Aim: To evaluate urea hydrolysis, volatile fatty acid (VFA) production (odour) and coliforms in cattle waste slurries after a urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) and a plant oil component (thymol) were added.
Methods and results: Faeces from cattle fed a diet of 70% corn silage and 30% alfalfa haylage, urine and distilled water in the ratio 50 : 35 : 15 were blended at high speed for 1 min. Triplicate aliquots of 750 ml were amended with NBPT plus or minus thymol and reblended for 1 min, and were poured into 1.6 l wide-mouth jars covered 90% with a lid. After 56 days, thymol (2000 mg kg(-1) waste) in combination with NBPT (80 mg kg(-1) waste) retained 5.2 g of an initial 9.2 g of urea in cattle waste slurries, compared with less than 1 g of urea retained when NBPT was the only additive (P < 0.05). Another experiment using excreta from cattle fed 76.25% high moisture corn, 19.25% corn silage and a 4.5% supplement, blended at a low speed, gave a similar response with urea hydrolysis; and the two treatments, thymol alone and thymol in combination with NBPT, reduced VFA production (P < 0.01) and eliminated all coliform bacteria by day 1. A third experiment indicated coliforms disappeared in the no addition treatment after 8 days; however, they were viable at 6.6 x 10(4) CFU g(-1) waste beyond 35 days in the NBPT treatment.
Conclusions: Thymol supplements the effect of NBPT by increasing the inhibitory period for hydrolysis of urea in cattle waste slurries and nitrogen retention in the waste.
Significance and impact of the study: Thymol and NBPT offer the potential to reduce odour and pathogens in cattle manure, and increase the fertilizer value.