Pig husbandry has a long history in Europe, where swine have served in both nutritional and religious roles [26]. Pigs have several benefits as livestock: they are particularly flexible in their diets, allowing them to be reared on poorer land than other livestock; and pig meat is well suited for long-term preservation as bacon and ham and an excellent source of fat [27].

Archeological evidence indicates that medieval pigs were generally slaughtered by their second birthday, since by that time they had gained sufficient weight [26]. An important aspect of the weight-gain process was fattening just before slaughter. The fruit of oak trees (acorns) and beechnuts served as a valuable high caloric food source in the fall months, generally late September through December. According to studies of modern Spanish autumn wood pasture feeding, pigs split their effort between eating grass and acorns equally, which provides a balanced fattening diet since the acorn kernel has a low concentration of crude protein but a very high content of crude fat whereas grass is exactly the opposite [28].

In the Middle Ages, this feeding was institutionalized in the practice of pannage, the feeding pigs on acorn or beech mast in the autumn in exchange for a fee, often as an in-kind payment [24,29]. By feeding on the crop of oak acorns or, in some areas beechnuts, the young pigs would be ready for slaughter in December. Because pigs quickly loose weight if food sources are depleted, owners would not want to keep them over the lean winter months unless they were breeding stock or still too small to be worth killing that year. The cycle of swine fattening on acorns followed by slaughter was so important within the medieval agricultural cycle that it became the standard calendar depiction for either October/November or November/December [30,31].

Numerous medieval artistic depictions provide insights into the practices that facilitated pig feeding. More than being simple tropes copied from one artist to another, these images show familiarity with swine management practices—something which almost everyone would have some knowledge of considering the pervasive medieval swine ownership patterns [26]—making the artistic sources useful for understanding historical agricultural practice [32].

Because medieval swineherds relied on acorns to fatten their herds before the onset of winter, they beat the nuts out of the trees with the help of sticks, shook the trees, or climbed up into them to knock down the precious fodder. In the hundreds of images that I have seen, the swineherd is always shown carrying a stick, other than in one case when the swineherd appears to be receiving money for his services [33]. The stick is used for three purposes: as an element for the herder to rest on; as a device to urge the pig to go in a desired direction; and for knocking down acorns/beechnuts. The third of these is by far the most common (Figure 1). The 13th century scholar Bartholomaeus Anglicus commented on this imagery choice saying that because November is the month for fattening swine, it is illustrated with a rustic beating the oak [34]. Ethnographic evidence from 20th century Spain indicates that modern swineherds still beat the oaks, using a long pole to which a smaller stick was attached via a rope to knock down acorns from cork trees [35].

Even with the stick, some acorns high in the tree may have been difficult to knock down, prompting the swineherd to climb the tree. We have several medieval illustrations depicting this activity (Figure 2). The Luttrell Psalter, an early 14th century English manuscript, shows a man sitting up on one of the highest branches using his stick to knock down the top-most acorns for the two hungry pigs below [37]. It has even been argued that the practice of climbing trees to knock down acorns was so common that the Old English poem Christ by Cynewulf refers to it [38].

It is possible that after acorns or beechnuts were knocked off the tree, some of them were collected for later use as fodder rather than being consumed on the spot. Some medieval images show the collection of nuts in baskets (Figure 3). Acorn collection is also recorded in a Roman treatise, Opus agriculturae: “At this time, pick the acorn crop and keep it: it is easy work for women and children” [40], advice which was later repeated in the Middle Ages by Vincent of Beauvais [41]. In his sixteenth century book on husbandry, the Englishman Thomas Tusser included a verse about this as well:

To gather some mast, it shall stand thee upon,

With servant and children, ere mast be all gone:

Some left among bushes shall pleasure thy swine;

For fear of a mischief, keep acorns fro kine.

If the trees had a particularly productive year or the herd was smaller than usual, collected acorns and beechnuts could be stored and fed to the pigs later.

