Variation in the Normalized Difference Vegetation Index (NDVI) in dairy farms in northern Antioquia

Anderson Bastidas Duque^*, Rolando Barahona Rosales¹ and Mario Cerón-Muñoz

Research Group on Animal Genetics, Breeding and Modeling (GaMMA), Facultad de Ciencias Agrarias, Universidad de Antioquia, Colombia.
grupogamma@udea.edu.co
¹ BIOGEM Research Group, Faculty of Agrarian Sciences, Universidad Nacional de Colombia, sede Medellín, Colombia.
^* Joven Investigador Colciencias

Abstract

Grassland vegetation cover is a factor of great importance to productivity and environmental impact of livestock, thus proper monitoring should be a priority for tropical livestock production. A novel way to do this is monitoring the normalized difference vegetation index (NDVI), calculated by the relationship between the red and near-infrared bands of satellite images, which allows identification of areas with high or low vegetation cover. The present study was undertaken with the purpose of analyzing the variation in NDVI in the north of Antioquia dedicated to dairy farming between 1995 and 2014. To do this, the vegetation index of 103 dairy farms in northern Antioquia was evaluated, using a generalized additive mixed model including the softened effects of the variables latitude, longitude, date and farm. The NDVI varied between 0.4 and 0.8, showing an average value of 0.61 and the municipalities with the greater vegetation index during the period evaluated were: San Pedro de los Milagros, Entrerrios and Donmatías. By contrast, Santa Rosa de Osos and San Jose de la Montana were the municipalities that had the lowest NDVI values. The NDVI data obtained suggests that following the implementation of dairy farming, a stable vegetation index has been maintained over time in the region studied, with no evidence of significant changes in vegetation cover. These findings should be corroborated following an on-site approach which should also be aimed at understanding the main factors underlying this response.

Keywords: livestock, pasture, remote sensing, vegetation cover

Resumen

La cobertura vegetal de las praderas es un factor de gran importancia para la productividad y el impacto ambiental de la ganadería, con lo que su monitoreo adecuado debe ser una actividad prioritaria para la ganadería tropical. Una forma novedosa de realizar este monitoreo es a través del índice de vegetación de diferencia normalizada (NDVI), calculado por medio de la relación entre la banda del rojo e infrarrojo cercano de imágenes satelitales, que permite identificar áreas con alta o baja cobertura vegetal. El presente estudio se realizó con el propósito de analizar la variación del NDVI, en la zona norte de Antioquia dedicada a la ganadería de leche entre el año 1995 y 2014. Para esto, se evaluó el índice de vegetación de 103 fincas lecheras del norte de Antioquia, mediante un modelo mixto aditivo generalizado incluyendo los efectos suavizados de las variables latitud, longitud, fecha y finca. El NDVI varió entre 0.4 y 0.8, presentando un valor promedio de 0.61 y los municipios que presentaron un mayor índice de vegetación durante el periodo evaluado fueron: San Pedro de los Milagros, Entrerrios y Donmatías. Por el contrario, Santa Rosa de Osos y San José de la Montaña fueron los municipios que presentaron los menores valores de NDVI. La información de NDVI sugiere que en la zona de estudio, desde la implementación de la ganadería de leche, el índice vegetación se ha mantenido estable en el tiempo, sin existir evidencia de cambios en cobertura vegetal. Estos hallazgos deben ser corroborados siguiendo una aproximación de sitio, que debe también ser dirigida a contribuir a la comprensión de los principales factores responsables de esta respuesta.

Palabras clave: cobertura vegetal, ganadería, pastos, teledetección

Introduction

Antioquia is the department with the greatest milk production in Colombia, producing of approximately 3.5 million liters / day; most of which (70%) are produced in the north subregion (MADR 2012), In recent years, milk production has been growing due to the increase of cattle inventory, together with the adoption of breeding programs and pasture improvement (CorlacR2 2012).

