<p><i><b>Context. </b></i>Large-ruminant production in developing countries is inefficient with low growth rates and declining weights, particularly in the dry season.</p> <p><i><b>Aims.</b></i>The impact of <i>ad libitum</i> supplementation of cattle with high-quality molasses blocks (20 kg) containing either 8% urea (UMB) or nil urea (MB), was examined.</p> <p><i><b>Methods.</b></i> Field trials on smallholder farms compared weight changes and average daily gains (ADG = g/day) data of young calves <8 months of age (<i>n =</i> 25)" growing calves 8–24 months (<i>n =</i> 35) and lactating cows (<i>n =</i> 46), of the indigenous breed when accessing either UMB or MB, with data being collected at Weeks 1, 4, 8 and 12. A pen study was also conducted at a research station involving mature, lactating crossbred cows (<i>n =</i> 37). Surveys of farming families experiencing use of the blocks was conducted (<i>n =</i> 20).</p> <p><i><b>Key results.</b></i> On smallholder farms, animals accessing UMBs were heavier than those accessing MBs at every collection day and in young calves these differences were statistically significant (<i>P </i>< 0.05). ADGs were higher in cattle accessing UMB than in those accessing MBs. Young calves had the highest ADG (251–265 g/day), followed by growing calves (198–237 g/day) and lactating cows (187–190 g/day), although differences in ADG between UMB and MB cohorts were not considered significant (young calves <i>P =</i> 0.562" growing calves <i>P =</i> 0.509" and lactating cows <i>P =</i> 0.993). Results from the pen study identified that ADGs were not significantly different (<i>P =</i> 0.933) between crossbred cows accessing MBs (236 g/day) and cows accessing UMBs (229 g/day). Surveys of farmers using blocks confirmed that their animals were calmer and healthier, and had better coat condition with minimal external parasites" these farmers wished to purchase the blocks and were willing to pay a mean up to US$6.5 ± 2.3 per block.</p> <p><i><b>Conclusions.</b></i> Provision of UMBs and MBs in Laos in the late dry season improved cattle growth rates, which is consistent with previous studies and far superior to the base-line data from Laos demonstrating declining ADGs. Farmers considered that the blocks contributed greatly to herd management and improved sale-ability of their cattle.</p> <p><i><b>Implications.</b></i> Provision of molasses blocks on low-input smallholder farms in developing countries significantly improves production efficiency, offering an 'entry point' intervention while forages are becoming established.</p>