Improving Alfalfa’s Leaf-to-Stem Ratio and Its Impact on Forage Quality and Fiber Digestibility

Don Miller, PhD.
Director of Product Development
Alforex Seeds

Plant Breeders are continually striving to improve the forages we feed our livestock. These genetic improvements and other factors has increased milk production per cow in the U.S. by 50-60% in the last half century. This large jump in production has been aided by the utilization of higher digestible feeds and high-intake forages such as alfalfa.

Despite its reputation for high forage quality, alfalfa can vary from high to low quality for a variety of reasons. In an effort to maintain a consistent level of forage quality there has been a concerted effort to understand what plant components influence these variations in forage quality.

The whole top growth of alfalfa is fed to the livestock. The nutrient content and digestibility of alfalfas two components, leaves and stems, figures greatly in its feeding value at harvest. Any reduction in forage quality, separate from harvest losses, is attributed to two factors that are related to plant maturity at harvest. First is a variation in the alfalfa leaf-to-stem ratio. The second is the increasing lignification of stem tissue during plant maturation.

The alfalfa leaf-to-stem ratio fed to the livestock greatly affects its forage quality. Thus, the adage more leaves = better quality, more stems = less quality. If we take a closer look at the chemical analysis of the plant top growth it soon becomes apparent that most of the feeding advantage of alfalfa is found in the leaves and to a lesser extent in the stems.

Benefits of high leaf-to-stem ratio

• Leaves have 2-3 times the protein content of stems.
• Leaves contribute the majority of feeding value in an alfalfa forage sample.
• The leaf fraction of alfalfa accounts for up to 70% of the Relative Feed Quality (RFQ) of a forage sample.
• Neutral Detergent Fiber(NDF) of leaves is more digestible than stem NDF.
• With increasing plant maturity alfalfa leaves remain relatively unchanged as far as digestibility is concerned. However, stems can become 30-40% less digestible.

Variation in the leaf-to-stem ratio:

The shift in the leaf to stem ratio of alfalfa as it matures is well documented. Immature alfalfa (prebud to bud) has the highest leaf to stem ratio (approx. 60/40), and as a result has the highest quality. The ratio approaches 50/50 at early flowering, and then reverses to having less leaves than stems (~40/60) at full maturity. This reversal of leaf to stem ratio adversely affects the feeding value of the forage.

The ratio of leaf-to-stem content in alfalfa is not constant. Leaf percentages can decrease as the crop matures. With the importance of leaf content on forage quality, knowledge of how the leaf-to-stem ratio changes with increasing plant maturity can be very important to the producer in obtaining optimal forage quality at harvest. The more alfalfa leaves a producer can capture at harvest the better. One tried and true method producers use to optimize leaf content has been to harvest the crop early (10% bloom). Early harvest sacrifices some yield but can improve forage quality in several ways. Immature alfalfa has a higher concentration of leaves, higher crude protein and a higher level of dry matter digestibility (DMD). Prebud alfalfa often has CP>22, ADF <30 and NDF < 40, where mature alfalfa can be significantly lower in quality with CP<17, ADF>36, and NDF>50.

Stem Lignification:

The second factor that can affect alfalfa forage quality is stem lignification. As the plant matures the lignin content of the stems increase as a means of supporting plant functions and keeping the top growth from lodging. However, with this increase in lignin in the plant tissue also comes a decrease in fiber digestibility. This is reflected in the fact that Acid Detergent Fiber (ADF) and Neutral Detergent Fiber (NDF) values increase with plant maturity. The increase in lignin content in the plant tissue makes the forage more difficult to digest in the rumen. This in turn slows down passage and animal feed intake. All of this factors negatively affects milk production.

Recent Innovations Improve Alfalfa Forage Quality:

Recent breeding efforts to improve forage quality has been directed at improving the alfalfa leaf-to-stem ratio and improving the fiber digestibility of the stem.

In regards to the alfalfa leaf-to-stem ratio, past improvements for leaf content involved an increase in the leaflet number and/or size of leaves (Multileaf alfalfa). The most recent improvement in forage quality has been in the area of modifying the plant architecture throughout the plant canopy. This includes the lower stems and the significant increase the overall leaf percentage. This improvement, coupled with an increase in leaf disease resistance, has improved overall leaf content and the leaf-to-stem ratio at harvest.

The second breeding innovation has been the improvement of the fiber digestibility of alfalfa’s stem component. A new class of alfalfa varieties with reduced lignin content is contributing to a significant improvement in whole plant fiber digestibility. Along with this improvement comes an increase in harvest flexibility. This allows the producer to harvest on their normal cutting schedule with better forage quality. They also have the option to delay harvest while maintaining acceptable forage quality and potentially increase yield.

Summary:

Digestibility of alfalfa decreases as the plant matures, mainly due to two factors, an increase in stem lignin and a negative shift in the leaf to stem ratio. Until now the best remedy to overcome these factors was to shorten the harvest interval to improve forage quality.

The recent release of alfalfa varieties with improvements in plant architecture that improve the leaf to stem ratio or improvements in stem fiber digestibility are giving producers more tools to increase its alfalfa’s overall forage quality. Depending on the amount of alfalfa in the dairy ration these new innovations have the potential of increasing milk production 2-3 lbs/ cow /day.

The alfalfa variety on shows improved leaf-to-stem ratio while the conventional variety on the left has not.

Additional Resources:

• You can find more information about Hi-Gest Alfalfa with high leafy-to-stem ratios on our Hi-Gest Alfalfa Technology page.
• Read more about Alforex Seeds research into higher leaf-to-stem ratios in this post, Alfalfa Breeding to Improve Leaf to Stem Ratio.
• Hi-Gest Alfalfa Technology – high leaf-to-stem ratio
• Article as published in the NAFA newsletter