Growing dairy heifers prefer supplementary long straw when fed a nutrient-dense ration in a limited amount

The objective of this study was to determine whether limit-fed heifers will choose to consume long particles, rather than short, of a low-nutritive feedstuff to ameliorate rumen function and meet foraging needs. Ten Holstein heifers 261.6 ± 39.2 (mean ± SD) d of age were exposed to each of 2 dietary treatments, in a random order, over 2 successive 7-d treatment periods (4-d adaptation period and a 3-d data collection period) using a crossover design. The treatments were (1) a provision of long particle oat straw (85% of particles>8mm; LS) and (2) provision of short particle oat straw (45% of particles >8mm; SS). Both treatments were offered following consumption of a limit-fed, nutrient-dense total mixed ration fed at 2.05% of body weight. Following each 7-d period, heifers were given access to both types of straw during an additional 2-d preference period; individual intakes were recorded daily. Feeding and lying behavior were recorded during the last 3d of each treatment period. Ruminal temperature was recorded during the last 3d of each treatment period using a telemetric acquisition system and rumen boluses. Dry matter intake of both the total mixed ration (6.3 kg/d) and straw (0.36 kg/d) was similar between treatments. Heifers fed LS spent more time feeding (197.7 vs. 175.2 min/d) throughout the day than heifers fed SS due to the increase in time required to consume long particles in the LS (59.8 vs. 34.2 min/d). Daily lying time (974.7 min/d) and time spent standing without eating (278.9 min/d) was similar between treatments. The preference period showed a strong preference ratio for LS rather than SS (preference ratio=0.83), with heifers consuming 0.43 ± 0.2 kg/d of LS and 0.07 ± 0.1 kg/d of SS (mean ± SD). Heifers maintained similar mean (38.3°C), minimum (35.1°C), and maximum (38.9°C) rumen temperature across treatments. The amount of time that rumen temperature was elevated over 38.6°C, 39.0°C, and 39.4°C was similar between treatments. In conclusion, heifers will consume similar amounts of supplementary long or short straw if provided to them alongside of a limit-fed TMR. Limit-fed heifers do, however, show a clear preference for LS when offered the choice, suggesting that they find LS to be more satisfactory for achieving rumen fill or meeting their behavioral foraging needs.