Foraging behaviour of brent geese, Branta b. bernicla, on grasslands: effects of sward length and nitrogen content

Feeding behaviour and preferences of brent geese, Branta b. bernicla were o bserved on pastures of different sward lengths and nitrogen contents. On sw ards of 2.0-6.0 cm captive geese took larger bites and had a higher intake rate when feeding on 6-cm swards than when feeding on shorter ones. In the field wild geese chose 6-cm swards in preference to both shorter and longer ones. For unfertilized pastures there was a strong negative relationship b etween nitrogen content and sward height: Il-cm swards contained 2.8% nitro gen, 3.5-cm swards 4.2% nitrogen. Application of 75 kg N ha(-1) organically based fertilizer at the end of October eliminated this relationship betwee n nitrogen content and sward height, swards of all heights then having a me an content of 4.1% N. On fertilized plots the geese preferred swards longer than 6 cm with no indication of a decrease in preference up to the maximum height investigated, 16 cm. Breaking strain of grass laminae was measured to give an indication of the proportion of strengthening tissues in the lea ves and hence their digestibility. Apical laminae from longer swards had a higher breaking strain than those from shorter swards. Unfertilized swards had a higher breaking strain than fertilized swards but the difference in b reaking strain between long and short swards was the same on fertilized and control treatments. These results are discussed in relation to the forage maturation hypothesis and are interpreted as indicating that the primary co nstraint on maximizing energy intake rates is not the reduced energy digest ibility of older foliage but the reduced nitrogen content in the higher-bio mass swards. We conclude that it is the balance between maximizing energy i ntake and nitrogen absorption rates which results in the observed preferenc e for intermediate-height swards.