select against old herbage. As this book does not cover herbage intake, this important subject is omitted, but see the proceedings of relevant conferences (Hodgson and Illius, 1998; Lemaire et al., 2000) and examples of experimental (Parsonset al., 1994) and modelling (Thornleyet al., 1994) reports.
Maintenance of stable ruminal function
It has long been known that excessive intakes of rapidly fermented materials have an adverse effect on ruminal function, leading to low-fat milk due to lack of acetate as a precursor of milk fat synthesis, and a condition known as subacute ruminal acidosis (SARA). Cows suffering from SARA significantly increased their preference ratios for long lucerne over lucerne pellets from 0.60 to 0.85 in an apparent attempt to alleviate the discomfort generated by SARA (Keunen et al., 2002). Further studies with cows in which SARA was induced by a high-starch diet showed no increase in their preference for drinking water that had been supplemented with sodium bicarbonate (an acid-neutralizing agent) at 2.5 g/l over plain water. They did, however, increase their total water intake in an apparent attempt to dilute out the excess acids in the rumen. Extreme acidosis can result in cattle ‘cud spitting’, i.e. dropping regurgitated boluses from their mouths.
Inclusion of sodium bicarbonate in the diet of sheep given a choice between foods with low- and high-energy density increased the proportion of high-energy food selected, as the bicarbonate ameliorated the low pH induced by the rapid fermentation of the high-energy food (Cooper et al., 1996). However, in subsequent experiments in which sheep were given an energy-dense food or a less dense food, with or without sodium bicarbonate supplementation, there was no effect of bicarbonate on the proportions of the two foods subsequently selected (James and Kyriazakis, 2002).
There was, nevertheless, an effect of dietary energy density such that sheep previously on the less dense food chose a significantly higher proportion of the more dense food than those previously on that high-density food. There was, however, a reduction in choice for urea-containing foods when the basal diet was higher in bicarbonate, explained by the need for urea as a buffer being reduced by the presence of bicarbonate in the diet (Jameset al., 2002). Effects of diet on buffering may override other diet selection objectives, such as the avoidance of an excess intake of RDP.
Stability of ruminal fermentation is an important requirement for the effective functioning of the microflora. Of particular importance is the regular and sufficient intake of fibrous material and it is observed that, given free choice between forage and concentrates, cattle and goats take about 20% of their DM intake as forage.
Castle (1979) offered dairy cows perennial ryegrass, with median chop lengths of 9, 17 and 72 mm. Voluntary food intake and milk yield increased as chop length decreased, while eating and ruminating times were reduced.
Despite this apparent nutritional advantage of the short-chopped grass, cows given a choice of all three ate 16% of the long and 32% of the medium-length grass. Similarly, lambs offered two foods do not totally avoid the bulkier one while lactating goats (J.M. Forbes, unpublished results) and growing heifers (J.H.M. Metz, personal communication) took about 20% of their DM intake as hay when they also had free access to concentrates. Growing lambs offered foods with different nutrient densities but similar ME:CP ratios ate some of the poorer food, and it was suggested that the better foods increased ruminal osmolality or reduced pH to an uncomfortable extent.
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Rapidly fermentable nitrogenous compounds also upset the ruminal balance and sheep avoid urea where possible (Kyriazakis and Oldham, 1993), even though this is a good source of nitrogen for the ruminal microflora. When two foods with protein contents above requirements were offered, the sheep consistently ate more of that with the lower content. However, when the choice was between a high-protein food and one supplemented with urea, the HP food was preferred, giving an overall crude protein content of the diet that was greater than required. The sheep preferred to eat too much undegradable protein than too much urea, presumably because of the more toxic effects of an excess of the latter. Thus, tactical adjustment to diet selection by ruminant animals that initially appears as a failure by the animals in obtaining a diet that both meets their requirements and avoids excess can be viewed within the overall feeding strategy of the animal to achieve its long-term goals that depend on healthy ruminal function.
When infused with acid or alkali to increase ruminal osmolality, sheep reduced their intake of a high-energy food but maintained their intake of a low- energy food, apparently in order to maintain the stability of conditions in the rumen (Engku Azahan and Forbes, 1992; Cooperet al., 1995).
In preference studies, Phy and Provenza (1998) offered lambs large (1200 g) or small (400 g) meals of barley followed by a choice between 200 g barley and 200 g lucerne pellets. Lambs given the small amount of barley did not express a strong aversion to it, whereas those given the large amount showed a marked preference for the lucerne. The adverse effects of the excess of readily fermentable carbohydrate provided by the high dose of barley induced an aversion, while the low dose of barley provided a readily available of source of energy without disturbing ruminal function unduly.
Although lambs preferred barley to lucerne when given a balanced basal food, this preference was reduced when they were given a basal food with an excessively high ratio of energy:protein (Villalba and Provenza, 1999). As the lucerne has a higher protein:energy ratio than barley (and higher than the basal food), this provides evidence that ruminants select to achieve a diet balanced for energy and protein. Similarly, their preference for whole over ground foods is modified if they need to eat more ground food in order to preserve a balance of energy and protein in their diet. Need for a particular macronutrient generated a stronger preference for a food providing more of that macronutrient, irrespective of its physical properties.
More recently Villalba et al. (2006) have shown that sheep learn to recover from overeating grain, tannins and oxalic acid, respectively, by ingesting sources of three medicines – sodium bentonite, polyethylene glycol or dicalcium phosphate – that specifically lead to recovery from illnesses caused by eating these three toxins. This is the first demonstration of multiple malaise–medicine associations.
There is thus ample evidence that ruminants do not always eat for maximum efficiency, as implied by Tolkamp and Ketelaars (1992), but strive to maintain sufficient intake of long fibre or chemical ameliorators of ruminal conditions to aid proper ruminal function.
Gut parasites
Gut parasites often reduce food intake by sheep and this is discussed in Chapter 17.
Tannins can offer some protection from gut parasites, as animals feeding on plants with tannins show lower nematode burdens, lower faecal egg counts and higher body weight gains than those eating similar plants without tannins.
Animals can learn to eat foods high in tannins and nutrients in order to overcome internal parasites (Hutchings et al., 2003). Gut immune function is also increased by the increase in protein passing through the rumen undegraded – caused by tannins in the diet.
Anti-emetic drugs
Daily treatment with the anti-emetic metoclopramide increased intake of endophyte1-infected, but not uninfected, tall fescue by sheep, suggesting that intake of the infected material is limited by feelings of nausea (Aldrich et al., 1993). In addition, anti-emetic drugs attenuated the aversion to several cereals induced by LiCl in sheep (Provenzaet al., 1994a), which does induce nausea in many species. Also, sheep receiving anti-emetic drugs ate more grain than those not receiving them; it thus appears that intake of cereal-based foods is limited by mild aversion to abdominal discomfort induced by overeating.
Lameness in chickens
Lameness is a common problem in intensively reared poultry, and administration of the analgesic carprofen allows lame birds to negotiate obstacles more quickly than untreated lame birds, although not as fast as normal birds, evidence that lameness causes pain. In order to discover whether broiler chickens could self- medicate with carprofen, lame and sound broilers were trained to discriminate between different-coloured feeds, one of which contained carprofen (Danburyet al., 2000). Lame birds selected significantly more drugged food than sound birds and, as the severity of the lameness increased, lame birds consumed a significantly higher proportion of the drugged feed. Clearly, they were selecting a dose of the analgesic that relieved the pain or discomfort of lameness.