The northern highland Antioquia forming the Central Cordillera north of the department of Antioquia, is characterized by having mild to moderate slopes crossed by a dense network of drainages (Escobar 2006) .The majority of soils are Hapludands and Fulvudands (Jaramillo 2002), which are poorly developed soils originated from black, sandy loam and friable volcanic ash, slightly acidic, of high cation exchange capacity, low base saturation, low available phosphorus and moderate fertility (Zapata 2006).

According to Gómez (2010), this area is covered mainly by grass pastures (37.9%), natural forests (22.1%), secondary vegetation (21%) and annual crops (9.7%). The Kikuyu grass (Cenchrus clandestinum (Hochst ex Chiov)) is the main grass in the specialized systems bovine milk production in the high tropics of Antioquia (Jaimes et al 2015), due to their habit of stoloniferous growth, high aggressiveness and resistance to trampling (Ramirez 2013; Ruiz et al 2014). It is a grass with high nitrogen requirements, so fertilization takes place after grazing to maintain an adequate supply of biomass (Mejia-Taborda et al 2014).

The floristic diversity of pastures is lower than that observed in the stubble and logged forests, due to the structural homogeneity of the vegetation. Plants from the Poaceae, Cyperaceae and Asteraceae families thrive there, at the edges of the pastures is common to find ferns and among the trees and bushes it is common to observe Cupressus lusitanica (cypress), Eucalyptus sp. (eucalyptus), Pinus patula (pine), Tibouchina lepidota (lasiandra), Myrcia sp. (myrtle) and Weinmannia pubescens (encenillo) among others (Robles 2006).

Over the years, this part of Antioquia has been subjected to different types of land use, with mining being one of the most common, from where soils are then devoted to agricultural activities which include livestock production, as it was the case in northern Antioquia. Together with these processes, there are changes in vegetation cover, either increasing or decreasing it and these variations are also influenced by climatic conditions and human activity (Escobar 2006).

Remote sensing is a technique that allows to gather information without a physical contact (Sobrino 2010). Through the use of satellites, images of the Earth's surface are obtained that can be used in various medium and long term studies of plant and soil processes, forest degradation analysis and detection of land use changes (Meneses-Tobar 2011).

The use of vegetation indices has become an important tool in agriculture, and it allows to monitor areas devoted to livestock production, as in the case of grazing systems used in specialized dairy farms where there is a periodic variation due to grazing (Barrachina et al 2009). The use of satellite images allows monitoring the productivity of pastures over time and identify periods of lower production of biomass, and the presence of diseases that reduce the vegetation index values in the pastures or areas evaluated (Escribano and Hernández Díaz-Ambrona 2013).

There are different techniques applied to the monitoring of vegetation, among which are those of radar, LIDAR and optical type. The radar wave sensors send signals that allow taking pictures in the presence of clouds and darkness. They also allow typifying the vertical structure of forest areas (Zapata and Anaya 2011; Marchionni and Cavayas 2014). LIDAR sensors use laser beams to measure directly the distribution of the vegetation canopy in three dimensions, in addition to terrain topography. These provide better information, but their current limitation are high costs and lower availability of images to study. Meanwhile, satellite images captured by optical sensors have a high temporal resolution and are inexpensive. Although these images are limited in clouded conditions and allow less accurate estimates in areas of very dense vegetation, there have been multiple studies on the variation of the growth of pastures through their use, specifically through the Normalized Difference Vegetation Index (NDVI) (Zapata and Anaya 2011). Although there are several methods for using satellite images to study changes in vegetation cover (Chuvieco 1998), the use of NDVI enables classification of different soil covers of a given area and differentiate active or vigorous vegetation from senescent or ill vegetation, by contrasting the amount of visible (60um - 70um) and infrared (70um - 90um) light which is reflected by green vegetation (López 2012).

One use of the NDVI is to evaluate how vegetation changes reflect the dynamics of the environment, as the vegetation is an important indicator of the effects of climate change, which is depicted by the distribution of vegetation in time and space (Aguilar et al 2012; Rodriguez-Moreno and Bullock 2013). In addition, NDVI is used to estimate the quantity, quality and growth of the vegetation (Hernández et al 2014). Thus, the use of satellite images to monitor vegetation is common and has been used to monitor the phenology in different crops (Hmimina et al 2013).

