Could the Persian Empire logistically support an army several million strong?

[Erpingham]

Why should the pack animals eat from their grain? If Texan cattle herds of thousands of animals can cover hundreds of miles and remain in good condition solely from living off the land, then a smallish caravan that isn't under the time constraints of an army on the march can surely do the same.

Jim has answered most of this.   A horse can spend 18 hours a day grazing.  So anything less than that, it will have difficulty eating its fill and will need supplements (e.g. grain).  The good news is horses will graze at night.  The advantage of cows is you don't need to deal with individual cows.  You can't just turn all your pack animals loose - you need to tether or hobble them and that will reduce their access to grazing and increase the fodder input.

[Justin Swanton]

Question: if a grain caravan eats all its grain in 10 days, how does an army, with many more men per pack animal, survive for 10 days?

It restocks regularly.  For example, if you restock every five days, it will only eat half its load, leaving half the load weight to provide for accompanying troops.

Doesn't quite add up. If a mule can carry 100kg of grain (a modest estimate), it can feed 20 men for 5 days. Assume 1 handler to each mule (a generous estimate) and the grain caravan can keep going for 100 days, on the assumption the animals graze off the land.

The rule of thumb is that a mule can carry 20% of its body weight, which is apparently 90kg but that will include the saddle.
One man per mule is merely standard.
So we'll take 80kg as the load (after pack saddle and the handlers bits and bods)
That's 176lbs which at 3lb per man will be about 58 man days,
BUT you've got to feed the mule, because even though animals graze working animals have to be grain fed

For feeding working horses the rule was half a pound of grain per 100lb weight of the horse. Your mule will weigh about 1000lb so if a horse would need 5lb of grain. Because it's a mule it needs less, but because it's working hard because you've got it heavily laden, I'd put it on the same ration as the man. So you'll have about 28 days before you run out. But of course if you're wisely carrying your grain in Amphorae as everybody insisted was necessary to stop it getting damp, the amphora weighs the same as its' load, so you'd end up 14 days from home having eaten everything but with a lot of empty amphorae to admire.

By picking mules you have picked the elite of the pack animal world and modern mules are apparently particularly well bred. The ancient world was more likely to use donkeys or horses which are a lot less efficient, needing to be fed more grain for the load carried.

OK, assuming the mule eats as much as the man, that's 2kg per day gone out of a load of 100 kg which means 50 days and the load is gone. To get most of the grain - say 3/4 - to its destination means a journey of 2 weeks tops. That's 20km x 14 days = 280km to the coastline or nearest navigable river.

Which gives us this area excluding Egypt:

[Justin Swanton]

  so if Egypt alone feeds the army, it is being asked to supply 15,56% of its annual harvest - or just the portion of the annual harvest necessary to feed its own people. Take its surplus into account and the grain supplied by the rest of the Mediterranean coastline and the Black Sea and it doesn't look so unreasonable.

No, it is being asked to increase its production
If you take away 15% of a national harvest and use if for something else people will starve!

You don't take away 15% of the national harvest. You forbid the export of the surplus and take that, plus a few percent of the local harvest - having asked the farmers to grow a bit more in the preceding years. Assuming Egypt supplies half the total needs you are getting the equivalent of 7,5% of its domestic harvest, largely from surplus and extra grain.

[Jim Webster]

OK, assuming the mule eats as much as the man, that's 2kg per day gone out of a load of 100 kg which means 50 days and the load is gone. To get most of the grain - say 3/4 - to its destination means a journey of 2 weeks tops. That's 20km x 14 days = 280km to the coastline or nearest navigable river.

Which gives us this area excluding Egypt:

no, it's not a 100kg. It starts off at 90 and gets down to 80 by the time you deduct saddle and kit. That's why I ended up with 28 days before you run out
Not only that but it's modern mules, so you're talking about the elite. The vast majority of the ancient world used donkeys

[Justin Swanton]

no, it's not a 100kg. It starts off at 90 and gets down to 80 by the time you deduct saddle and kit. That's why I ended up with 28 days before you run out

Fine, so 2/3 of the load reaches its destination. Bear in mind that the handlers would have eaten half that anyway.

Work also on the assumption that the grain would be stored in amphorae but carried in sacks.

[Jim Webster]

You don't take away 15% of the national harvest. You forbid the export of the surplus and take that, plus a few percent of the local harvest - having asked the farmers to grow a bit more in the preceding years. Assuming Egypt supplies half the total needs you are getting the equivalent of 7,5% of its domestic harvest, largely from surplus and extra grain.

there isn't a surplus. What happens is 100% is earmarked.
Hopefully in year one you get 105% and you quietly store away the 5% so it doesn't crash the market
In year two you get 95% and you release the 5% onto the market with a smile and nobody dies.

If you want to produce an extra amount, you have to grow an extra amount. Over a three or four year period there is no surplus.

[Justin Swanton]

You don't take away 15% of the national harvest. You forbid the export of the surplus and take that, plus a few percent of the local harvest - having asked the farmers to grow a bit more in the preceding years. Assuming Egypt supplies half the total needs you are getting the equivalent of 7,5% of its domestic harvest, largely from surplus and extra grain.

there isn't a surplus. What happens is 100% is earmarked.
Hopefully in year one you get 105% and you quietly store away the 5% so it doesn't crash the market
In year two you get 95% and you release the 5% onto the market with a smile and nobody dies.

If you want to produce an extra amount, you have to grow an extra amount. Over a three or four year period there is no surplus.

Egypt was an exporter of grain.

