The effect of the energy density (ED) of feeds offered as a choice on the diet selection of sheep, and the relationship between the rumen environment and the diet selected from feeds of different ED were investigated in two experiments. In the first experiment two feeds, L and H, and their mixture M (3: 1 w/w) were formulated. AU feeds had similar calculated metabolizable protein:metabolizable energy (ME) ratios, but differed in ED (7-4, 8 1 and 10.1 M J ME/kg fresh feed for L, M and H respectively). The feeds were offered ad lib. either singly or in paired choices (L/M, L/H and M/H; n6 per treatment) to growing sheep. Although the rate of live-weight (Lwt) gain on feed H was higher than on feeds L or M, and the daily rate of feed intake lower, the sheep on feed choices did not consume only feed H. Instead they selected a mixture of both feeds offered, such that the total amount of H consumed per kg fresh feed was similar on choices L/H and M/H. The rate of Lwt gain of sheep on choices L/H and M/H was not different from that achieved on feed H alone. In the second experiment the choice L/H was offered to fistulated sheep (10 months of age, mean Lwt 57.5 kg) in an 8 x 8 Latin square, with 7 d periods. Treatments were infusions into the rumen (total volume 1 litre) over 4 h on days 1-4 of each period of acid (HCl; Acid 1,400; Acid 2, 300 and Acid 3,200 mmol/l), alkali (NaOH; Alk 1,316; Alk 2, 212 and Alk 3, 109 mmol/l) and control (NaC1; Con 1, 315 and Con 2, 209 mmol/l). Infusate osmolalities (mOs/kg) were 795 (Acid l), 585 (Acid 2, Alk 1 and Con l), 390 (Acid 3, Alk 2 and Con 2) and 200 (Alk 3). Infusion treatment significantly affected the diet selection of the sheep (P < 0.05) according to the osmolality of infusate, but not according to rumen pH. During infusions intake of feed H tended to decline with increasing treatment osmolality, whereas intake of L remained constant. The effects on diet selection and feed intake were of a short duration with no carry-over effects. These results show that sheep given a choice between two feeds of Werent ED select a substantial quantity of the low-ED feed; this diet selection is affected by short-term manipulations of their rumen environment, in a manner that is consistent with the maintenance of effective rumen conditions.

We proposed an hypothesis that ruminants attempt to select a diet that promotes high levels of feed intake by maintaining optimal ruminal conditions. Three tests of the hypothesis considered whether the diet selection of sheep given either a choice of two high energy density (ED) feeds or a choice between a high and a low ED feed is affected by 1) sodium bicarbonate inclusion (NaHCO3; 1, 2, and 4% [wt/wt]) in the high [ED] feed); 2) dietary carbohydrate source (barley-based, B and sugar beet/ barley, S) of the high ED feed; or 3) physical form (alfalfa: pelleted, ALFP and long chop, ALFL) of the low ED feed. To conduct these, 42 lambs were used in seven 6 x 6 Latin squares, which were either foods ALFL and ALFP offered alone and paired with feed B or S and their NaHCO3 derivatives as a choice, or foods B and S offered alone and paired with their NaHCO3 derivatives as a choice. Each Latin square period lasted for 3 wk. For the choices between a NaHCO3-supplemented high ED feed and either a high or a low ED one, the inclusion of NaHCO3 increased feed intake. Its level of inclusion had a marked effect on the proportion of unsupplemented high ED feed selected (P < .05) but not on the proportion of low ED feed (alfalfa) selected. More alfalfa was selected when the high ED feed was based on barley (B) rather than sugar beet/barley (S) (P < .05). The physical form of the alfalfa affected diet selection because its selection was greater (P < .001) when ALFP (pelleted) was offered in the feed choice rather than ALFL (long-chop). The significant effects of NaHCO3 level, dietary carbohydrate, and physical form on the diet selection and feed intake of the sheep are consistent with the proposed experimental hypothesis.

No abstract

We have shown previously (Cooper and Kyriazakis, 1993) that lambs offered a choice between a low energy density (ED) food and a high ED food, do not solely consume the high ED food, as it would have been expected from an optimal foraging perspective. It is proposed that the lambs offered this choice have to balance the benefits of consuming a high ED food (high growth rate) against the metabolic costs incurred, which may be excessive disturbances to the rumen environment, such as a low rumen pH and an increased osmolality (this has been shown to be the case in the diet selection of sheep between concentrates and hay in an experiment by Engku Azahan and Forbes (1992)). The aim of this experiment was to test which of these two rumen conditions have an influence upon diet selections made by sheep offered foods of different ED.