Forage Allowance as a Target of Grazing Management: Implications on Grazing Time and Forage Searching

This work aimed to evaluate the following hypotheses: 1) the daily grazing time (GT) and 2) forage searching are more associated with the sward structure than with the levels of daily forage allowance (FA). To this end we proposed a model that was tested through an analysis of the sward structure, grazing time, and displacement in grazing by heifers on the natural grassland of the Pampa Biome (southern Brazil), which has been managed by FA levels since 1986. For three seasons, between January 2009 and February 2010, we evaluated the effect of FA on the main descriptors of the sward structure (herbage mass, sward height, and tussocks frequency) and the effect of these on the GT, displacement rate (DR), and daily displacement (D) in grazing. The data were analyzed with the use of regression and descriptive analyses from three-dimensional contour graphs with the data of the sward structure and GT. The DR was not associated with the FA levels or sward structure; however, the DR presented a positive linear relationship with the D and GT. The incremental change in the GT was accompanied by an increase in the D. Lastly, independently of the level of the FA and season evaluated, the lower values of GT were always associated with the following structural configuration: forage mass between 1 400 and 2 200 kg DM · ha−1, sward height between 9 and 13 cm, and tussock levels not exceeding 35%. Outside these limits, a penalty occurred in the GT and displacement patterns of the heifers. We found evidence that a better understanding of the cause–effect relationships between the sward structure and the ingestive behavior of the animals demonstrates the possibility of increasing the performance of domestic herbivores with important economic and ecological consequences. El objetivo del estudio fue evaluar las siguientes hipótesis: (i) si el tiempo de pastoreo diario (TP) y (ii) la búsqueda de forraje están más estrechamente relacionados a la estructura del pasto que a los niveles diarios de oferta de forraje (OF). Con este fin, propusimos un modelo que se puso a prueba en base al análisis de la estructura del pasto, el tiempo de pastoreo y el desplazamiento en pastoreo en terneras sobre un pastizal natural del Bioma Pampa (sur de Brasil) que, desde 1986, se ha manejado con distintos niveles de OF. En tres épocas, entre Ene/2009 y Feb/2010, se evaluó el efecto de la OF sobre los principales descriptores de la estructura del pasto (biomasa de forraje, altura y frecuencia de mat