Systems, component, and modelling studies of pasture-based dairy systems in which the cows calve at different times of the year : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University, Palmerston North, New Zealand

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Massey University
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The New Zealand's dairy system is characterised by a concentrated calving period in late winter-early spring, which aims to synchronise cows' feed requirements with the seasonal pattern of pasture growth, but which also results in an uneven distribution of milk supply to the factories. Changing the calving season of some herds from spring into autumn could improve the overall efficiency of the dairy industry. However, pasture-based autumn-calving systems are usually perceived to be less "efficient", because of the lack of synchrony between feed supply (grazed pasture) and feed requirements. One conclusion of the literature review (Chapter 1) was to hypothesise that autumn- and spring-calving systems would perform at similar levels provided that sufficient supplementary feed was available during wintertime. This thesis integrated three experimental approaches (system, component, and modelling) in order to test the above hypothesis, and to investigate the physical performance of pasture-based dairy systems that differed in their calving dates. A 3-year system study conducted at No 1 Dairy Farm, Massey University, in which autumn, spring, and autumn/spring calving systems were compared, showed that all systems achieved similar performances and overall efficiencies (Chapter 2). A key factor for this was the greater total yields by the autumn-calved cows, due mainly to their greater yields in mid and late lactation and their longer lactations (Chapter 3). A new technique that combines the n-alkanes and 13C methods in order to quantify herbage and maize silage DM intakes by individual grazing cows which are given access to the silage as a group, was developed and validated (Chapter 4), and re-evaluated in a separate study (Chapter 5). Overall, individual cows differed considerably in their intakes of maize silage DM, but this variation was not always related to variation in milk yields. An innovative, dynamic, interactive simulator of seasonal pasture-based dairy farms (IDFS) was developed as part of this thesis (Chapter 6). The model allows computer experiments to be run, with pastures and cows managed on the basis of logical decision rules; therefore, it resembles real farm management. The user makes decisions (which paddocks are to be grazed, pre- and post-grazing herbage mass, supplement feeding, etc) continuously, and can see the impact of his/her management decisions on the graphical interface provided. Based on comparisons with actual data, it was concluded that IDFS simulates the main components of seasonal dairy farms with reasonable realism (Chapter 7), although the model is at an early stage of development and has not been completely validated. In conclusion, this thesis has 1) demonstrated that pasture-based systems with contrasting calving dates can achieve similar physical performances provided that supplementary feeds are available, and 2) developed two new tools (quantification of herbage and maize silage intakes by individual cows, and the IDFS model) that can be applied in future systems research.
Dairy cows, Calving, Grazing management