Voluntary intake of dry matter and water of sheep fed low dry matter forages and consequences for nitrogen excretion

Low dry matter (DM) forages (<16%) have been reported to restrict voluntary intake. However, New Zealand temperate grazing systems use a wide range of low DM% forages in high production systems. Interest in mitigating nitrogen (N) losses from farming systems has increased use of low DM% pasture forbs to dilute urine N concentrations. Voluntary DM intake (DMI), water and N intake, urine volume and N excretion were measured in young growing sheep fed a range of autumn-harvested, low DM% plantain, chicory, and ryegrass cultivars. Four diet-treatment groups of eight young male sheep were used in each of three consecutive runs, with 7 d total urine and faecal collection in metabolism crates. Feed, refusal, urine and faecal assessment of DM% and N% enabled DM, water and N intake, urine and faecal water, and N excretion to be calculated. The association of DM% and DMI, and both water intake and suspected phyto-diuretics with urine output, were also assessed. Voluntary DMI considered high (>25g/kg BW) were observed in all diet-treatments (DM%: 8.5-16.0%), was above 30g/kg BW in eight of ten diet-treatments (8.5-13.3%), and was lowest in the highest diet-treatment DM%. The association of DM% and DMI was negative, weak (r 2= 0.22) but significant (P < 0.05). Total water intakes were high across all diet-treatments (126-390 ml/ kg BW/ d), but least in a ryegrass diet-treatment. Urine volumes were high (3697-11654 ml/ d), and greatest in a chicory diet-treatment. The association of feed DM% with both total water intake (r 2=0.83) and urine output (r 2 >0.90) was negative, strong and significant (P<0.001), while that of forage concentrations of suspected phyto-diuretics with urine output was negative, weak (r 2 = 0.20-0.44), but significant (P < 0.01). Urine DM voided was high with greater volumes, suggesting higher costs. Total N output and urinary N output were greater in plantain diet-treatments, while the proportion of excreted N in urine was lower (P < 0.05) in all plantains compared to chicory and ryegrass diet-treatments, and the urine N% was greatest in ryegrass, and similar among the plantain and chicory diet-treatments. Conclusions: high DMI can be achieved with low DM% forages in young growing sheep, and the feed water intake, not phyto-diuretics, produces the observed high urine volume. Low DM% temperate forages are therefore suitable for N excretion mitigation via increased urine volume and urinary N% dilution, but urine energy and mineral costs may increase.