Novel drivers of voluntary intake of low dry matter forages in sheep : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Fresh forages account for a significant part of the ruminant’s diet in pasture-based systems. Previous studies have suggested dry matter content below 16% reduce dry matter intake, but contemporary NZ temperate grazing systems using a range of low DM% forages are reported to have strongly positive production features, so, an experimental series (1, 2 and 3) was designed to assess DMI using fodder beet (FB), plantain, chicory and ryegrass cultivars using young sheep. In experiment one, DMI, water intake, urine volumes and mineral (Na, K, Ca, Mg, P) balance were measured with young sheep using metabolism crates and fresh harvested FB to represent a total diet DM% below 14%, with grass silage (GS) as a control. Eight (49±1.75 kg) Coopworth hogget rams were fed one of the three treatments, sequentially in runs 1 to 3; treatment 1 (ad libitum GS) - FB 0%: GS 100%; treatment 2 (restricted FB) - FB 30%: GS 70%; treatment 3 (ad libitum FB) - FB 90%: GS 10%. Total diet DM% were 43.21, 30.44, and 13.56%; total diet CP% were 13.38, 13.97, and 12.18%; and total diet NDF was 57.19, 43.13, and 22.29%, respectively for runs 1, 2 and 3. In run 3 the mean daily DMI was greater at 2.74% of LWT compared to run 2 (2.21%) and run 1 (2.49%) (p<0.05). Mean daily water intake in run 3 was 239 ml/kg of LWT, fourfold higher (p<0.05) than observed in run 1 (61 ml/kg of LWT). Mean daily urine production in run 3 was 160 ml/kg LWT, significantly greater (p<0.05) than observed in run 1 or run 2. The mean daily Na intake in run 3 was 21.27 (g/d), significantly greater than runs 1 and 2 (p<0.05), with greater (p<0.05) urinary Na concentration and mean daily urinary Na output in run 3, increasing from 105 to 2090 (mg/L) and 0.14 to 17.89 (g/d) runs 1 to 3. Mean daily N intake was not different between runs, while mean daily urinary N output (g/d) was significantly lower in run 3 (6.19g/d) compared to run 1 (7.88 g/d). Urinary N% was greater in run 1 than run 2, and run 3 (0.6, 0.28 and 0.08% respectively). The N utilization (N retained/total diet N) was greater in run 3 (67%) compared with run 1 (35%) and run 2 (33%). The DMI observed in run 3 was high by comparison with other pen fed harvested forage studies, but lower than the intake observed in grazed forages of similar DM%, and neither the NDF intake nor the CP content of the diet likely limited intake. There was no evidence of rumen dysfunction, and it was concluded that, when FB was fed ad libitum, despite being a low DM% forage, high DMI could be achieved, and the very high urine volumes observed suggested a role for Na in excreting the considerable water load consumed with these DMI. However, it was not clear whether all these effects could be related to an effect of increased Na intake in ad libitum FB diet. Plantain and chicory are low DM% forages that have higher Na content compared to ryegrass, and comparatively high N content compared to FB. Therefore, the second experiment was conducted to investigate the role of Na in DMI in low DM% forages, using eight different plantain varieties, chicory, and ryegrass, in four different sequential runs. In each run, 32 young sheep (41±7kg) were used in four groups of eight animals, with the first two runs using the same sheep and forages on a 14d delay and were conducted as sequential runs. In runs 1 and 2 the treatments were tonic, agritonic, ecotain (plantain varieties) and ryegrass, and in run 3 the treatments were tonic, captain, SF and oracle (plantain varieties), and in run 4 the treatments were tonic, tuatara, boston (plantain varieties) and chicory. In all four runs, ad libitum feed was given (refusals > 20%). Despite low DM% (8.47 to 16.56%), plantain and chicory treatments recorded significantly higher (p<0.05) DMI compared to the grass treatments where DM% varied from 13.06 to 16.17%. The DMI/fasted LWT varied from 3.1 to 5.0, 3.5 and 2.6 to 3.2% respectively for plantain, chicory and grass, and the mean daily NDF intake for plantain and chicory varied from 0.74 to 1.64% (NDF/LWT), and for grass 1.17 to 1.22%. The mean daily urine production was highest in the chicory treatment (11654 ml/d), and expressed against fasted LWT, 177 to 294 ml/kg for the plantain and 98 to 118 ml/kg for the grass treatment. Mean daily Na intake in plantain varied (0.3 to 10.57 g/d) across runs, but all except boston and tuatara treatments were >3g/d, while the chicory treatment was 3.5g/d, and both mean plantain and chicory values were significantly greater (p<0.05) than the mean of the grass treatments (0.43 g/d). The plantain treatments in runs 1 and 2 had greater (p<0.05) mean daily urinary Na concentration and mean daily urinary Na outputs (g/d) compared to the grass treatments. The mean daily Na intake (3.51 g/d) was lower in the chicory treatment, and the mean daily urinary Na output for chicory treatment (3.51 g/d) was equal to the intake. Despite the plantain DM% doubling from run 1 to run 2, the mean daily total water intakes were closely similar between runs within plantain treatments (range 378-398 vs 366-376 ml/fasted LWT, respectively for run 1 and run 2). When the mean daily DMI was expressed against LWT, run 2 plantain treatments