A question came to me from a crop consultant. His alfalfa grower asked him how he could increase crude protein (CP) in his alfalfa. The buyer of the alfalfa, for the most part, is happy with the hay. For example, the buyer is happy with the total digestible nutrients (TDN), but he would like to see a little higher CP. The consultant said that the grower is generally on a 28-day cutting cycle and is generally cutting the hay pre-bloom. He wondered if nitrogen (N) fertilizer would help to improve CP.
The best way to improve CP is to: 1) cut early, 2) choose a more dormant variety (but give up yield), and 3) manage the harvest to retain the leaf fraction. Since this grower is already cutting pre-bloom, and since the grower is not yet replanting, that would leave option 3. Retaining the leaf fraction is important because the protein content of the leaves is higher than that in the stems. Trying not to rake the hay when it is especially dry might help to retain the leaves.
Let's now focus on the consultant's question of whether N fertilizer could help to improve CP. The UC Irrigated Alfalfa Management production manual states that N fertilizer has resulted in higher CP in some instances but that an equal or higher number of trials showed no improvement to CP with N fertilizer. Dan Putnam, Rollie Meyer, Vern Marble, and other forage researchers have, for a long time, recommended against fertilizing alfalfa with N. This is based upon field data, economics, and logic!
1. Forage Quality. While N fertilizers can (in some cases) increase the apparent CP of the forage by a point or two, this "protein" is not actually well utilized by ruminants. N fertilizer usually results in a higher non-protein nitrogen content (NPN) which is NOT protein, but free N in various forms (e.g. nitrate or free amino acids), which is immediately released into the rumen upon ingestion and forms ammonia. The ammonia must be excreted by the animal at a metabolic cost (ATP), so it is actually costing feed energy. It also results in excess urea in the manure. So, even though it looks like the protein is a little higher, it isn't actually. Remember, CP is measured N content (not just protein) multiplied by 6.25.
2. Economics. Small differences in yield are sometimes (not always) observed with applied N; however, those are rarely economically advantageous. Remember that the uptake levels of alfalfa are very high. A 10-ton Central Valley alfalfa crop will remove about 700 lbs of N, which with losses, one would need to apply close to 1,000 lbs N/year to meet the N needs of the crop. One could never cost-effectively fertilize to satisfy this need.
3. Losing your free N. N applications or high soil N have the tendency of suppressing N2 fixation by making the Rhizobium lazy. Fertilizers would mostly just replace fixed N. Atmospheric N contributions to alfalfa growth are a major environmental benefit, and it's a shame not to take advantage of it.
4. Weeds. N applications encourage weeds, especially grasses. This negatively impacts quality.
5. Trade-off with Energy. Keep in mind that some alfalfa hay crops that have low N and low CP also have high TDN (energy values) such as the well-managed Intermountain spring cut hays grown under cool temperatures. This is due to dilution - if carbohydrates accumulate in the leaves, (e.g. 5-8 percentage points higher), then CP (and NDF/ADF) will be lower. When something goes up, something else goes down. Since energy tends to be more valuable in the marketplace, however, this is a good thing!
Having said all of this, there are some rare situations where N fertilization may be helpful to get the crop going after the roots have been compromised, but even these are unusual. Rachael Long detailed this in the blog When is N fertilization to alfalfa beneficial? Almost Never!
Dan Putnam does have a lingering question about applying N fertilizer in alfalfa and that is whether very small amounts (e.g. via drip irrigation) might be effective at hastening regrowth after each cutting. Growers using drip have done this, and Dan thinks it could work with overhead sprinklers or with buried drip lines where the N can be 'spoon fed' and carefully managed. We need data, however, to prove whether or not this would be effective. Dan suspects the differences would be minor.