I've had some discussion on the grazing lists recently about pasture fertilization. A comment I made got some back channel inquiry asking for more explanation. The comment was to the effect that feeding out hay on pasture was the best way to fertilize. To explain why this is so a good place to begin is with this short overview of the N cycle on pasture from Dairy New Zealand.

About 15% of the N eaten by a cow is converted into milk and meat with the rest being excreted, particularly in urine . . .

For beef cattle like I raise even more of the N is excreted since they milk less, or not at all in the case of steers being raised for meat. In terms of the N budget for the pasture it's much the same as if I'd spread the N value of the forage as little white granules, but the cattle also get the nutritional benefit of the carbohydrates in the forage. The cattle grow and the grass grows too.

It's not quite that simple since the N is deposited in the form of dung and urine, organic N, which is not useful to plants until nitrified by soil bacteria. This takes time and there are some N losses in the process. The urea in the urine is made available the quickest, but the bacterial conversion is a multi-step process that at one step produces ammonia gas. Some of that is lost to the atmosphere, almost as much as the cattle retain. The N in dung takes longer to nitrify and there is less of it, less than half as much as in urine, and some of it is lost as ammonia gas too.

The end stage of nitrification is nitrate, the primary plant nutrient, the same stuff produced by bacteria in symbiosis with legumes. Some of it is lost to leaching below the root zone or away from the pasture in surface flows, and some is lost to denitrifying bacteria which consume it, converting it to various forms such as N2O, a GHG, and N2, completing the cycle back to the atmosphere.

There are chemicals that can be added to pastures that suppress denitrifying bacteria, but it isn't clear that they are cost effective, and the idea of suppressing soil life seems contrary to good soil management. But that's part of the beauty of using hay as an N source. Since it is fed out a little at time in a near continuous stream there is a constant supply of nutrients in various stages of nitrification, but never a great surplus of nitrate that can leach away or denitrify.

Looked at from another angle the dung and urine is forage for soil life. They contribute a great deal to soil health. It is their "wastes" that feed the plants. The hay has fed the cattle, fed the soil life, and fed the plants giving a triple benefit whereas the little white granules do less.

Having all the burgeoning life in the soil makes it deeper, better able to hold moisture when it's scarce and drain excess water when it's not. It's cooler when it's hot, warmer when it's cold. This can contribute as much to forage growth as the extra nutrients.

In short, feeding hay nourishes the cattle, fertilizes the grass and increases soil organic matter. To properly determine the cost/benefit of hay feeding all of these factors must be counted.

In practice, when I have the hay available, I try to be just a little bit overstocked for the amount of forage that I can grow and make up the shortfall with hay. Forage growth varies with the season so it's a constantly variable pasture substitute. Also, the animals' consumption varies over time. A young animal grows and needs more forage to keep growing as metabolic overhead increases with size. A cow's needs vary greatly during gestation and later lactation. It's takes close attention but using hay as a substitute for forage to top up the animal's needs allows all of the available grown forage to be consumed, which reduces waste.

Taking all of this together, using hay as the fertilizer source in a management intensive grazing system increases grazing efficiency by getting more of the available grown forage into the animals, reduces nutrient loss by doling it out a bit at a time, and improves pastures in a variety of ways so that more forage can be grown in future. Pastures become ever better for many years, allowing stocking rates to continually rise year over year while pasture health increases.

This doesn't mean that there is no place for legumes such as clover for home grown nitrates, and it doesn't mean that there is no place for N concentrates. Home grown N is cheaper, and the soil bacteria that decompose organic matter and nitrify organic N work very slowly when temperatures are low, so you may need to dose pastures in early spring to get a proper response. Timing matters. A light application of ammonium nitrate in spring is immediately plant available and gets the year off to a good start. Even the legumes like that, and they will pay you back later.