The donkey

The domestic donkey is a descendant of the African wild ass and was first domesticated in approximately 3000 BCE (Rossel et al 2008). Current estimates of the worldwide donkey population are approximately 44 million (Starkey & Starkey 2000) with the majority of donkeys providing transport and draught power in developing countries. Donkeys are tractable animals that come in a variety of sizes, with breeds ranging from miniatures of less than 91 cm to mammoth jacks and Andalusian donkeys reaching over 1.6 m (Svendsen 2009). The donkey evolved in desert areas and has adapted to eating poor quality fibrous plant material (Izraely et al 1989). The donkey and its hybrid offspring the mule and hinny are renowned for their stoic natures and ability to survive in tough environments on poor quality food making them the work animals of choice in inhospitable areas of the world (Svendsen 2009, Starkey & Starkey 2000).

Donkeys and mules are also used for leisure and competition in developed countries and are popular as children’s ride and drive animals or as mounts for trail riding. Keeping donkeys and mules in temperate environments as leisure animals can, however, put them at risk of diseases associated with obesity or inappropriate management. They therefore require careful feeding to help to prevent conditions such as laminitis, hyperlipemia, and gastric ulceration.

Donkeys, for many reasons, should not be considered as if they were small horses; studies have shown physiological (Hill et al 2001, Liberatore et al 2001) as well as pharmacological and pharmacokinetic differences between donkeys and horses (Lizarraga et al 2004). Unfortunately, however, little specific detailed information about the nutritional needs of donkeys and mules is available and although some fundamental research has been carried out it is still far behind the field of horse nutrition. Much of the information in this chapter draws on scientific research but also the extensive experience of the authors in managing herds of both working and non-working donkeys as well as mules.

The structure of the donkey’s gut

The structure and function of the donkey’s gut is similar to that of the horse. In post-mortems carried out on working donkeys in Ethiopia, the intestine of the average sized donkey (100 kg) was ~24 m long and has a total maximal capacity of ~160 liters (S. Yoeseph, personal communication, Ethiopian Agricultural Research Organisation). Donkeys may have enhanced digestive efficiency compared with the horse due to improved digestibility of dry matter (DM), energy and fiber; longer gastrointestinal mean retention times of feed residues on high-fiber diets (Pearson et al 2006), higher volatile fatty acid (VFA) production per kg DM intake in the large intestine and enhanced recycling of urea (Suhartanto et al 1992, Faurie & Tisserand 1994).

One additional way in which donkeys may adapt to high-fiber diets, as mentioned above, is to use the full capacity of their large intestine (cecum and colon) so forage can be retained for longer and be digested more thoroughly. This increased capacity of the large intestine may give donkeys a “pot belly”, which conspires with their angular body frame to give the appearance of ill thrift. Consequently, donkey keepers should always use a body condition scoring system developed specifically for donkeys.

Key Points

• Donkeys have increased digestive efficiency when compared to horses

• Donkeys use the full capacity of the cecum and colon so forage can be retained for longer and digested more thoroughly

• Donkey’s teeth should be checked by a qualified professional and rasped if appropriate once per year or more frequently if problems dictate

Physical breakdown

Before digestion can start, donkeys must first physically break down their food by the action of chewing. It has been recommended that every kilogram of hay that the donkey consumes should be chewed more than 2000 times in order to reduce the forage to fragments of approximately 1.6 mm in length (Frape 2004). Smith (1999) compared the number of chews per kg of feed ingested in donkeys, ponies and cattle across three different diets (straw, haylage and chopped alfalfa). All species of herbivore chewed the more fibrous forages for longer, but on a given diet, donkeys tended to chew food less than ponies and obtained a faster rate of intake relative to body size (Smith 1999).

Concentrate feeds require less chews per kilogram (1000–1500) whilst poorer quality feeds like straw require more chews per kilogram (2500+). The condition of the teeth has a large effect on chewing efficiency; donkeys with dental disease will spend longer chewing and chew less efficiently than a donkey with healthy teeth. It is therefore very important that owners keep a careful eye on the condition of their donkey’s teeth, especially in mature donkeys that have all their permanent teeth. Donkeys should have their teeth rasped at least once a year by a qualified equine dental technician; the frequency of rasping will need to increase as the donkey gets older. Checking the donkey’s mouth should be an essential part of any routine veterinary examination of working donkeys; surveys have shown up to 62% of working donkeys have dental disease (Du Toit et al 2008a, b). A few minutes spent rasping the teeth of a working donkey can make an immeasurable long-term improvement to its tough life.

