The objective of this study was to investigate the relationship between nitrogen (N) partitioning and isotopic fractionation in lactating goats consuming diets with a constant high concentration of N and increasing levels of water soluble carbohydrate (WSC). Eight lactating goats were offered four different ratios of WSC : N in the diet. A two-period incomplete cross-over design was used, with two goats assigned to each treatment in each period. N balance measurements were conducted, with measurement of feed N intake and total output of N in milk, faeces and urine. Treatment, period and infusion effects were tested using general ANOVA; the relationships between variables were analysed by linear regression. Dietary treatment and period had significant effects on dry matter (DM) intake (g/day). DM digestibility (g/kg DM) and N digestibility (g/kg N) increased as the ratio of WSC : N increased in the diet. No treatment effect was observed on milk urea N concentration (g/l) or urinary excretion of purine derivatives (mM/day). Although dietary treatment and period had significant effects on N intake, the change of N intake was small; no effect was observed for N partitioning among faeces, milk and urine. Milk, plasma and faeces were enriched in 15N compared with feed, whilst urine was depleted in 15N relative to feed. No significant relationship was established between N partitioning and isotopic fractionation. This study failed to confirm the potential to use N isotopic fractionation as an indicator of N partitioning in dairy goats when diets provided N in excess to requirements, most likely because the range of milk N output/N intake and urinary N output/N intake were narrow.

The natural abundance of 15N in animal proteins (δ15Nanimal) is greater than that in the diet consumed by the animals (δ15Ndiet), with a discrimination factor (Δ15N = δ15Nanimal− δ15Ndiet) that is known to vary according to nutritional conditions. The objectives of the present study were to test the hypothesis that Δ15N variations depend on the efficiency of nitrogen utilisation (ENU) in growing beef cattle, and to identify some of the physiological mechanisms responsible for this N isotopic fractionation in ruminants. Thus, we performed the regression of the Δ15N of plasma proteins obtained from thirty-five finishing beef cattle fed standard and non-conventional diets against different feed efficiency indices, including ENU. We also performed the regression of the Δ15N of different ruminant N pools (plasma and milk proteins, urine and faeces) against different splanchnic N fluxes obtained from multi-catheterised lactating dairy cows. The Δ15N of plasma proteins was negatively correlated with feed efficiency indices in beef cattle, especially ENU (body protein gain/N intake) and efficiency of metabolisable protein (MP) utilisation (body protein gain/MP intake). Although Δ15N obtained from different N pools in dairy cows were all negatively correlated with ENU, the highest correlation was found when Δ15N was calculated from plasma proteins. Δ15N showed no correlation with urea-N recycling or rumen NH3 absorption, but exhibited a strong correlation with liver urea synthesis and splanchnic amino acid metabolism, which points to a dominant role of splanchnic tissues in the present N isotopic fractionation study.

In quantitative feeding experiments, larvae of Grammodes stolida (Fab) consumed significantly more of mature leaves than young and senescent leaves of Lannea coromandelica (Houtt). The maximum leaf consumption was by the third instar. The amount of food consumed by the penultimate and final instars comprised 72.6% of the total larval consumption. Food utilisation indices, namely approximate digestibility (AD), efficiency of conversion of ingested food (ECI), and efficiency of conversion of digested food (ECD) showed a significant decrease during the second larval stage. The bionomics of the moth were studied in relation to its survivability and reproductive potential on Lannea coromandelica which appears to be a potential host which the pest could colonise during periods of non-availability of other crop hosts.