Evaluation of refractive index for measuring urinary nitrogen concentration in a sensor worn by grazing female cattle

Figures & data

Figure 1. AgResearch urine sensor assembly (left) and attachment on a cow (right).

Table 1. Summary of nitrogen, potassium and sodium concentrations of the urine samples from different feed types used to evaluate refractive index. K and Na concentrations were not measured in urine from the mixed pasture. The average least significant difference value (LSD) for P = 0.05 is presented.

Feed		N	K	Na	K:N ratio
		(g/L)	(g/L)	(g/L)
Clover	n	48	24	24	24
	mean	6.9	11.0	0.3	1.6
	range	2.0–11.4	3.6–15.0	<0.1–1.5	0.9–2.5
Ryegrass	n	48	24	24	24
	mean	7.7	14.6	0.2	1.8
	range	2.4–12.2	1.4–18.7	<0.1–0.9	1.3–2.3
Turnips	n	46	24	24	24
	mean	2.1	13.2	0.2	6.3
	range	0.8–3.7	6–17.5	<0.1–0.8	4.5–10.7
Mixed	n	26
	mean	1.9
	range	0.6–5.0
LSD		1.06	3.01	0.37	5.0
P		<0.001	ns	ns	<0.001

Figure 2. The relationship between N and K concentrations in urine from cattle fed monocultures of clover, ryegrass or turnips. Prediction confidence intervals for the 2.5th and 97.5th percentile intervals are shown.

Figure 3. The regression between urine sensor refractive index (RI) value and urinary-N concentration from cows fed on ryegrass (n = 48 samples from six cows), clover (n = 48 samples from six cows), turnip (n = 46 samples from six cows) or a mixed ryegrass/clover pasture (n = 26). Prediction confidence intervals for the 2.5th and 97.5th percentile intervals are shown.

Table 2. Regression coefficients (with standard error) for the relationship between refractive index and urinary nitrogen and urinary potassium concentrations for each feed type.

Feed	Intercept	Slope	P value (slope)
Nitrogen
Clover	−1112 (5.9)	835 (4.4)	<0.001
Ryegrass	−887 (4.3)	666 (3.3)	<0.001
Turnips	−590 (7.8)	442 (5.8)	<0.001
Mixed	−1085 (13.2)	814 (9.7)	<0.001
Potassium
Clover	−1399 (15.7)	1051 (11.7)	<0.001
Ryegrass	−1258 (11.3)	946 (8.4)	<0.001
Turnips	−2768 (22.6)	2078 (16.9)	<0.001

Figure 4. The regression between urine sensor refractive index (RI) value and urinary-K concentration from cows fed ryegrass (n = 24 samples from three cows), clover (n = 24 samples from three cows) or turnip (n = 24 samples from three cows). Prediction confidence intervals for the 2.5th and 97.5th percentile intervals are shown.