Field and modelling studies of the effects of herbage allowance and maize grain feeding on animal performance in beef cattle finishing systems : a thesis in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University, Palmerston North, New Zealand
The objetive [sic] of the work described in this thesis was to develop a mathematical model designed as a tool for research intended to improve the efficiency of finishing systems for 1-2 year old beef cattle under intensive grazing mangement [sic] on sown pastures in Argentina. The work involved a) three experiments in Argentina carried out to define the effects of herbage allowance and maize grain supplementation on herbage intake and animal performance, b) one experiment in Argentina following a preliminary study in New Zealand of seasonal variation in the composition and nutritive value of intensively managed beef pastures, and c) an exercise to develop a model of beef cattle production incorporating modules dealing with aspects of pasture production and utiisation [sic], herbage intake and animal performance. The results from the series of short-term grazing studies showed consistency in the comparison of the effects of increasing herbage allowance and supplementation on herbage intake and animal LWG (Chapter 4). A method combining the use of n-alkane and 13C method proved to be accurate for quantitative estimates of herbage and maize grain intake, and allowed estimates of a substantial variation in individual maize grain intake (between 31 to 41 % CV) when animals are supplemented in groups. The substitution rate (SR) measured in these studies varied little across experiments or level of grain at a herbage DM allowance of 2.5 % LW d-1 (0.36 and 0.38 kg herbage DM per kg grain DM for Chapters 3 and 4 respectively). Increasing level of herbage DM allowance increased quadratically the SR from 0.38 to 0.83 and 0.87 kg herbage DM per kg grain DM. The n-alkane method was effective in providing estimates of diet digestibility. Different methods for estimating diet composition, such as microhistological evaluation of faeces, differences in nutrient and component selection indexes and n-alkanes were used in the initial grazing trial (Chapter 2) but they were not considered to be reliable and they were too laborious for continued use under field conditions. The outcome of the studies on seasonal variation in herbage quality initially was useful in establishing a database of the range of values observed, and in demonstrating their relative robustness, at least under conditions of good pasture management. In these studies, herbage nutritive value did not seem to be a limiting factor for growing beef cattle, at least in terms of the minimum observed content of metabolisable energy (10.8 MJ ME kg DM) or crude protein (17.3 % DM). Additionally, significant relationships were established between morphological and maturity estimates and herbage nutritional variables in a pasture under grazing conditions. These relationships showed promise for future use in the prediction of herbage nutritive value, but require further work. The model developed ("BeefSim"), represents the main biological dynamic processes of the target system of this thesis, together with additional management decision and financial estimates. It was shown that the model presents adequate flexibility and can be interrogated in terms of its response to different management conditions, scenarios and timeframes. Pasture management and grain feeding were controlled in an interactive management module responding to deviations in pasture conditions and animal liveweight from pre-determined targets. Two key outcomes of the model, liveweight gain and herbage intake were accurately predicted when compared against experimental information under different levels of herbage allowance and maize feeding. System comparisons developed with the model showed agreement with the literature, and maize grain feeding associated with the monitoring procedure demonstrated an effective use of grain in the system. The model provides a good biological basis for a holistic appraisal of the effects of "process technologies" such as grain feeding in beef cattle finishing systems, and will be developed further.