Effect of feeding time and time of harvest of fresh pasture on urinary and faecal nitrogen excretion patterns in sheep

ABSTRACT Under New Zealand grazing systems, a high proportion of the nitrogen consumed by ruminants is excreted. This experiment examined the effect of feeding morning and afternoon harvested annual ryegrass, fed shortly after harvest, on the timing of nitrogen excretion in one‐year‐old sheep during winter. Dry matter and water‐soluble carbohydrate (WSC) content increased in the afternoon‐harvested ryegrass. This resulted in increased protein and WSC intakes and a decrease in nitrogen loss in the sheep fed in the afternoon with afternoon‐harvested grass. The pattern of urinary nitrogen excretion was related to feeding time with peak urinary urea excretion in the period 4–8 hours after feeding, with over 60% of the daily urinary urea excreted in the 12 hours after the feed was offered. This study suggests that shifting stock onto new pasture breaks in the afternoon could reduce nitrous oxide production by increasing the efficiency of nitrogen utilisation in animals and reducing the total amount of nitrogen excreted. It may be possible to utilise the potential diurnal differences in pasture composition to reduce nitrogen losses. This practise could also improve animal performance as well as change the timing of nitrogen excretion.