We offered heifers the choice between a tall abundant reproductive sward (RS), evolving from ear emergence to full flowering in the course of the experiment, and a short (8-cm high, treatment S) or tall (14-cm high, treatment T) vegetative sward (VS). This aimed to create a trade-off between intake (maximal on RS) and diet quality (maximal on VS) to test heifers motivation to keep grazing the better quality forage (VS) as its accessibility and the quality of the alternative decreased. Six heifers were allocated per treatment and placed in experimental choice conditions for three successive 10-day periods. Feeding choices were recorded for 2 days per period from dawn to dusk. Total intake and diet quality were estimated from nitrogen and chromic oxide contents in faeces sampled during the last 5 days of the periods. Additional short-term intake rates were measured on all swards once per period. As expected, preference for VS increased as RS matured in both treatments (S or T). The behavioural response of the heifers to the decrease in VS height depended on RS maturity: the heifers increased their preference for RS in the first period and did not modify their choice in the second period. In the last period, not only they did not increase their grazing time on RS, but increased that on VS, even though potential DM and DOM intake rates remained higher on RS. They thus clearly expressed a strong preference for the vegetative sward. Increases in grazing time and biting rate enabled them to maintain both total intake and diet digestibility at the day scale. This emphasises the importance of the time scale used when observing diet choices of ruminants, particularly when these are offered a choice between foods of different digestibility and fill effect in the rumen. The results also suggest that if cattle are required to consume tall ageing RS swards in extensive grazing systems, then any VS patches should be very short or the animals should have high nutritional requirements and/or be given shorter daily grazing times.