Dried citrus pulp is a common ingredient in ruminant diets and compound feeds throughout the world. In the tropics, it can enhance tropical beef and dairy production systems based on low to medium nutritive value grasses and other forage sources (Villareal et al., 2006).
Energy value and digestibility
Due to its relatively high digestibility (OMD in the 85-90% range) and energy value (ME about 2900 kcal/kg DM, 85-90% that of maize and comparable to barley ME), citrus pulp is used as a cereal substitute in concentrate diets (Bampidis et al., 2006; Villareal et al., 2006). Unlike cereals, its energy is not based on starch but on soluble carbohydrates and digestible fibre. Citrus pectins are easily and extensively degraded, producing acetic acid, which is less likely than lactic acid to cause a fall in pH resulting in acidosis (Wing, 2003). Due to its high fibre content, the long rumination of citrus pulp produces large quantities of saliva that has a buffering effect on rumen pH (Mertens, 2000; Faria et al., 2008). Citrus pulp is therefore considered as a safer feed than cereals for animals fed high-concentrate, low-roughage diets, such as high-yielding dairy cows (Crawshaw, 2004). In rations containing low digestibility forages (hay or straw) or based on roughages such as maize silage or sorghum silage, citrus pulp seems to have a positive effect on fibre digestibility, perhaps due to a longer rumen retention time (Arthington et al., 2002; Barrios-Urdaneta et al., 2003).
The digestibility of the protein of citrus pulp is low and variable (from 37% to 70%) and, therefore, including large amounts of citrus pulp in diets containing protein-rich forages may cause a general decrease in protein digestibility. Its low soluble nitrogen content may result in a decrease in rumen ammonia. Supplementation with urea or ammonia can be a valuable strategy. Citrus pulp contains highly fermentable carbohydrates that may promote a more efficient use of N, by increasing the production of microbial protein by the rumen bacteria (Rihani, 1991). However, true protein sources can be more efficient than non-protein N (Kim et al., 2007).
Due to the low phosphorus content and to the Ca:P imbalance, phosphorus supplementation is an important consideration for balanced diets containing citrus pulp (Arthington et al., 2002).
As citrus pulp has a low content of vitamin A, green leafy roughage is an important ingredient in rations with high levels of citrus pulp (Göhl, 1982).
The palatability of dried citrus pulp is variable, as it may have a bitter taste due to limonin and other compounds present in the seeds and peels. It can result in a decrease in intake if introduced too quickly in the diet, and a more progressive introduction is recommended. It is well accepted by animals that are accustomed to it and can then increase intake (Rihani, 1991; Wing, 2003; Göhl, 1978). In fattening lambs, palatability decreased when diets contained more than 40% dried citrus pulp (Bhattacharya et al., 1973).
Citrus pulp is a valuable feedstuff for dairy cows. The extensive acetic acid production in the rumen helps maintain milk yield and milk fat content when forage is scarce (low fibre diet) or when high energy is required (e.g. as a replacement for cereals) (Arthington et al., 2002). A comprehensive review of past literature indicates that dried citrus pulp has no useful dietary properties other than its nutrient content, which limits its use in dairy cattle concentrate rations (Wing, 2003). It is not a full substitute for cereals and while it does have roughage-sparing properties, it cannot be used in place of the entire roughage allowance (Wing, 2003; Crawshaw, 2004).
A level of 40% of the total ration from dried citrus pulp has been considered feasible (Wing, 2003). However, inclusion rates lower than 20% (diet DM) are recommended, and higher levels may alter negatively DM intake, milk parameters and diet digestibility. Dried citrus pulp, included at 20% DM, as a concentrate substitute in a 50-60% maize or sorghum silage-based diet did not change DM intake, milk yield or milk protein content (Belibasakis et al., 1996; Assis et al., 2004a). Milk fat content increased (Belibasakis et al., 1996) or did not change (Assis et al., 2004a). Feed efficiency (carbohydrate conversion) increased while maintaining the same milk yield (Miron et al., 2002). Below 20%, neither rumen parameters nor digestibility were altered (Assis et al., 2004b). Between 20% and 24% inclusion in mixed dairy rations, rumen parameters remain unaltered but milk yield and milk protein content may be reduced, while milk fat content remains equal or increased (Leiva et al., 2000). Beyond 24% of the total diet, dried citrus pulp decreased total dry matter intake, and total dry and organic matter digestibility (Salvador et al., 2008).