were observed to have the highest for any treatment (4.62 to 5.08% of LWT) in all four runs, and it was concluded this water intake was an effective upper physiological limit, thereby limiting DMI. The results from this experiment do not support the hypothesis that forage DM% below 16% restrict DMI, and it was concluded that a total water intake of approximately 370-400ml/ kg LWT represents a physiological upper water intake limit, and that a daily urinary Na concentration of approximately 300mg/L, and a daily urinary Na output of approximately 3.5g was required to enable corresponding urine outputs to achieve the DMI observed. It was also concluded that diet Na content and daily intakes facilitated water excretion, and that only sufficient Na intakes would therefore enable high DMI with low DM% forages. The third experiment was conducted to investigate the effect of Na on DMI and urine production when the animals were externally dosed with Na while feeding on low DM% pasture that was originally low in Na. Twenty-four young sheep (39±5 kg) were divided into three groups of eight and three dietary treatments were used in two sequential runs in a cross-over design (grass and grass + salt treatments). The ad libitum FB treatment was maintained as the control treatment in both runs, while the other dietary treatments were grass and grass + salt (12 g of Na was added via oral drenching to achieve a daily intake similar to that of ad libitum FB feeding treatment). The mean daily DMI and the mean daily DMI/fasted LWT were significantly lower (p<0.05) in the FB treatment in both runs 1 and 2, and no significant differences (p<0.05) were observed between the grass treatment or grass + salt treatments in DMI or DMI expressed against LWT. The mean daily NDF intake expressed against the LWT was significantly higher (p<0.05) in the grass treatment and grass + salt treatments compared to the FB treatment and varied from 1.14 to 1.22% from run 1 to run 2. The mean daily urine production (ml/d) was not significantly different (p<0.05) between treatments in run 2, but in run 1 the grass + salt treatment was greater than the FB treatment. The mean daily total water intake expressed against fasted LWT were significantly higher (p<0.05) in grass and grass + salt treatments than the FB treatment, while in run 2, none of the treatments were found to be significantly different. The mean daily feed Na intake (g/d) was significantly higher (p<0.05) in the grass + salt treatment than in grass treatment in both runs. Despite the five to tenfold greater mean daily Na intake in the grass + salt compared to the grass treatment, there were no significant differences in mean daily urine production among the grass and grass + salt treatments in run 1 or 2. The urinary Na concentration was significantly increased (p<0.05) in the grass + salt treatment compared with the grass treatment (1316 vs 223 mg/L). The mean daily urinary Na mass output (g/d) was significantly higher (p<0.05) in the grass + salt treatment in both runs compared to the grass treatment. In both run 1 and run 2, grass + salt treatment had significantly higher (p<0.05) urine production (ml) and urinary Na output (g) in the early part of the diurnal period compared to that with the latter part. However, both grass and the FB treatments did not significantly differ in urine production (ml) and urinary Na output (g/d) during the diurnal period. It was concluded that supplemented Na does not necessarily increase the DMI or the urine volume in the grass + salt treatment despite the grass treatment being low in DM% (<16%), when the DMI was likely to be initially restricted by the NDF intake (1.14 to 1.22% NDF/LWT), and the excess of Na intake could have led to increase the urinary Na concentration once the minimum Na intake has been reached without necessarily increasing the urine production. It was concluded the lower DMI observed in the FB treatment was most likely a result of the CP content of the FB diet (8.06-8.38%) limiting the RDP of the diet. From the results of this series of experiments it was concluded that when FB was fed ad libitum, despite being a low DM% forage (<16%), high DMI could be achieved, and therefore high urine volumes were associated with this intake. It was also concluded that low DM% in forages did not restrict DMI if the animals consumed sufficient daily Na intake to enable high urine outputs of a minimum Na concentration (c.300mg/L) required for diuresis, and if the animals remained below a physiological upper water intake limit of approximately 370-400ml/ kg LWT, which was observed with the DMI observed in very low DM% forages such as some plantain and chicory treatments in this study. This process required a daily Na output of approximately 3.5g to enable corresponding urine outputs allowing the DMI to be increased in low DM%. However, supplementary Na does not necessarily increase the DMI or the urine volume despite the forage being low in DM% (<16%) if the DMI is restricted by the NDF intake, suggesting Na supplementation on most NZ ryegrass pastures will not be effective. In such cases, the excess of Na intake leads to an increase in urinary Na concentration once the minimum Na intake has been reached without necessarily increasing the urine production or the DMI.