to the AM cutting time in the second and third harvest events (Table 1). Drought stress during July (Fig. 2) may explain the lower sugar content in the third harvest because plants close their stomata in dry conditions to conserve moisture, reducing the supply of carbon dioxide necessary to synthesize carbohydrates during photosynthesis (13). Data from the post-fermentation analyses indicates that the lactic acid concentration of alfalfa silage in the PM cut was only higher than that of the AM in the second cut. The excessively dry forage in the first and second cuts likely decreased the activity of fermenting bacteria causing lower lactic acid. Silage pH was not different between AM and PM treatments for any of the three cuttings. The PM cut alfalfa in the first year resulted in higher sugar content. These results concur with several studies conducted in other states in the U.S. such as Allen et al. (1) in Iowa, and the second year data from Thomas et al. (18); Lactic acid concentration was not affected by cutting time which contrasts findings by Melvin (16) in Melbourne, Australia. There was no effect of cutting time on forage quality parameters of NDF, ADF, and CP (Table 2).

Fig. 2. Monthly precipitation during the 2001 and 2002 growing seasons compared to the 30-year average for East Lansing, MI.

Table 1. Mean values of sugar content, dry matter, lactic acid and pH level for ensiled alfalfa harvested in the AM and PM in the 2001 and 2002 growing seasons.

Mean values within columns for each year, for AM and PM cutting time, and for each harvest number followed by different letters are significantly different.

 * Tukey, significant at P < 0.05.

** Tukey, significant at P < 0.01.

Table 2. Forage quality analyses of fresh alfalfa harvested in the AM and PM in the 2001and 2002 growing seasons.

Mean values within columns for each year, for AM and PM cutting time, and for each harvest number followed by different letters are significantly different.

 * Tukey, significant at P < 0.05.

 ^x ADF = Acid detergent fiber.

 ^y NDF = Neutral detergent fiber.

 ^z CP = crude protein.

Second year (2002). The second year also showed a higher sugar content in the PM cut alfalfa compared to the AM in all cuttings. Although there was greater sugar content in the PM fresh cut, there was no difference in lactic acid concentration between AM and PM treatments. Lactic acid concentration in alfalfa silage harvested in 2002 was much greater than that in 2001, which may be due to better control of dry matter content at ensiling. Silage pH was higher in the AM than the PM treatment in third harvest only.

There were no differences between AM and PM cutting times for NDF, ADF, and CP with the exception of ADF and NDF of cut 3 (Table 2). This may have been due to variation in alfalfa maturity within the study area.

Summary

Cutting alfalfa in the PM resulted in higher sugar content compared to alfalfa cut in the AM, which has been shown to increase palatability and animal preference in alfalfa hay. However, in this study, even though we found higher sugar content in the PM cut alfalfa, it did not increase lactic acid content in the resultant silage. In addition, alfalfa cut in the PM did not affect forage quality factors including CP, NDF, and ADF compared to AM cut alfalfa. Weather conditions in the Great Lakes region during alfalfa harvest for silage should take precedence over the decision to delay the cutting of alfalfa to the PM.

Acknowledgments

The authors gratefully acknowledge Tammy Webster at the Rumen Fermentation Profiling Lab, West Virginia University, for performing the sugar analysis. We wish to thank Dr. Steven Rust’s lab, Department Animal Sciences at Michigan State University, for their cooperation in providing the laboratory facilities for silage analyses.

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