Most of the images in medieval manuscripts are clearly oak trees with oak leaves and acorns (Figure 1 and Figure 2). However, there are drawings that might be representing beech (e.g., Figure 3 and [44]). In a text written in the late 1700s, the author says hogs were taken “especially among the oaks and beeches” of a wood pasture and open forest area of the New Forest called Boldrewood during the pannage season [45], indicating that the nuts of both tree species were used as pig fodder at the time. The relative importance of the two different trees is not certain for medieval wood pasture, but according to pollen records, oak was much more dominant until the last 200–300 years in the New Forest [46,47].

While the artistic and literary evidence indicates that pigs were fed on oak acorns, it is not forthcoming about the size of the pig herds. For that, we have to turn to medieval financial records. The most well known source attesting to pannage numbers are Domesday Book and Little Domesday Book, a series of documents that summarized data gathered as part of an audit of the royal fisc in the 1080s [48]. In the inquest documents, the scribes often recorded the size of a woodland based on how many swine could be fed in it (silva n porcis), which made sense as part of a financial audit because the king as a landholder could assess pannage fees from swine owners. These records can be useful for indicating the relative importance of swine versus other livestock: for example, in the Norfolk records, sheep are by far the dominant stock with over 46,000 recorded, but pigs at a little over 8000 are likewise much more prevalent than cows (23) or horses (106 domesticated and 139 wild) [49]. The herd sizes recorded in Domesday could be quite large, as in Luton, Bedfordshire, which is listed as holding two woods that could feed 2050 swine total [50]. A few of the detailed entries for holdings in the New Forest listed multiple moderately-sized herds, including Eling which had “wood yielding 280 swine for the pannage,” Holdenhurst with wood yielding 129 swine, and Ringwood with wood yielding 189 swine [51]. An extensive study of English manorial agriculture from 1250 to 1450 showed that swine were most important as an agricultural product in northern England and the northwest midlands and in the pastoral systems of the Weald in Kent and Sussex [52]. Although medieval herd sizes cannot always be correlated to woodland quantity available at a specific manor [53], swine herding appears to be generally associated with woodland and wood pasture in medieval England.

Rackham argued that Domesday records could not be recording actual swine numbers feeding in the woods because acorn crops are variable [54]. But this might not be the case. Although recent oak silviculture studies demonstrate that acorn production ranges from very poor, in which a small proportion of the trees have acorns, to very good, in which nearly all trees have a heavy crop [55], the studies show that moderate crop sizes are just as common as extreme years [56]. Excellent crop years (called “mast” years), for the pedunculate oak occur every two to six years [57]. Thus acorns were probably available almost every year in the medieval English oak woods even though the amount was highly variable. While the numbers of pigs listed in Domesday might not be accurate for each and every year, they reflect the approximate size of the herds that could be fattened in the area in spite of acorn variability.

Although pigs were deliberated fattened on acorns and beechnuts, tree fruits could not be their only source of nutrition because of the limited seasonality of the crop. For the rest of year, swine could graze on grass, tubers, and roots; eat stubble of harvested grain crops such as barley and wheat; and consume leftovers such as brewery dregs or household food wastes [26]. Feeding swine grain fit for human consumption was not a preferred strategy because of the increased cost, according to a 15th century agricultural tract [58]. According to a 14th century husbandry handbook titled Seneschaucy, owners were encouraged to provide food directly to their herd only during severe frost; the rest of the time, the pigs should find their own food in forest, wood pasture, marsh, or untilled fields [59]. However, manorial records from the estates owned by Peterborough Abbey in eastern England indicate that pigs were regularly fed on grain and legumes there, probably because most of the nearby land was in crop production [53].

Grasslands for swine pasture must have been abundant in most places and the quality of pasturage was likely key to swine rearing in the Middle Ages [60]. Parsons noted that pasture quality and quantity was just as important as the size of the acorn crop in valuation of pig pastures in southwestern Spain, where woodland-fed swine were still common in the early 1960s [35]. Even when acorns are present, pigs consume large quantities of grass, as well as roots and rhizomes of plants such as bracken fern (Pteridium sp) and couch-grass (Agropyrum repens) [28,54]. Palladius, a Roman writer from the fourth century who was translated into Middle English verse, noted that pigs did well in oak woodlands because of the acorns as well as herbage in the grasslands underneath [61]. Rooting was likewise acknowledged in medieval times as a critical feeding strategy [62]. Tusser recorded a verse highlighting the role of both acorns and braken fern rhizomes (“brake-rootes”) as swine food:

Get pole, boy mine!