Early research reported that citrus pulp could cause milk taint under certain conditions, but this has not been confirmed experimentally (Crawshaw, 2004).
Beef cattle and growing cattle
Dried citrus pulp is also a valuable feed for beef and growing cattle and can partly replace cereal energy sources. It can be safely included in rations at 20-30% of the DM (Crawshaw, 2004), but higher values are feasible. It was possible to include up to 40% dried citrus pulp in the diet of fattening cattle without altering animal health (Oliveira et al., 2002; Oliveira et al., 2005). Up to 55% dried pulp in the diets of young bulls (replacing 86% of maize grain) did not affect live-weight gain and carcass yield, though there was a decrease in backfat thickness (Henrique et al., 2004; Henrique et al., 2006). In beef cattle fed low-quality star grass (Cynodon nlemfuensis), increasing the amounts of dried citrus pulp up to 2.5 kg/day/animal (as fed), equal to 30% of the diet DM, reduced forage intake but increased energy intake (Villareal et al., 2006). Urinary calculi have been observed in steers fattened with rations of more than 30% citrus meal (Göhl, 1978).
Citrus pulp at 30% of the diet is acceptable in rations for calves over two months old, but it is not recommended for younger calves because of depressed intake (Wing, 2003). A 45% inclusion rate in calf rations was also reported (Göhl, 1978).
Sheep can adapt quickly to a diet containing 20% dried citrus pulp (Crawshaw, 2004). Several experiments have reported that citrus pulp can be included up to 30-40% in the concentrate diet of fattening lambs with no ill-effects on growth and carcass quality (Bhattacharya et al., 1973; Martinez Pascual et al., 1980; Caparra et al., 2007). A positive effect on feed efficiency and daily gain has been reported (Rodrigues et al., 2008). Dried citrus pulp included at 36% in a cassava flour-based diet gave higher diet digestibilities, live-weight gains and average daily gains than cassava peels, groundnut hulls and maize cobs (Aregheore, 2000). However, daily weight gain was found to be maximized with 15-20% citrus pulp in the concentrate (Martinez Pascual et al., 1980). Rates above 30% can decrease digestibility (Göhl, 1978). Higher rates (more than 40-60%) resulted in overall performance decreasing and an increase in the severity of rumen parakeratosis (Martinez Pascual et al., 1980; Rodrigues et al., 2008), and lower palatability (Bhattacharya et al., 1973).
Inclusion of 30% citrus pulp in the concentrate given to lactating ewes fed alfalfa hay and straw did not affect milk yield and milk composition (Fegeros et al., 1995). Another experiment reported that replacing 33% to 100% of barley with citrus pulp in diets based on ammoniated straw resulted in a linear decrease in milk yield, but not milk composition, and a lower daily weight gain of lambs (Castrillo et al., 2004).
In male goats, supplementation with dried citrus (lemon) pulp and urea of a diet based on barley straw and alfalfa hay increased DM intake and apparent dry matter digestibility of the straw (Madrid et al., 1997). However, a later experiment showed that digestibility decreased when a supplement of 300 g/day/animal was given (Madrid et al., 1998). In growing kids, replacing maize grain with 40% citrus pulp gave the best daily weight gain, DM intake and feed conversion while higher levels were thought to have deleterious effects on mineral metabolism (Bueno et al., 2002).
Dry citrus pulp can also be incorporated into goat rations as a dry season supplement to foliage from leguminous browse plants (Enterolobium cyclocarpum), replacing up to 50% of the concentrate (dried brewer's grains) (Oni et al., 2008).