Nov. 13, 2023 11:27 AM

Despite the fact there is no symbiotic relationship between alfalfa and ruminants, the nutrient composition of alfalfa complements the nutritional needs of ruminant animals surprisingly well.

While we know more about some of alfalfa’s nutrient components, such as neutral detergent fiber (NDF), rumen undegraded protein (RUP), starch, fat, and minerals than we do about others, such as pectin, water soluble carbohydrates (WSC), and rumen degraded protein (RDP), all play an important role in the nutrition of ruminant animals.

Alfalfa contains the highest protein of any forage. For formulating ruminant diets, the crude protein is further subdivided into that which is degraded in the rumen (rumen degraded protein, RDP) and the remainder that escapes ruminal degradation (rumen undegraded protein, RUP) to flow into the small intestine as part of the animal’s absorbed, metabolizable protein (MP) supply.

The conventional belief is that most of the protein that is degraded in the rumen is eventually broken down to ammonia that is absorbed across the rumen wall into the blood stream, ultimately excreted as waste urea nitrogen in urine. While this is partially correct, most of the RDP is used to support microbial protein synthesis in the rumen. This microbial protein flows into the small intestine as the other major contributor to the ruminant’s metabolizable protein supply.

A peptide advantage

The RDP in alfalfa has been shown to be a rich source of peptides, derived primarily from a photosynthetic enzyme, ribulose-1,5-bisphosphate carboxylase (Rubisco). Peptides have been shown to stimulate microbial protein synthesis, in vitro.

The benefit of this was recently demonstrated by researchers at the Miner Institute in New York, where high-producing dairy cows were fed diets that were similar in nutrient content but contained five different ratios of alfalfa hay to corn silage in the forage portion that constituted 62% of the diet dry matter (DM). Resulting milk protein production was maximized somewhere between a 30:70 to 50:50 ratio (DM basis) of alfalfa to corn silage in the forage portion of the diet.

Presumably, this optimal ratio range resulted from improved ruminal microbial growth and protein synthetic activity from alfalfa being present in the diet at these levels, supporting the value of alfalfa RDP in the rumen. The other interesting takeaway from this study was that DM intake and milk production were unaffected across a wide range of alfalfa hay to corn silage ratios, even as high as the 90% alfalfa forage treatment, dispelling the myth that lactating cows cannot perform well on high alfalfa forage programs.

We know much about the largest of alfalfa’s nutrient components, NDF, and its digestibility (NDFD). Taken together and expressed as ruminal undigested NDF (RuNDF), it can have a profound impact on optimizing rumen fill while improving intake and ruminal digestion of the entire diet. For a particular forage, the amount of RuNDF is calculated by multiplying the undigested NDF (100% - NDFD, expressed on an NDF basis) by the NDF content of the forage. The sum of the RuNDF amounts from each of the forages in the diet represents an approximation of rumen fill.

Multiple carb sources

As a forage, alfalfa is well suited in this respect since its RuNDF content is relatively moderate compared to most other forages because of its lower NDF content and its high rate of NDFD. Recent genetic modification of the lignin content in HarvXtra-traited alfalfa varieties has allowed greater flexibility in fine-tuning the NDFD advantage. Furthermore, while high-quality alfalfa can have an NDF content similar to corn silage, it contains much less starch (usually only 3%) than corn silage, which makes alfalfa less prone to contribute to subacute ruminal acidosis.

Instead of starch, alfalfa contains pectin (considered by many as “soluble fiber”) and water-soluble carbohydrates. Taken together, these two “nonfiber carbohydrates” constitute around 28% of the alfalfa plant’s DM, but we know relatively little about their potential nutritional benefits other than being a source of energy for the animal.

Pectin is rapidly degraded by rumen microbes producing acetate and propionate, but not lactate like rapidly fermented starch. It can be assumed that the WSC fraction also has a high rate of ruminal digestion. Therefore, the criticism that alfalfa lacks a rapidly, ruminal digestible carbohydrate fraction like that found in corn silage may be unfounded since the rapidly digestible carbohydrates (pectin + water-soluble carbohydrate + starch) amount can rival the starch content of an average corn silage.

One thing we do know about pectin, however, is that it has a high cation exchange capacity (CEC). This, along with much of the mineral cations in the ash of alfalfa, gives alfalfa a high CEC. This high CEC is linked to alfalfa’s contribution to the diet’s greater buffering capacity, which promotes greater milkfat synthesis by the cow.

A nutritional powerhouse

While there are many factors contributing to alfalfa’s nutritional value in diets, it’s apparent that NDF, NDFD, RUP, RDP, and ash are important nutrient components contributing to its feeding value for ruminants. The content of NDF, and its digestibility, can have a major impact on intake, digestibility, and feed efficiency through their contribution to the RuNDF content of the diet.

The amount of RUP and RDP will contribute to the metabolizable protein content of the diet both directly and indirectly through supporting ruminal microbial protein synthesis. Knowing the proportions of RUP and RDP in the CP of alfalfa could help optimize the correct dietary balance to maximize the metabolizable protein supply to the ruminant at the greatest efficiency of CP use.

Furthermore, contrary to popular misconceptions, we are beginning to better understand that:

  1. Average to high-quality alfalfa does not depress DM intake or milk production and can be used at high levels in the forage programs of high-producing lactating dairy diets.
  2. The RDP fraction of alfalfa is high in peptides that can stimulate milk protein production, apparently by stimulating microbial protein synthesis, and is not useless to the animal.
  3. Alfalfa contains a rapidly, ruminally digestible carbohydrate fraction that can rival the starch content of an average corn silage.
  4. Alfalfa possesses one of the highest CEC values of any forage. This enhances the diet’s buffering capacity, which can promote greater milk fat synthesis by the cow.

Most computer software formulation programs do not account for all the benefits of alfalfa in ruminant diets. Nutritionists who don’t know this will allow their formulation programs to undervalue alfalfa and reduce the level in the diet. Other nutritionists who know the value of alfalfa in dairy diets will be sure to include a minimum of 20% of diet DM (based on multiple studies) as alfalfa.

If you didn’t think so before, hopefully now you will believe that alfalfa is truly amazing.

This article appeared in the November 2023 issue of Hay & Forage Grower on pages 12-13.

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