Beate hawes to swine.

Drive hog to the wood,

Brake-rootes be good.

In the foundation charter of Lessay abbey in Normandy, which at that time was part of the Anglo-Norman kingdom, the monks were permitted “to have pasture for all their animals and for all their pigs, as well as pasture for the animals of the servants who take care of the monks’ herds; if the pigs are sent to graze inside the park, the monks shall have the right freely to graze one hundred pigs from Martinmas until Lent” [63]. In this example, the grazing was over the winter and spring after the acorn-feeding period, showing that grazing in wood pastures was integral to medieval pig husbandry.

Wood pasture with pedunculate oak pollards would have been an appropriate agricultural landscape in the Middle Ages to maximize the combination of acorn and grass production. This choice is supported by two factors.

First, acorn production may be increased by pruning and cultivating trees as pollards in wood pasture rather than high forest. Although pollards in northern Europe might be thought of as fodder producers through branch and leaf harvesting or wood producers (e.g., [64]), the pollarded oak stands in Spain are currently managed for their acorn production [35,65]. In the dehesa system, the oaks are pruned to encourage outward growth rather than upward growth so that the light is increased and the acorn-bearing branches are at a more convenient height, creating an open landscape with trees that have open and productive canopies [35]. Although there are ecological differences between the systems—the dehesa is dominated by holm oak (Q. ilex) and cork oak (Q. suber) instead of Q. robur and Q. petraea—the link between medieval tree management techniques and acorn production for swine feeding may have been similar. Acorn production occurs most often on branches exposed to light [55]. Thus, oaks do especially well in open pastures where they are not shaded. Pruning or pollarding the tree increases light exposure of the branches, which is believed to lead to better acorn crops on the dehesa [35,65], although one more recent study has questioned increased productivity [66]. In any case, the result of the management is an open landscape with widely spaced open canopy trees.

When medieval illustrations show more than one tree in pannage scenes, they most often depict pigs feeding among widely spaced trees rather than dense forest [67]. Keeping in mind that artistic images reflect stylistic choices as well as reality, some of the images of pannage indicate managed trees with the lower branches of the trees pruned away [68]. An illustration in a 12th century manuscript shows a swineherd knocking down acorns from a more clearly pollarded tree (Figure 4).

Second, of the two primary types of oaks in Britain, pedunculate oak (Q. robur) and sessile oak (Q. petraea), the pedunculate has a more suitable acorn configuration. Pedunculate oaks have larger acorns, heavier acorn crops, and bear crops at a younger age than sessile oaks, and the larger pedunculate acorns are also easier to store over the winter [57]. Most importantly, in pedunculate oak, the acorn grows on a stem that extends from the main branch (called the pedunculate), making it easier to knock off. Because of this difference, swineherds would have preferred to feed their swine in pedunculate oak woodlands, a fact shown in the medieval illustrations. Although many illustrations do not show the acorns on the trees, when they are present, they are unmistakably on pedunculates, such as in the Luttrell Psalter [37] and the Hours of Jeanne d’Evreux [70].

Although we do not know the extent of human selection for pedunculate oaks over sessile, we can assume that the pedunculate’s favorable characteristics prompted people in the Middle Ages to try to maximize their presence [57]. This could have taken place through selective planting or by leaving only good producing trees in wood pastures. Rackham noted that pedunculate oak was much more common than sessile oak in remanent medieval wood pastures like Epping and Windsor Forests even though the opposite would be expected from the soil characteristics of the site [24]. This could be because of medieval selection pressure on seedlings. Certainly by the 16th and 17th centuries, evidence exists that pedunculate oaks were preferred for planting in England’s oak plantations [57]. It is therefore likely that medieval people also made sure that most of the oaks in wood pastures were pedunculate oaks in order to feed their swine most effectively.