For thousands of years in the Northern climates mankind had to slaughter a large proportion of their livestock due to a shortage of winter fodder.
What prevented mankind from using silage/haylage until the 19th Century?
Was it just that no-one had the idea until the 19th Century?
Judging by History of Silage by Wilkinson et al (2003), the premise behind the question is somewhat inaccurate.
Silage making is probably more than 3000 yr old. The ancient Egyptians and Greeks stored grain and whole forage crops in silos. Reviews of the history of silage refer to the mural in the Naples Museum, which shows whole-crop cereals being harvested and loaded into a small stone-built silo (Shukking, 1976; Woolford, 1984). Kirstein (1963) mentioned that silos were found in the ruins of Carthage, indicating that forage was ensiled there at around 1200 Be. Cato (cited by Shukking, 1976) noted that the Teutons in the first century stored green fodder in pits in the ground and then covered the pits with dung.
Little is known about silage making between about AD 100 and the eighteenth century, although it is likely that ensiling of forage crops continued to be practiced on a small scale throughout this period. According to Shukking (1976), grass was ensiled in Italy in the thirteenth century, and ensiling was practiced in the northern Alps, Sweden, and the Baltic region early in the eighteenth century. By the middle of the nineteenth century, interest in the ensiling of grass, sugarbeet tops (Beta vul- garis L.), and other crops had spread beyond the Baltic and Germany to most other European countries.
There's a small caveat here, which is that the authors of the article seem to file just about anything that's in a silo as silage. By contrast, I'm assuming you're meaning the sauerkraut version of hay that is used to feed livestock when they can't graze outdoors.
I've only skimmed the couple of pages that followed, but if I'm not mistaking this captures why it caught the interest of late 19th century farmers:
The initial enthusiasm for silage in the UK was reflected in the encouragement for the adoption of the technology from government, agricultural societies, and leading landowners, who recognized that the main advantage of silage was that it was less susceptible to total failure than either hay or root crops.
Put another way, before that they'd simply use plain hay, with all of its associated problems -- of which there are, apparently, many. (I'm no specialist by any stretch, so I'll direct you to the paper for the details.)
The paper doesn't say why it only spread like wildfire almost overnight in the late 19th (which it basically did) or why it did only then. One might speculate that hitherto benefits of a procedure known in the Baltic and Germany spread slowly in that area until, suddenly, the railroad came along and then it spread it far and wide. But that's just speculation.
TL,DR: Maybe proper ensilaging is only feasible with some degree of mechanisation
Ensilaging crops (whole crop silage is actually pretty common as fodder) or grass is somewhat involved: You need to chop the plant matter down to a few cm, spread in layers < 30 cm and the press is to avoid air pockets. Air pockets mean mold. During the actual ensilaging (the fermentation), a lot of excess water with a high acid content is set free and needs to be kept somehwere. When the silage is fed, there's a certain minimum amount per day - i think the silo face (the sruface you feed from) should advance a meter per day at least because air will enter the silage stack, again leading to mold. In modern aplications the silo is usually covered by a foil, but that is not strictly neccessary (no foil means the top layer spoils).
A silo can be as simple as a blacktop area, more typically it's blacktop with sturdy concrete walls (with some protection) an three sides and a drainage system. However the structure has to handle the weight of the silage and the vehicles, as well as the agressive leachate - I've once met the operator of a biogas plant, his silo was in a limestone area and he had to ecavate the ground 2 o 3 m deep and haul in granite to ensure a lasting foundation (maybe the structural engineer and geotechnician where too cautios).
The point of the matter is that ensilaging has an economy of scale aspect and also requires some mechanization. I don't know for sure, but I would suspect that some form of harvester is required to do the chopping. The first harvester was invented in the 1830ties, with large comine havesters becoming common in the late 19th century.
