Improving alfalfa crude protein with nitrogen

From time to time, the question is raised about fertilizing alfalfa with nitrogen (N) to either boost yields or improve crude protein (CP) concentration. With alfalfa being a nitrogen-fixing legume, such a practice runs counter to both logic and research findings.

Michelle Leinfelder-Miles, an extension farm advisor with the University of California-Davis (UC) based in Stockton, recently addressed the question of fertilizing alfalfa with nitrogen in a UC-Extension Alfalfa & Forage News blog. She had the question posed to her by a hay grower whose customer was seeking higher CP alfalfa hay.

“The best way to improve crude protein is to cut early, choose a more dormant variety, and manage the harvest to retain the leaf fraction,” Leinfelder-Miles notes. “Retaining the leaf fraction is important because the protein content of the leaves is higher than that in the stems.”

The farm advisor outlines several reasons why applying N fertilizers to alfalfa is generally a bad idea.

“While N fertilizers can in some cases increase the apparent crude protein of the forage by a point or two, this ‘protein’ is not actually well utilized by ruminants,” Leinfelder-Miles writes. “Nitrogen fertilizer usually results in a higher nonprotein nitrogen content (NPN), which is not protein but free N in various forms such as nitrate or free amino acids.”

She explains that these free forms of N are immediately released into the rumen upon ingestion and are incorporated into ammonia. This ammonia must be excreted by the animal at a metabolic cost, reducing feed energy. This also results in excess urea in the manure.

Economics also comes into play when considering a N fertilizer application to alfalfa. Though Leinfelder-Miles notes that yield bumps are occasionally seen with applied N, they rarely are economical.

She also points out that applying commercial N comes at the expense of reducing N fixation by the alfalfa. Mostly, applied N would just replace the fixed N.

“Atmospheric N contributions to alfalfa growth are a major environmental benefit, and it's a shame not to take advantage of it,” Leinfelder-Miles notes. “Nitrogen applications to pure alfalfa stands also can encourage the encroachment of grassy weeds,” she adds.

Finally, the farm advisor points out that a higher CP content may come at the expense of a lower energy content. Thus, it may not make sense to maximize CP values if this means lower concentrations of total digestible nutrients (TDN).