Simulation of the nitrogen use efficiency in pasture-finished steers in southern Chile

The high content of crude protein (CP) of pastures in southern Chile creates a nutritional imbalance that results in large amount of nitrogen (N) being excreted into the environment. Two regression equations and a dynamic model, available from the literature, were utilized to simulate the N use efficiency (NUE) in pasture-finished steers. A medium frame animal (Angus breed) with an initial and final body weight (BW) of 350 and 500 kg, respectively, was used to estimate nutrient requirements and excretions. Average DMI was fixed at 2.2% of animal BW with 150 days on feed (October to February). Energy and protein requirements, as well as the retained protein, were estimated by using Beef NRC software (tabular system level 1), while the nutritional characteristics of the pasture were obtained from studies based in Chile. The quality of the pastures was good enough to allow daily weight gain (GDP) of 1.0 kg/d. However, metabolizable protein balance was positive across the whole finishing period (average of 149 g/d). Total production of microbial protein increases with time because of the greater DMI, even when the PC content in forage decreases with time. This also explains the lower contribution of DIP during the last months of the finishing period. The estimated N intake was 123 g/d, whereas N excretion was 106 g/d. In summary, the combination of high levels of CP of Chilean southern pastures and the low NUE of the finishing cattle (13.4 - 16.3%), represent a strong challenge in terms of contamination and production.

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