[Jim Webster]

no, it's not a 100kg. It starts off at 90 and gets down to 80 by the time you deduct saddle and kit. That's why I ended up with 28 days before you run out

Fine, so 2/3 of the load reaches its destination. Bear in mind that the handlers would have eaten half that anyway.

Please stop playing with perfect figures. These are what I gave you, this is the elite and a modern elite. It's to show you how good you can get under perfect conditions.
In the real world people budgeted on ten days to a fortnight because that's what worked with the livestock they had at the time.
Even the Byzantines seem to have worked on similar figures

[Justin Swanton]

no, it's not a 100kg. It starts off at 90 and gets down to 80 by the time you deduct saddle and kit. That's why I ended up with 28 days before you run out

Fine, so 2/3 of the load reaches its destination. Bear in mind that the handlers would have eaten half that anyway.

Please stop playing with perfect figures. These are what I gave you, this is the elite and a modern elite. It's to show you how good you can get under perfect conditions.
In the real world people budgeted on ten days to a fortnight because that's what worked with the livestock they had at the time.
Even the Byzantines seem to have worked on similar figures

Do you have the Byzantine figures?

[Erpingham]

'Cultural racism' (or we can call it 'cultural vanity' in order to avoid using an -ism) is the imposition of our own outlook on previous culture(s), which I regret some people do in spades,

But fortunately is rare in this forum.  However, if application of critical thinking, based on a Western tradition, is cultural vanity then many of us will be guilty as charged.   As to "imposition of our own outlook", it is something we all do.  The "sources first" approach is equally an "imposition of our own outlook".  So, maybe, stepping back and being a little less judgemental  of others legitimate intellectual approaches may be the way to go.

[Jim Webster]

Egypt was an exporter of grain.

Funnily enough I noticed.
The exported grain isn't surplus. It isn't being bought by people who buy it purely for the joy of allowing Egyptian grain run through their fingers. it's bought to eat.
Every year people from the same cities will turn up in Egypt and will buy pretty much the same amount every time. Some time a little more, some time a little less. That's how grain markets work. Egyptian peasant farmers know when they sow their grain that a proportion of it will go out of Egypt. Temple warehousemen will expect to see the foreigners every year buying, often the same foreigners. They're part of the market.
If you're saying that Xerxes is going to have his person stand at the docks and ban grain being exported to the hungry citizens of his empire we're back to the argument I had with Patrick probably about page one or two.

[Jim Webster]

Do you have the Byzantine figures?

I remember them being quoted in Slingshot many years ago, they stuck in mind mainly because they were so close to Engels. But then why not. Engels almost certainly knew of them

[Justin Swanton]

Egypt was an exporter of grain.

Funnily enough I noticed.
The exported grain isn't surplus. It isn't being bought by people who buy it purely for the joy of allowing Egyptian grain run through their fingers. it's bought to eat.
Every year people from the same cities will turn up in Egypt and will buy pretty much the same amount every time. Some time a little more, some time a little less. That's how grain markets work. Egyptian peasant farmers know when they sow their grain that a proportion of it will go out of Egypt. Temple warehousemen will expect to see the foreigners every year buying, often the same foreigners. They're part of the market.
If you're saying that Xerxes is going to have his person stand at the docks and ban grain being exported to the hungry citizens of his empire we're back to the argument I had with Patrick probably about page one or two.

The surplus grain was exported to areas outside the Persian Empire, so the hungry citizens remain fed with the domestic harvest. The buyers from outside Persian borders have to find their grain from elsewhere, it's not Xerxes' problem.

Xerxes forbids the export of the surplus grain and buys it himself or takes it as a tax, along with the extra grain he orders to be grown for the campaign.

[Jim Webster]

The surplus grain was exported to areas outside the Persian Empire, so the hungry citizens remain fed with the domestic harvest. The buyers from outside Persian borders have to find their grain from elsewhere, it's not Xerxes' problem.

Xerxes forbids the export of the surplus grain and buys it himself or takes it as a tax, along with the extra grain he orders to be grown for the campaign.

that's the problem with having a really big empire. Apart from a handful of Greeks, some of whom may have Medized to guarantee their grain supplies, the grain is bound for within the Empire.
If hungry citizens could be fed with the domestic harvest, then they wouldn't buy grain from Egypt in the first place.
So there's still no surplus grain. He's got to order more and find a way of increasing the manpower and farmable land of Egypt by a considerable percent

[Justin Swanton]

The surplus grain was exported to areas outside the Persian Empire, so the hungry citizens remain fed with the domestic harvest. The buyers from outside Persian borders have to find their grain from elsewhere, it's not Xerxes' problem.

Xerxes forbids the export of the surplus grain and buys it himself or takes it as a tax, along with the extra grain he orders to be grown for the campaign.

that's the problem with having a really big empire. Apart from a handful of Greeks, some of whom may have Medized to guarantee their grain supplies, the grain is bound for within the Empire.
If hungry citizens could be fed with the domestic harvest, then they wouldn't buy grain from Egypt in the first place.
So there's still no surplus grain. He's got to order more and find a way of increasing the manpower and farmable land of Egypt by a considerable percent

I'm working on the assumption that most of Egyptian surplus grain was not destined for the Persian Empire but for the Greek cities and other customers on the Mediterranean littoral. But do we have source material on this?

Even presuming Egypt didn't export a single bushel beyond the Empire's borders, at most it is being asked to supply something like 7,5 percent its annual harvest of a single year. Is it asking too much from Egyptian farmers to farm 1,9% more land over 4 years?