Growth monitoring and yield estimation of forage based on multiple phenological indicators

doi:10.17521/cjpe.2024.0174

Abstract

Abstract:

Aims Smart agriculture requires real-time monitoring of crop growth and accurate yield prediction. In this study, we aim to investigating the capacity for using multiple phenological indicators from phenocam images to monitoring forage growth and predicting forage yield.
Methods Phenocam and unmanned aerial vehicle (UAV) images were taken during a one-year field experiment on the forage production of two forage species, silage maize (Zea mays) and oats (Avena sativa), under different fertilizer treatments. Based on the green chromatic coordinate (GCC) extracted from phenocam images, and the normalized difference vegetation index (NDVI) and leaf chlorophyll index (LCI) extracted from UAV images, we explored the capacity of using phenocams in tracking the growth status and estimating forage yield at the site scale.
Important findings (1) Nitrogen application rate affected the phenological metrics and harvest index of the forage grasses. The longest growing period ((68 ± 5) d) and the highest dry matter yield ((28 548.30 ± 4 269.30) kg·hm^-2) for silage maize were observed under high fertilizer treatment, while for oats, the longest growing period ((59 ± 1) d) and the highest dry matter yield ((5 180.70 ± 1 939.05) kg·hm^-2) were found under medium fertilizer treatment. (2) GCC and LCI showed high correlation with plant height. Before reaching the position of peak greenness (POP), GCC can well capture the dynamics of the plant height (R² was 0.86 for silage maize and 0.49 for oats), and had the smallest bias in capturing the plant height dynamics of silage maize. (3) The phenological indicators from phenocam can effectively predict forage yield (R² was 0.829 for silage maize, 0.935 for oat). This study confirmed that phenocam can effectively capture the dynamics of forage growth and predict yield. The phenological indicators from phenocam could provide an effective way for real-time monitoring of forage growth and for informing management practices.

Key words: phenocam, green chromatic ccoordinate, forage, yield estimation, precision agriculture

YAN Wen-Xiu, ZHAO Shi-Han, ZHENG Chun-Yan, ZHANG Ping, SHEN Hai-Hua, CHANG Jin-Feng, XU Kang. Growth monitoring and yield estimation of forage based on multiple phenological indicators[J]. Chin J Plant Ecol, 2025, 49(7): 1096-1109.

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URL: https://www.plant-ecology.com/EN/10.17521/cjpe.2024.0174

https://www.plant-ecology.com/EN/Y2025/V49/I7/1096

Figures/Tables 10

Fig. 1 Unmanned aerial vehicle (UAV) image of the experimental field with plots of treatments and locations of the phenocams, and the illustration of the region of interesting (ROI) from the phenocams. A, UAV image on field trials. B-E, Phenocam images and ROI mapping of green up phase (B, D) and brown down phase (C, E). L, M and H represent low, medium and high fertilizer level, respectively.

Fig. 2 Processing flow of unmanned aerial vehicle (UAV) images and phenocam images. GCC, the green chromatic ccoordinate; LCI, the leaf chlorophyll index; NDVI, the normalized difference vegetation index; ROI, the region of interesting.

Fig. 3 Seasonal dynamics of the Green chromatic coordinate (GCC) and the retrieved events of phenology and phases of phenology during the whole growing period under different nitrogen fertilizer levels. A, Zea mays. B, Avena sativa. H, high fertilizer; L, low fertilizer; M, medium fertilizer. DD, downturn date; EOS, end of season; POP, position of peak greenness; SD, stabilization date; SOS, start of growing season. DOY, day of year.

Table 1 Phenological indicators and harvest index under different nitrogen fertilizer application levels (mean ± SD)

