Interpreting hay analysis results
Crude protein (CP) is simply a measure of the amount of nitrogen in the sample, the assumption being that most of the nitrogen is in the form of proteins (organic molecules composed of amino acids).
Most grass hays from well-tended hay fields have a CP content of between 8% and 12% (as sampled). From Equi-Analytical's records of over 92,600 samples of grass hay from across the US between 2000 and 2016, the average CP (as sampled) was 10%, and the range was 6.4% to 13.6%.
In my experience, grass hays that fall below 8% CP (as sampled) often have other nutritional inadequacies - not to mention poor palatability - and when fed as the main or sole component of the horse's diet they result in protein malnutrition, even in sedentary adult horses. If all else about the hay is acceptable, then this problem can be remedied by adding a little alfalfa to the horse's diet. (Good quality alfalfa is typically at least 16% CP.)
At the other end of the spectrum, grass hays that are well above 14% CP (as sampled) make me suspicious of heavy fertilization with synthetic nitrogen-containing fertilizer. In such circumstances, plant growth may be so rapid that nutritional quality, particularly mineral content, can be sacrificed. The hay may look and smell great, but its mineral profile may leave something to be desired - and to be addressed with supplementation.
Remember, too, that 'crude protein' is simply a measure of nitrogen content; it does not measure protein specifically. So, heavily fertilized grass hays with CP values of 15% or 16% may not necessarily be meeting all of the horse's protein needs.
Grass hays with a CP in the 12% to 14% range tend not to be a problem, particularly if the reason for the slightly higher CP content is because there are some legumes in the mix. Wisely fertilized hay fields may also yield grass hays with these CP values, and with good mineral profiles.
So, while I consider 8% CP a 'hard limit' at the lower end of the range for all horses, I generally don't mind grass hays nudging up above 12% CP, as long as the hay otherwise meets the horse's needs. However, there is one circumstance in which I may want to set a 'hard limit' of 12% CP at the upper end of the range:
In very hot conditions, horses on high-protein diets can have trouble with heat regulation, particularly if they are having to work in hot, humid conditions. Under such circumstances, limiting the CP for the total diet to between 10% and 12% meets the working horse's needs for maintenance and recovery without interfering with heat regulation.
Lysine is an essential amino acid that is used as a marker of protein quality in horse feeds. Lysine cannot be made in sufficient quantity by the horse, so it must be consumed in the diet (hence the term essential amino acid). For the average adult horse who is not doing much and fed mostly hay, an estimated lysine content of at least 0.3% (as sampled) is adequate, although higher is better.
Crude Protein and Estimated Lysine