The effects of sodium and potassium fertilizers on the grazing behaviour of dairy cows

Sodium fertilizer has been observed to increase the grazing time of dairy cows kept on pastures of restricted availability, but it is unclear whether this is a behavioural response to changes in pasture composition or whether it is an indirect effect of a reduction in herbage availability caused by increased cow production and appetite. Sodium fertilizer was applied in an experiment utilising tall pasture and low stocking rates so that availability would not be restricted. Cows on the pasture with sodium fertilizer still had longer grazing times than those on pasture where no sodium fertilizer had been applied. They had more feeding bouts but bout length was reduced, and they had faster biting and walking rates while grazing. They ruminated for longer, with a faster chewing rate, and spent longer standing idle and less time lying idle. They had longer drinking bouts but a similar number of them, compared to the cows on pasture without sodium. The fixed and variable energy costs of grazing and drinking bouts were estimated, these being the energy requirements for activities that are independent and dependent of bout duration, respectively. It is proposed that for grazing bouts the low ratio of fixed costs (getting up and lying down) to variable costs (standing and walking) favoured an increase in bout number to increase the grazing time of cows grazing pasture fertilised with sodium. For drinking bouts, the high ratio of fixed costs (getting up, lying down, standing and walking to the trough) to variable costs (standing at the trough) favoured bout elongation to increase the drinking time of cows grazing pasture fertilised with sodium. A second experiment with unrestricted herbage availability between morning and afternoon milking compared the grazing response to sodium and potassium fertilizers with a control where no fertilizer was applied. Grazing time was increased most by sodium and a smaller response occurred with potassium. The proportion of herbage that was rejected by the cows decreased with sodium and to a lesser extent potassium. It is concluded that changes in herbage composition with sodium fertilizer increase feeding and drinking by the grazing dairy cow, with the increase in feeding being achieved by increased bout number but reduced bout length, and the increase in drinking being by increased bout length. (C) 1999 Elsevier Science B.V. AU rights reserved.