After an adaptation period ruminants readily consume raw potatoes in spite of their bitterness (Snowdon, 2017). Potatoes are a good source of energy, comparable to good maize silage for ruminants. Their main component is starch, a valuable form of energy for both fattening and milk producing animals (Maynard, 1929). Potato protein, though present in small amounts, is considered to have a high biological value with favourable amino acid composition (Halliday, 2015). Raw potatoes, with their high water content, may have a laxative effect. Therefore they should be introduced gradually (Göhl, 1982). The low levels of both protein and fibre of potatoes suggest that they should be supplemented by protein rich feeds and roughages in ruminant diets. Because they are bulky material with almost no fibre, potatoes should be considered as high moisture grain replacer rather than roughage.
Dairy cows can receive up to 15 kg of raw potato a day and beef cattle up to 20 kg a day. If cattle are given no more than 20 kg a day, it is a good feed that will in no way impair health (Göhl, 1982). Though it was possible to include up to 50% (DM basis) potatoes in cattle diets, and though some cows have been reported to consume over 45 kg cull potatoes per day, it has been recommended to limit potato intake to no more than 30% for optimal animal performance (Snowdon, 2017).
Studies assessing the inclusion of culled potatoes in ruminant diets are rarer than those evaluating the inclusion of potato by-products. Since some by-products, such as screen solids, may have a composition close to that of raw potatoes, data from potato by-products were also used to assess nutritional aspects of culled potatoes for ruminants (Charmley et al., 2006).
Nutritive value
Culled potatoes are characterized by a low DM content, between 16 and 25% (Fiems et al., 2013; Eriksson et al., 2009; Charmley et al., 2006; Bradshaw et al., 2002). They are mainly composed of starch, with relatively low contents of protein, minerals and fibre, and they are used as energy concentrates (Charmley et al., 2006). The metabolizable energy (ME) value of culled potatoes is high, similar or slightly higher than that of barley with reported values between 11 and 14 MJ/kg DM (Nelson, 2010; Charmley et al., 2006; NRC, 2001; NRC, 2000; Sauvant et al., 2004).
Starch from uncooked potato is more resistant (slowly degraded) in the rumen than that of barley or wheat likely, because starch granules are larger in potatoes than in barley or wheat and contain higher proportion of amylopectin (Monteils et al., 2002). This relatively slow rate of rumen metabolism should result in a more balanced rumen pH and better utilization of nutrients by rumen micro-organisms and a better utilization of starch by the animal (Charmley et al., 2006). However, due to the low fiber content of potatoes, it cannot be excluded that a high inclusion rate can result in ruminal dysfunction if the basal diet does not contain enough fibre (Fiems et al., 2013). Cooking potatoes strongly increased rumen fermentation rate of starch, while deep freezing did not alter the properties of raw potatoes (Eriksson et al., 2004).
Growing steers
Culled potatoes are above all used as a high energy feed for beef feedlots. It has been considered that increasing the inclusion of culled potatoes and potato by-products beyond 20% DM of the diet resulted in a decline of DM intake (Charmley et al., 2006) but huge discrepancy about this inclusion rate exits between studies (Nelson, 2010). A reason for that may be that the adaptation to a potato-based diet lasted approximately 6 weeks, which may contribute to the reduction in DM intake in the early feeding phase (Bradshaw et al., 2002; Charmley et al., 2006).
The inclusion of culled potatoes at 30% (DM basis) in the diet resulted in higher intake of finishing cows when culled potatoes replaced the concentrate, whereas this level of inclusion had not significant effect on intake when culled potatoes replaced maize silage (Fiems et al., 2013). Culled potatoes did not affect daily gain, body weight, most carcass parameters (weight, composition, lean and fat content) and meat quality (colour, moisture, fat and protein content, pH, water-holding capacity, WB shear force). Only carcass dressing percentage was lowered with the inclusion of culled potatoes. Feed efficiency was also unaffected by the inclusion of culled potatoes (Fiems et al., 2013).
Finishing bulls were fed on a diet containing a fixed amount of culled potatoes (about 50% dietary DM) and concentrate with maize silage given ad libitum. This diet was compared to diets containing only a fixed amount of concentrate with maize silage offered ad libitum or a fixed amount of concentrate with a mixed silage composed of maize and beet pulp. Dry matter intake was lower on the diet including culled potatoes but energy content on this diet was higher. Daily gain and feed efficiency were also higher on that diet including potatoes compared with the diet including maize silage alone. Carcass and meat characteristics did not differ between the diets containing either culled potatoes or maize silage alone (Boucqué et al., 1994).
Dairy cows
Because potatoes are bulky material with nutrients diluted in high water content, it has been questioned if a high producing cow can consume sufficient amounts of potato to fully replaced grain (Eriksson et al., 2009). In a comparison between 4 sources of starch for dairy cows, potatoes could not compete with cereal grain such as wheat or maize for either milk yield or milk fat content. Cows fed on potatoes had also the lowest milk protein, milk lactose and milk solid non-fat yields. Starch from potatoes was not more valuable than cereal starches (Mosavi et al., 2012). Moreover, large amount of potatoes (25% DM) can be difficult to accept for some cows (Eriksson et al., 2009). A good use of potato in lactating cows was as a partial replacer of cereal grains in order to reduce feed costs (Jans, 1989).
In Colombia, medium-yielding Holstein cows (15 kg milk/d) in their second period of lactation that were grazing on good quality pasture (kikuyu grass (Pennisetum clandestinum) at 26% CP) could be supplemented with potato tubers at 6 or 13 kg/d/cow. The two supplementation levels resulted in higher milk yield, higher milk protein and higher true milk protein. However, it was not necessary to feed the higher level of potatoes as the animal performances were similar at 6 and 13 kg potatoes (Montoya et al., 2004).