Features Agronomy Fertility and Nutrients
Reduce forage frustrations with fertility

“Good management is the most important factor in profitable, productive forages,” says Joel Bagg, forage specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs, “because without good management, we pay for it in years like this one, where growers have been hit with a challenging growing season.” According to Bagg, good fertility is essential to both yields and the persistence of forage crops.  

Know your soil nutrient needs
Forage crops remove a lot of nutrients; therefore, they have a high nutrient requirement. One of the most important areas of forage fertility is phosphorus (P) and potassium (K) management, an area often overlooked by growers. “P and K are fundamental to producing enough forage to feed livestock and stay competitive in today’s market,” says Bagg, who illustrates the importance of P and K with an example of nutrient value loss in an alfalfa-grass mixture. A typical amount of P and K removed per tonne of hay harvested in an alfalfa-grass mixture is equivalent to 13.5 lb., or 6.1 kg of phosphate (P2O5) and 64 lb., or 24.6 kg of potash (K2O), the nutrient value of the removal is currently close 2¢ per lb., or $44 per tonne of dry hay harvested.

The key to making informed management decisions is determining what soil nutrients are needed, and Bagg says soil testing is a fundamental part of forage management. “Knowing how much P and K are available in the soil to start with is critical,” explains Bagg who recommends soil samples be taken at least every three years. Growers should use the results of a representative soil sample to determine optimum fertilizer rates. And with higher fertilizer prices, targeting your fertilizer applications more strategically will benefit not only your hay, but your bottom line too. Bagg also notes that, with the increasing cost of commercial fertilizers, the time and effort it takes to take a soil sample can easily be offset by the potential yield return.

“Unlike nitrogen, forages cannot generate P or K,” says Bagg. “And without replacing these important nutrients with manure or commercial fertilizer, soil tests will drop quickly.” Interpreting the soil test is just as important as the test itself, he notes. Growers should check the P and K levels, because if soil tests drop below optimum levels, forage yields can be significantly reduced. Research has shown significant yield losses in alfalfa when P soil test levels fall below 12 ppm and K levels fall below 120 ppm. Bagg says yield curves at low soil test levels are steep and a positive yield response from applying fertilizer will be seen when tests are below these optimum levels. “But don’t expect any extra yield from applying fertilizer once the soil tests have been built up to higher levels,” he says, pointing out that, in these cases, you can choose to apply fertilizer to replace the nutrients removed to prevent future nutrient deficiencies, “but don’t expect extra yield from any maintenance applications.”

Fertilizing with manure
“Livestock producers have an advantage in maintaining soil fertility where manure is available to apply during the rotation,” says Bagg, who notes that while dairy farms that apply a lot of manure typically have high P and K levels, K deficiency has become more common. For livestock farmers applying manure, he also recommends manure sampling to determine nutrient levels. Commercial fertilizer applications should be reduced according to the amount of P and K within manure sampling results. Bagg highlights potential advantages of applying liquid manure to forage crops, including yield and quality benefits, spreading of the workload, reduced manure storage requirements, prevention of soil compaction and reduction of environmental risk.

As a forage specialist, Bagg has seen a wide range of soil fertility levels in hayfields across Ontario. And it’s no surprise that some hayfields with very low soil fertility and yield are typically those that are infrequently or never rotated, or seldom receive manure or commercial fertilizer. At lower soil test levels, “soil mining” can occur. This should not be acceptable in a properly managed field rotation, yet, Bagg says, it goes on in many hayfields every year. He reminds growers that the short- and long-term costs of poor fertility are much higher than the cost of the fertilizer.

Shorten forage stand life in rotations
The optimum rotation length of forages depends on a number of factors, including land value and other crops within the rotation. “With high forage prices and land costs, forage yields and returns have never been more important,” says Bagg.
While the ideal maximum age of an alfalfa stand will vary, Bagg recommends a three-year forage rotation. Hayfields are often left for too long in rotations, resulting in significant yield losses. It’s also important to maximize establishment year forage yields because alfalfa yields are typically at their highest the year following establishment, and gradually decline with stand age due to diseases, loss of vigour and plant thinning. Yields can often decline to approximately 75 percent of the maximum yield by the fourth year following establishment. The decline can be even more rapid and significant with aggressive cutting schedules or soil fertility loss. “This yield loss wouldn’t be tolerated in any other crop without doing something about it, so neither should it be accepted with forages,” says Bagg.  

Growers should consider shortening the number of years of forage in rotations and using legume nitrogen credits when rotating into corn. Forage stands with greater than 50 percent legume content enable growers to deduct 100 lb. per acre of nitrogen (N) from the N requirements in the corn crop the following year. Stands with one-third to one-half legume content get an N credit of approximately 50 lb. per acre. Research shows that, in addition to the N credit, there is a significant rotational benefit of approximately 10 to 15 percent increase in corn yields when corn follows alfalfa. That can be equivalent to more than $125 per acre of additional corn. Bagg points out that the additional forage establishment costs incurred by shortening the rotation are more than offset by the N credit and corn yield benefits.

Forage fertility is made up of a number of tools and management decisions, and fertility is just one piece of the larger management puzzle. After all, forage yield and profitability is determined in part by management, and if forages are given the same level of management that is given to other crops, they can certainly boost a grower’s bottom line.