School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemIntake and digestion of cereal straws by sheep( 1986)This thesis reports on a series of experiments which examined the utilization of cereal straws by sheep. Cereal straws are considered low quality feeds as their intake and digestibility are low. The intake and digestibility of cereal Straws can be influenced by many characteristics of the straws themselves, by genotype/species of animal consuming the straw and by the physiological state of these animals. The effects of these factors on straw utilization are reviewed in Chapter 1. It is apparent that the intake and digestibility of straws can be increased by various pretreatments or by supplementation. The experiments conducted were intended to provide new and useful information on the effects of calcium hydroxide [Ca(OH)2] or urea pretreatments on the intake and digestion of cereal straws by sheep. The objectives of the first study were to investigate the effects of Ca(OH)2 treatment of a wheat straw on : (i) its intake and digestibility in sheep and the effect of restricting the intake of the treated material on its digestibility (Chapter 2) (ii) the effects of high calcium levels in the treated straw on the utilization of other macro-minerals (Chapter 2) and (iii) the effects of this treatment on digestive processes within the reticulo-rumen (ReRu) and on the sites of fibre digestion (Chapter 3). Pretreatment of the straw with Ca(OH)2 increased organic matter intake (WI) (685 vs 398 g/day) and digestibility (OMD) (62 vs 54%) resulting in an increase in digestible organic matter intake (DOMI) of 98%. Restricting the intake of the treated straw, which was intended to increase retention time in the ReRu, resulted in only small ( 2% units) increases in OM or neutral detergent fibre (NDF) digestibility. Although the treated straw diet contained high levels of calcium (21 g/kg DM), most of this was excreted in faeces and there were no adverse effects on phosphorus or sulphur balance. As the experiment was of 49 days duration and the ad libitum intake of the treated straw diet was maintained throughout this period, it was apparent that no serious mineral imbalances were imposed. Further, the sheep consuming the Ca(OH)2-treated straw diet lost less weight (40 vs 140 g/day) than the control animals. As regards the measurements of the amount and composition of digesta in the ReRu and the rates of digestion and passage from that organ, there was large variability between sheep within treatments. Consequently, comparisons of the dietary treatment effects were not significant due to the small number of animals used and the fact that these treatments were imposed on different animals. However, there were significant positive relationships between dry matter intake (DMI) and DM load in the ReRu and between DMI and DM flow through the abomasum. The apparent mean retention times (MELT) of DM in the ReRu for the treated straw diet fed at near ad libitum intake was 22 h and this tended to be less than that of 27 h in sheep given the untreated straw. There was also a tendency for increased fractional digestion and passage rates of the NDF of the alkali-treated compared to untreated straw. Hence, the increase in intake of the Ca(OH)2-treated straw was associated with apparent increases in ReRu DM load (40% increase) , and with non-significant increases in fractional digestion (110% increase) and passage (40% increase) rates. The amount of non-ammonia-nitrogen (NAN) apparently digested in the intestines was greater (4.8 vs 2.9 g/day) for the treated compared to untreated straw, but there were no differences when considered as the amount of crude protein apparently digested (DCPi) per 100 g DWI (7.8 vs 9.1 g DCPi/100 g DOMI). Urea pretreatment, which involves spraying straws with a urea solution followed by a period of air-tight storage, is known to increase the intake and digestibility of these feeds. However, the extent to which these improvements are due to the effects of the added nitrogen or to an alkali effect per se are not clear. An experiment was carried out in two periods in which a rice straw was fed to sheep untreated or treated with urea. In the first period both diets were supplemented with urea and sodium sulphate, while these supplements were not given in the second period. The effects of pretreatment and/or supplementation on straw intake and digestibility are reported in Chapter 4, while the processes of digestion in the ReRu and the sites at which digestion occurred in sheep given the two supplemented diets are presented in Chapter 5. Urea and sulphate supplementation of the untreated rice straw increased OMI (803 vs 576 g/day) and (MD (54 vs 48%). While the intake of urea-treated straw, which was airdried prior to feeding, was similar to that of urea-supplemented-untreated straw (779 vs 803 g/day), the OND (61 vs 54%) and NDF digestibility (60 vs 49%) were increased. The nitrogen intake on these two diets was similar and the increase in digestibility was presumably due to an alkali effect during storage. Supplementation of the treated straw with urea and sulphate further increased intake to 932 g OM/day, but had no additional effect on digestibility. Thus, while urea supplementation reduced the rate of liveweight loss of sheep from 138 to 20 g/day, sheep fed the treated and supplemented straw maintained weight (+ 38 g/day). For this particular straw, it is concluded that 50% of the improvement in nutritive value gained by pretreatment was due to the additional nitrogen added. These results are discussed in relation to the likely effects of treating or supplementing straws of higher or lower nutritive value than the one used in the present work. There were no differences in the quantity or composition of digesta in the ReRu of sheep fed untreated straw supplemented with urea and sulphate or urea-treated straw given with similar supplements (Chapter 5). The MRT of DM in the ReRu was less (18 vs 21 h) in sheep given the treated rice straw. In addition, treatment increased fractional digestion rate (3.4 vs 1.9 %/h), but did not affect fractional passage rate (2.7 vs 2.9 %/h) of NDF. While the higher intake of the treated and supplemented straw was associated with increased amounts of NAN reaching (17.6 vs 12.6 g/day) the small intestines, there were no differences in the amounts of NAN apparently digested in the intestines (9.3 vs 8.0 g/day or 9.8 vs 11.2 g DCPi/100 d DCtII). These results are discussed in relation to current theories on the regulation of intake in sheep. In the final experiment (Chapter 6), the effect of body condition of sheep on the intake and digestion of the urea-treated rice straw used in the previous experiments was examined. The sheep used were those involved in previous experiments and they were allocated to two groups. Prior to the experiment, one group was fed a forage (grass hay + urea-treated rice straw in the proportion 0.75 : 0.25) and lupin grain diet (0.84 forage : 0.16 grain, DM basis) ad libitum, while the other group was given the same diet at a restricted level. At the commencement of the experiment the sheep weighed 57.7 versus 42.6 kg fleece-free liveweight and had 15.8 versus 9.7 kg total body fat. At this point the sheep in both groups wire offered the treated straw ad libitum and were given infusions of urea and sodium sulphate into the rumen. Feed intake was measured continously for 51 days, while the quantity and composition of digesta in the ReRu and flowing through the abomasum were measured on four occasions (about days 10, 18, 32 and 50). The thin sheep consumed consistently more (1170 vs 1040 g/day) feed throughout the experiment, but there were no differences in digestibility. Associated with the increased intake were larger ReRu fills (8.59 vs 7.67 kg) and DM loads (1040 vs 960 g) in the thin animals. However, the fractional digestion (3.3 vs 3.4 %/h) and passage (2.0 vs 2.0 %/h) rates of NDF were not different in the two groups of sheep. These results are discussed in relation to the likely effects of body condition on the intake of less digestible straws where nutrient limitations might exist. In the concluding chapter (Chapter 7) the implications of these findings are discussed in relation to other information on the utilization of cereal straws.