Water requirements

On average approximately 60% of the donkey’s body is made up of water, about 82% of the blood volume is water and even 25% of the weight of bone is water. Donkeys have evolved in semiarid environments and are well adapted to cope with thirst and rapid rehydration (Maloiy 1973). They are able to tolerate body water losses of up to 30% of their normal body weight, then rehydrate rapidly by drinking 24–30 liters of water in 2–5 minutes without apparent ill effect (Maloiy 1973). Donkeys are more thirst tolerant than ponies, and will maintain food intake even when deprived of water for long periods (Mueller et al 1997). Work has suggested that donkeys may be able to reduce water and energy turnover rates, as well as sweating rates and reduce water excretion whilst maintaining feed intake (Yousef 1991, Sneddon et al 2006). It has been reported that when given access to water after withdrawal of food and water donkeys will eat first and then drink (Houpt 1993). However, this short-term tolerance of thirst should not be confused with the long-term requirement for water.

The overall water requirement of donkeys is believed to be similar to that of horses. The general rule is that animals should always be provided with free access to clean water throughout the day. The best way to provide this is by self-filling water troughs that should be regularly cleaned. Donkeys can be particularly finicky about water temperature so care must be taken to provide water that is not too cold (>15°C) especially to geriatric animals (generally those older than 20 years). Donkeys may refuse to drink rather than take water from a trough with icy water, leading to problems such as impaction colic. Provision of water for working donkeys and mules is an essential part of preventive healthcare, donkeys and mules should be taken to water as frequently as possible during the day and at least every 4 hours.

Key Points

• Water requirements for donkeys are the same as those for a similar sized horse

• Donkeys are more thirst tolerant than horses and are capable of maintaining food intake even when deprived of water for relatively long periods

• Water provided must be clean, palatable and of a reasonable temperature, in particular, very cold water will often be refused leading to potential health issues such as impaction colic

Energy requirements

It is important to appreciate the energy requirements of donkeys in order to avoid under- or over-feeding. Research carried out by the Donkey Sanctuary has established scientifically validated guidelines for donkeys kept in temperate and in tropical climates (Wood et al 2005, Carretero-Roque et al 2005).

Mature donkeys that are kept at maintenance levels require between 80 and 95 kJ of digestible energy (DE) per kilogram of live weight per day (see Table 16-1). The upper value will apply during winter months when the energy requirement of donkeys tends to increase. The lower value will apply during the height of summer.

Table 16-1 Digestible Energy Requirements and Dry Matter Intake of Donkeys of Different Body Weights Fed on a Low Energy Density Diet Such as Straw (5.5–6.5 MJ/kg DM of Food)

Donkey live weight (kg)	Daily requirement for digestible energy (MJ/day)	Daily dry matter intake (kg/day)
150	12–14	2.3
175	14–17	2.6
200	16–19	3
225	18–22	3.4
250	20–24	3.8

In order to formulate an appropriate ration for donkeys it is necessary to estimate how much dry matter a donkey will eat per day. In a study at the Donkey Sanctuary, total daily dry matter intakes of between 1.3–1.7% of live weight were measured in donkeys fed on straw and hay (straw was available ad libitum). Other published studies have reported dry matter intake values in donkeys of between 0.9 and 2.5% that were given a variety of feeds (Smith & Pearson 2005) the higher values in this range were recorded in donkeys fed chopped lucerne. Much higher values of daily dry matter intakes have been recorded in horses fed chaff based diets (Dugdale et al 2008) and in donkeys fed chopped lucerne (Smith 1999). However, for donkeys fed unchopped forage a reasonable estimate of the appetite limits of a typical donkey is approximately 1.5% of its live weight in dry forage per day (Smith & Pearson 2005).

It is important to satisfy the energy requirement of donkeys, their appetite and their psychological need to spend much of the day foraging. For most of the year a ration that contains 70–75% barley straw or other fibrous forage such as maize stover and 25–30% of moderate quality grass hay, grazing or green fodder will supply all the energy requirements of donkeys. During the winter when energy requirements increase the proportion of hay or green fodder may need to be increased to 50–75% and the proportion of straw fed decreased to 25–50%. In practice, donkeys select hay in preference to straw, therefore by limiting the amount of hay fed and offering straw ad libitum animals are unlikely to exceed their energy requirements. It is important to select straw with few cereal heads or retained loose grain in order to prevent excess energy and starch intake, this is particularly important when owners are trying to manage the weight of obese animals and in order to prevent laminitis. A mineral supplement will also be required and some source of vitamins either in the form of a small quantity of fresh green fodder (200 g of chopped fresh alfalfa) or a commercial vitamin mix.

Little work has been done to compare the metabolism of donkeys with horses. As both species are hind-gut fermenters they have been said to exhibit better glucose tolerances than fore-gut fermenters such as ruminants (Frape 2004). Preliminary work by McLean et al (2009) suggests that donkeys have similar insulin sensitivity to adult horses of similar body condition score; this is particularly interesting when contrasted to adult ponies who generally have lower levels of insulin sensitivity when compared to both groups.

In terms of energetics donkeys have higher energy requirements for maintenance per kilogram of body weight than ponies but expend less energy per metre travelled than ponies relative to their body weight (Smith et al 1994). This greater locomotive efficiency may reflect the relatively larger proportion of “endurance” muscle fibers in donkey muscles (Marlin & Nankervis 2002). Endurance muscle fibers use metabolic energy more efficiently and fatigue more slowly than sprint muscle fibers.

