Rehydrating whole maize grain (WMG) causes swelling that may alter feed efficiency and digestibility in ruminants. This study assessed the effects of dry and rehydrated WMG on performance, feed efficiency, ingestive behaviour, blood parameters, and carcass traits of feedlot Nellore cows compared to a diet based on whole-plant maize silage. Twenty-four cull cows (351 ± 8.96 kg) were allocated to three dietary treatments: (i) whole-plant maize silage, (ii) rehydrated WMG, and (iii) dry WMG, in a completely randomised design over a 60-day evaluation period. Dry matter intake was lower in animals fed rehydrated WMG (6.29 kg/day) than in those fed maize silage (9.09 kg/day), while dry WMG diet (8.15 kg/day) was intermediate and did not differ significantly from either (P = 0.021). No differences were recorded between diets for daily average gain and final weight (P = 0.384). Feed efficiency was higher in cows fed rehydrated WMG (0.20 g/g) than in animals receiving dry WMG (0.12 g/g) (P = 0.040). Blood glucose, creatine kinase, and cortisol concentrations were unaffected by the diets (P = 0.134). Cows fed maize silage spent more time ruminating (15.98 min/h) than those fed with rehydrated (4.61 min/h) or dry WMG (4.43 min/h) (P = 0.009). Rehydrating WMG in nonroughage diets improved feed efficiency without compromising animal performance and carcass traits. Higher feed efficiency in cull cows supports the sustainability of beef production by reducing concentrate use without impairing feedlot system.