School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemRoot-shoot interactions in the growth of irrigated white clover( 1993)White clover pastures support the dairy industry in the irrigated area of northern Victoria. However, pasture production is low because conditions for root growth are sub-optimal, particularly under flood irrigation. This thesis investigated the possibility that the growth of white clover can be increased by reducing the limitations to root growth. A series of experiments examined the response of white clover plants to various soil-based treatments and quantified relationships between root and shoot growth. Plants were grown in intact soil cores in the greenhouse with shoot and root growth measured by destructive harvest. The cores were collected from a range of field sites that were characterised by their different soil physical properties and the variation in pasture yield they supported. Other cores contained a sand-based potting mix in which the conditions for root growth were superior to the most productive field soil. Despite the large effects of soil treatment on white clover production, the growth of shoots and roots was highly correlated (R2>0.95). A prerequisite of high shoot yield is, therefore, a large root system. In one experiment, soil drying or defoliation perturbed the correlation but this disruption was only temporary. In another,experiment, the repeated cycles of drought stress that accompanied a series of extended irrigation intervals had no effect on the relationships between shoot and root growth. In field soils, the restrictions to root growth could not be overcome by intensive irrigation and fertiliser management. However, plants in the treatments in which the soil physical properties had been modified produced 4.0 - 6.5 times as much shoot DM compared with the least productive treatment. This suggests that the potential to improve pasture yield by amelioration of the soil physical properties is very large. Two further experiments were conducted in which either the soil texture or the frequency of irrigation varied between the upper and lower sections of the soil cores. In both cases the production of shoots was correlated with total root production. However, when `unfavourable' conditions restricted the growth of roots in one layer, extra growth of roots in the `favourable' layer was not sufficient to compensate. As a consequence, both total root and shoot growth were reduced. Taken together, these results suggest that there is a large scope to improve the yield of white clover by removing the restrictions to root growth that currently exist in field soils. This will probably entail both amelioration of the soil physical properties and careful management with respect to water and fertiliser applications. However, if the experiments reported here accurately reflect the field situation, then the growth of white clover pastures can only be maximised if the entire root zone is modified.
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ItemImprovement of utilization of white clover by supplementing with maize silage for lactating cows( 1991)In a 10 day cross over design experiment, six Friesian cows, in late lactation, were fed either white clover or white clover plus maize silage on a 50:50 DM basis in metabolism cages from continuous feeders. Data for intake, digestibility, nitrogen balance, milk production and rumen pH, volatile fatty acids (VFA), ammonia, fluid flow rate, pool sizes and protozoa population were collected. Replacing 50 % of white clover with maize silage reduced nitrogen intake significantly (P<0.001) and increased neutral detergent fibre (NDF) intake (P<0.01) and acid detergent fibre (ADF) intake (P<0.05). Digestibility was reduced significantly as follows: dry matter digestibility (P<0.05), organic matter, NDF, and ADF digestibilities (P<0.01) and crude protein digestibility (P<0.001). There was no significant difference between diets for dry matter intake (DMI), organic matter intake (OMI) or digestible organic matter intake (DOMI). Milk yield and composition were similar for both diets. Rumen fluid ammonia concentration and urea nitrogen in urine were reduced significantly (both at P<0.001) by adding maize silage to white clover. There was no significant difference in nitrogen in milk or nitrogen balance. Rumen protozoa population increased significantly in the cows fed white clover plus maize silage diet due to an increase in Entodinium, No/ml (P<0.05) and total protozoa No/ml (P<0.01). There was no significant difference in rumen fluid VFA, pH, or fluid flow rate. It was concluded that feeding white clover with maize silage improves the efficiency of utilization of nitrogen in white clover which potentially can increase milk production per cow and/or per hectare without changing the milk composition or altering the rumen environment.
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ItemThe nutritive value of white clover pasture for cows in mid-lactation when supplemented with maize silage( 1992)In a completely randomised grazing trial, 8 cows in mid-lactation were offered white clover pasture (35kg DM/cow/day) with 4 of the cows given a supplement of maize silage (8 kg DM/cow/day). Factors evaluated over a 28 day period were total DMI, pasture DM/maize silage DM substitution rate, herbage selection in the pasture, milk yield, fat and protein concentration. Supplementing cows with maize silage increased total DMI by 22% and lowered pasture herbage DMI by 21%; equivalent to a substitution rate of 0.5 kg pasture DM /kg of maize silage DM. Selection of herbage in the pasture was not affected by supplementation, but cows on both treatments selectively grazed the white clover. Supplementing cows with maize silage increased milk yield by 34%, fat yield by 35% and protein yield by 35% (P<0.05), but did not affect milk fat and protein concentration. In a metabolism trial, 6 rumen fistulated cows in mid-lactation were fed similar diets in a crossover experiment but with white clover fed ad libitum. Digestibility, nitrogen balance, milk yield, fat and protein concentration were evaluated over a 7 day period. Maize silage DM and white clover DM digestion in the rumen were evaluated by DM disappearance rates from nylon bags over a 96 hr period. Measurments of rumen fluid volume, fluid mean retention time, fluid outflow, pH, ammonia and VFA concentration and the acetate/propionate ratio were estimated over a 24 hr period. Supplementing cows with maize silage increased total DMI by 34%, without affecting DM digestibility (DMD). However, OMD was reduced by 5% and fibre digestion by 18% (P<0.05). Supplementing cows with maize silage did not affect the rate of digestion of maize silage DM or white clover DM in nylon bags, but white clover was digested at a faster rate than maize silage (P<0.05). The increased DOMI by cows supplemented with maize silage was highly correlated with a 24% increase in milk yield (P<0.05; iI=61 ) but had no effect on milk fat and protein concentration. At similar nitrogen intakes cows supplemented with maize silage produced 2.3 kg more milk (P<0.050='66) but there was no effect on milk fat and protein concentration. Rumen fluid volume, mean retention time and fluid outflow rate were not significantly altered by the maize silage supplement, although rumen pH and ammonia concentration were significantly lower (P<0.05). Total VFA concentration was not significantly different between the two diets, but acetate concentration was significantly lower (P<0.05), while propionate concentration was significantly higher (P<0.05) in supplemented cows. The reduction in acetate and the corresponding increase in propionate, lowered the acetate/propionate ratio. Butyrate concentration was not affected. It was concluded that for cows in mid-lactation fed on good quality white clover dominant pasture, maize silage supplementation can increase milk yield, without affecting milk fat and protein concentration. The increase in milk yield was attributed to increased apparent energy intake as nitrogen intake did not change markedly. However further work is required to quantify whether increased nitrogen utilization occurred and whether the lower ammonia concentration in maize silage supplemented cows were due to improved rumen microbial synthesis. The possibility also exists that improved milk yields were due to by-pass rumen starch, thereby providing glucose to the animal, after enzymatic digestion in the small intestine.