There is little published research on the microbial activity of the donkey gut. One paper suggested that in the caecum the microbial cellulolytic activity is higher in donkeys than in ponies (Faurie & Tisserand 1994), whilst the same group has shown that greater quantities of VFAs are produced in the cecal fluid of hay-fed ponies 12 hours feeding than in donkeys fed the same diet (Tisserand et al 1991). In terms of the diversity of microbial species and their total number there is little difference across a wide variety of mammal herbivore species including fore and hind gut fermenters although they did not include the donkey (Stevens & Hume 2004). Gut flora is affected by diet, and horses that are adapted to a hay diet are reported to digest fiber more efficiently than horses that are adapted to a grain diet (Frape 2004). As donkeys tend to be fed on higher-fiber diets than horses, better microbial adaptation would be expected. When fed straw alone or with corn, in one study the VFA production was significantly higher in donkeys than in ponies (Suhartanto et al 1992) and the relative concentrations of butyric, isobutyric, valeric and isovaleric acids were higher. The pH of the cecal fluid was lower (6.7–6.9) than in ponies (7–7.3) when on the same diet. Explanations for the improved digestibility of high fiber forages in donkeys compared with ponies have previously included higher dry matter intakes per day coupled with faster mean retention time (Pearson et al 2006) and more efficient microbial digestion in the cecum (Suhartanto et al 1992, Faurie & Tisserand 1994). However, more recently the improved digestibility was associated with reduced dry matter intakes but longer gut retention times (Smith & Pearson (2005). Whether these differences in the explanations for the improved digestibilities recorded are due to differences in diet, intake, experimental study design, feeding regimen or breed is unknown. One study, in which the ponies and donkeys were fed almost identical dry matter intakes of either pelleted hay or straw, suggested a higher digestibility of the organic matter and the cell walls in the donkey (Faurie & Tisserand 1994) (%organic matter digestibility of 41.7 ± 1.2 and 48.3 ± 2.3 for straw in the pony and donkey respectively). More work is needed in this area.

The adaptation of the donkey to consume highly fibrous diets, especially those high in lignin, would suggest that the microbial community is specific to these substrates; an important consequence of this adaptation may be that this community is even more sensitive to dietary change to feedstuffs high in starch. This may make the feeding of high starch diets difficult to achieve safely in donkeys due to the sensitive nature of their microbiota; such diets are therefore actively discouraged.

Feeding for pregnancy, lactation and growth

When donkeys are growing, pregnant or lactating extra energy is needed. Research in this area is still to be conducted but general guidelines are provided below.

Key Points

• Mature donkeys that are kept at maintenance levels require between 80–95 kJ of digestible energy (DE) per kilogram of body weight per day

• For donkeys fed unchopped forages a reasonable estimate of the appetite limits of a typical donkey is approximately 1.5% of its body weight in dry forage per day

Ideally immature growing donkeys should be provided with sufficient energy to allow them to grow at a steady rate, avoiding periods of rapid growth or retardation. Providing too much energy to a growing donkey, especially when not balanced with adequate protein, calcium and phosphorus may result in the development of orthopedic problems. The available time for young donkeys to attain their mature stature is 2–3 years. Growing donkeys may face problems at weaning and during their first winter and some supplementation with concentrate feed may be required in order to avoid prolonged growth checks or permanent stunting. Pasture-fed immature donkeys are unlikely to require energy supplements, but should be given access to mineral licks formulated for equines, use of molassed mineral licks is discouraged as donkeys are likely to gorge on these products and may risk developing laminitis.

In pregnant donkeys the demands of the growing fetus only exceed the normal requirements in the final three months of pregnancy. Current advice, based on horse/pony information but shown to be effective in practical situations, is that digestible energy allowances should be increased by 11% above maintenance in the 9th month, 13% in the penultimate month and 20% in the final month of pregnancy. Pregnant and lactating jennies should have new feeds introduced very gradually over a period of 4–6 weeks, consideration should always be given to supplying additional requirements by increasing the hay or haylage proportion of the ration before introducing less suitable grain or concentrate rations.

Lactating jennies are likely to lose weight (–1 body condition score point/5; based on Fig. 16.1) during the first 2 months that their foals are suckling, even when they are receiving moderate feed supplementation. Preparation for this loss should be made in the final months of pregnancy by allowing pregnant jennies of body condition score 3 to gain to 1 point of body condition by additional feeding (i.e., at foaling they should have a body condition of –4). If the pregnant jenny is already in such condition then attention must be paid to maintaining this whilst not allowing the jenny to become more overweight as this may increase the risk of her becoming hyperlipemic. The jenny should receive sufficient additional feeding during the first 2–3 months of lactation to minimize body weight losses aiming at a stable body condition score of 3 for the remainder of the preweaning period.