animal health consulting

The amine–behaviour hypothesis

I began to develop this hypothesis when I went looking for an explanation as to how the autumn pasture — whose NSC had dropped since the summer, whose simple sugar content had remained unchanged, and whose fructan content was less than half that of the summer hay — could have triggered these episodes of abnormal behaviour.

In case you missed it, The Problematic Pasture contains two graphs that illustrate these seasonal changes in nutrient content.

The thing that stuck out to me was how high the crude protein (CP) content was in the autumn pasture (29.3% of dry matter). When I calculated how much surplus protein the horses would be consuming from the pasture alone, these two horses would have been consuming more than 2 kg of extra protein each day.

. . .

That is not a typo. As promised (or threatened, depending on your outlook ☺︎), here are the maths:

1. CP content. The autumn pasture contained 293 grams of crude protein per kilogram of dry matter (293 g/kg DM).

29.3% = 29.3 grams per 100 grams = 293 grams per 1000 grams = 293 g/kg

2. DM content. While I did not measure the water content of the sampled pasture, lush grass is typically at least 80% water, or at most only 20% dry matter.

Rather than send the fresh grass to the lab, I first dried it in my food dehydrator (4 hours at 35 C) to avoid spoilage between collection and analysis. That's the setting recommended for drying fresh herbs. I should probably have gone a bit longer, because the lab result showed that the grass was still 18% water, when I was aiming for a water content of less than 10% after drying, same as for a well-cured hay.

Anyway, let's proceed on the assumption that the autumn grass the horses were eating was only 20% dry matter, even though it may well have been even lower (i.e., > 80% water).

3. Grass intake. The average adult horse, grazing established pasture full-time consumes an average of 20 grams of dry matter per kilogram of body weight per day (20 g DM/kg bwt/day).[11,12]

Depending on pasture height and quality, DM intakes range from 19 to 34 g/kg bwt/day. But let's opt for the lower end and proceed with an average of 20 g/kg bwt/day.

And remember that we're still talking about dry matter intake, not yet about fresh grass intake.

At this rate, the average 500-kg adult horse would eat approximately 10 kilograms of dry matter per day (10 kg DM/day).

20 g/kg bwt x 500 kg bwt = 10,000 g = 10 kg

Assuming that the fresh grass is only 20% dry matter (80% water), the average 500-kg horse would eat about 50 kilograms of fresh grass per day.

I mentioned earlier that I'm mathematically challenged. I still have to do “if and then” sums to make these sorts of calculations. Here it is:

First, I converted the percentage (20%) to a decimal (0.2).

Then I  created my little “if and then” square…

If 1 kg of fresh grass = 0.2 kg DM (that's 20% DM)

Then ___ kg of fresh grass = 10 kg DM (the average daily DM intake)

Multiply the two knowns on the diagonal (1 kg fresh grass x 10 kg DM), then divide by the other known (0.2 kg DM) to fill in the blank.

(My primary school maths teacher must surely be rolling in her grave…☺︎)

The answer is 50: (1 kg fresh grass x 10 kg DM) ÷ 0.2 kg DM = 50 kg fresh grass

As I mentioned, daily DM intakes for horses on pasture range from 19 to 34 g/kg bwt. That works out to be a range of 47–85 kg of fresh grass per day, depending on pasture height and quality.

But let's continue with an average of 50 kg fresh grass/day.

4. Crude protein intake. Putting it all together, these horses would have been consuming an average of 2,930 grams of crude protein per day.

293 g CP/kg DM x 10 kg DM/day = 2,930 g CP/day

We didn't actually need to take that little gambol through the fresh grass intake, because the calculations above are made using dry matter figures. But I did it anyway because I was curious — and because the sheer volume (well, weight) of grass that horses may consume on lush pasture fairly took my breath away! And all because of its high water content. The horse has to eat a lot just to meet his daily energy needs, let alone all the rest.

5. Maintenance protein requirement. The recommended daily intake of crude protein for maintenance in the average 500-kg adult horse is 630 grams.[13]

It ranges from 540 grams to 720 grams. But note that we're talking about grams, not kilograms. The maintenance requirement is less than 1 kg/day.

6. Surplus protein intake. These horses will have been consuming an extra 2,300 grams of crude protein each day.

2,930 grams intake – 630 grams requirement = 2,300 grams surplus

That's an extra 2.3 kg CP/day.

And that's just from the pasture component of their diet.

At the time, they were also on three separate high-protein supplemental feeds, each around 20% CP, the reason being that the summer hay was so low in CP (6.2%). By my rough estimate, these other feeds added another 560 grams of crude protein each day.

The horses also had free-choice access to the summer hay, which while low in protein, still contributed some additional protein. So, the grand total comes to almost 3 kg of surplus protein, per day.

We cut way back on those supplemental feeds once we got the pasture results back. Even so, the horses were eating more than 2 kg of surplus proten each and every day, just from their pasture, which of course they favoured over the summer hay.

. . .

So, I got to wondering: Could a high-protein diet cause abnormal behaviour in horses? If so, how?

That's when I came across the research on food- and gut-derived amines in humans and lab animals.

To cut right to the chase, Figure 6 (below) is a pictorial summary of how I think it all works in horses on lush pasture, when both the NSC and crude protein content of the grass are relatively high.