	青贮玉米 Zea mays				燕麦 Avena sativa
	L	M	H	p	L	M	H	p
物候期 Events of phenology (DOY)
UD	173 ± 7	180 ± 2	174 ± 2	0.271	173 ± 3	174 ± 1	175 ± 2	0.584
SOS	189 ± 4	198 ± 8	190 ± 5	0.314	179 ± 3	180 ± 3	186 ± 10	0.587
SD	202 ± 11	211 ± 10	204 ± 10	0.683	183 ± 4	186 ± 7	189 ± 10	0.753
POP	219 ± 9	227 ± 4	225 ± 2	0.480	202 ± 1	202 ± 8	203 ± 6	0.970
DD	242 ± 2	244 ± 3	243 ± 5	0.835	225 ± 1^ab	229 ± 3^a	221 ± 2^b	0.025^*
EOS	255 ± 2	256 ± 2	257 ± 3	0.501	232 ± 1^b	240 ± 2^a	228 ± 4^b	0.017^*
RD	266 ± 3	269 ± 3	282 ± 26	0.579	237 ± 0^ab	250 ± 6^a	234 ± 7^b	0.049^*
物候阶段 Phases of phenology (d)
变绿期 Green up phase	13 ± 7	13 ± 2	14 ± 5	0.971	4 ± 0	6 ± 3	3 ± 2	0.489
稳定期 Stable phase	39 ± 12	33 ± 13	40 ± 14	0.837	42 ± 5	43 ± 7	32 ± 10	0.309
枯黄期 Brown down phase	13 ± 4	12 ± 1	14 ± 7	0.907	6 ± 0	11 ± 3	7 ± 2	0.229
LOS	65 ± 3	58 ± 9	68 ± 5	0.326	53 ± 3	59 ± 1	42 ± 10	0.127
物候斜率 Slope of GCC during the phases of phenology (× 10³ GCC·d^-1)
PRR	48.06 ± 22.15	35.58 ± 9.97	38.99 ± 14.40	0.742	94.28 ± 0.87	57.29 ± 63.87	80.60 ± 42.65	0.704
PSR	-43.39 ± 11.13	-39.19 ± 6.51	-44.32 ± 25.10	0.946	-75.50 ± 4.27^b	-21.54 ± 18.75^a	-69.01 ± 24.19^b	0.043^*
收获特征 Harvest index
产量 Yield (kg·hm^-2)	15 985.20 ± 4 605.30	19 737.15 ± 5 929.20	28 548.30 ± 4 269.30	0.106	4 293.30 ± 274.20	5 180.70 ± 1 939.05	5 036.60 ± 787.95	0.830

Fig. 4 Plant height of forage during the growing season and the normalized vegetation index: green chromatic coordinate (GCC), normalized difference vegetation index (NDVI), leaf chlorophyll index (LCI). A, Zea mays. B, Avena sativa. DOY, day of year.

Fig. 5 Linear relationship between plant height and vegetation index: green chromatic coordinate (GCC), normalized difference vegetation index (NDVI), leaf chlorophyll index (LCI) for two forage species, the gray ellipses indicate the 95% confidence intervals. A-C, Zea mays. D-F, Avena sativa. H, high fertilizer; L, low fertilizer; M, medium fertilizer. POP, position of peak greenness.

Table 2 R2 and adjusted R2 (adj.R2) of the fitted models between yield and vegetation index for Zea mays and Avena sativa

植被指数 Vegetation index	青贮玉米产量R²R²of Zea mays yield	青贮玉米产量adj.R²adj.R²of Zea mays yield	燕麦产量R²R²of Avena sativa yield	燕麦产量adj.R²adj.R²of Avena sativa yield
GCC	0.032	-0.106	0.031	-0.131
NDVI	0.008	-0.134	0.059	-0.098
LCI	0.001	-0.141	0.031	-0.130
GCC × NDVI	0.034	-0.288	0.091	-0.272
NDVI × LCI	0.008	-0.322	0.247	-0.054
GCC × LCI	0.062	-0.251	0.096	-0.265
GCC × NDVI × LCI	0.062	-0.501	0.447	0.033

Fig. 6 Pearson correlation coefficients between phenological indicators and harvest index of two forage species. Plant height was measured at August 12. Browndownphase, days of brown down phase; DD, downturn date; EOS, end of season; Greenupphase, days of green up phase; LOS, length of growing season; POP, position of peak greenness; PRR, peak recovery rate; PSR, peak senescence rate; RD, recession date; SD, stabilization date; SOS, start of growing season; Stablephase, days of stable phase; UD, upturn date.

Table 3 The best fitting models between multiple phenological metrics and harvest index, and the corresponding adjusted R2 (adj.R2) and akaike information criterion (AIC)

收获指标 Harvest index	拟合方程 Fitting equation	调整后R² adj. R²	赤池信息量准则 AIC
青贮玉米株高 Plant height of Zea mays	y = 2610.663 - 9.479SD - 0.0492PRR - 4.934LOS	0.328	91.03
青贮玉米产量 Yield of Zea mays	y = -36427.42 + 224.29Greenupphase + 155.50UD + 119.23LOS	0.829	126.66
燕麦株高 Plant height of Avena sativa	y = 464 - 1.692DD + 0.8107Stablephase - 0.0014PRR	0.975	20.65
燕麦产量 Yield of Avena sativa	y = 8463.209 - 37.717SOS - 26.772Stablephase - 0.0325PRR	0.935	78.19

Fig. 7 Explanatory contribution of each factor in the best-fitting models derived from the contribution of each factor to the total R2. DD, downturn date; Greenupphase, days of green up phase; LOS, length of growing season; PRR, peak recovery rate; SD, stabilization date; SOS, start of growing season; Stablephase, days of stable phase; UD, upturn date.

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