The aim of this study was to analyze the variation of Normalized Difference Vegetation Index (NDVI) in areas in the north of Antioquia dedicated to dairy farming between 1995 and 2014.

Materials and Methods

Information of NDVI was calculated from images between 1995 and 2014 of the satellites Landsat 5,7 and 8 on 103 dairy farms located in the municipalities of San Pedro de los Milagros, Entrerrios, Santa Rosa de Osos, Donmatías and San José de la Montaña, in the northern region of Antioquia department (see Figure 1). These farms were associated to the "Strengthening of the dairy production chain in the Northern District of Antioquia" project carried by University of Antioquia and the National University of Colombia - Medellín and funded by the Colombian General Royalties System and the Government of Antioquia.

Figure 1. Location of the municipalities in the department of Antioquia, where the vegetation index data were obtained.

A total of 88 satellite images were obtained which were subjected to an atmospheric correction on the B3 (red) and B4 (near infrared) bands when images came from Landsat 5 and 7, and bands B4 (red) and B5 (near infrared) when images came from Landsat 8, using the dark object subtraction method to estimate the amount of light dispersion due to particles and aerosols in the atmosphere and subtract it digitally. In addition, a mask of valid pixels was applied, where the pixels are classified into three categories: Clouds, no-clouds and gaps and edges. This was carried out through a mask generated by an unsupervised "k-means" classification, whose fundamental principle is the classification of the pixels in k number of classes by proximity to their means. Subsequently, the image was cut to the size of the study area. To do this, a survey of each farm was obtained by GPS and a shape of the perimeter to each farm was created. The index was calculated by the following relationship:

NDVI: (ρNIR – ρR) / (ρNIR + ρR)

Where: ρNIR: near infrared band and ρR: red band

To model the relationship between vegetation index and the other variables, the following information of the image was taken into account: latitude (m, between 6.39 °N and 6.87 °N), longitude (p, between -75.68 °W and -75.35 °W), year (between 1995 and 2014), day of the year (d, between 1 and 365) and Julian date (j) and located on the farm (f). Analyses were performed using generalized additive mixed models (GAMM), using the MGCV library (Wood 2011) of the R Software project (R Core Team 2015). A total of 18 models were evaluated; in which the variables were subjected to different interactions and smoothing effects to determine the model that better relates the vegetation index with the other variables (Table 1).

Results and discussion

The value of the obtained NDVI interval varies between minus one (-1) and plus one (+1). Of these, only the positive values correspond to areas of vegetation, values close to zero signify no vegetation, and negative values belong to clouds, snow, water, areas of bare soil and rocks (Escribano and Hernández Díaz-Ambrona 2013).

The model with the greater R2 (0.32) and lower BIC, was the one that considered the combination of year (a_i) and the day of the year (d_j) as a smoothed function and the combination of latitude (m_k) and longitude (p_l):

y_ijklm = α+ s(a_i, d_j) + s(m_k, p_l) + e_ijklm

As shown in Figure 2, in the dairy region of northern Antioquia, the vegetation index presented low values (<0.50) in January, February, November and December throughout the period evaluated. More specifically, between 1995 and 2002, the NDVI showed its lowest value (0.40) in days 1 to 20 (January) and 320 to 345 days (November and December), showing highly significant differences (p <0.01). In analyzing figure 2, it must be remembered that IDEAM (2007; 2012) reported that between 1997 and 1998 occurred the strongest El Niño of the millennium in Colombia, resulting in a considerable reduction in the volume of rainfall in the region. In addition, during 2002 and 2010 occurred other droughts of lower intensity. Increased vegetation index (> 0.60) between days 60 and 100 (March and April) coincided with the start of the rainy season reported for the Andean region (IDEAM 2012). De la Casa and Obando (2006) found a relationship between vegetation index and precipitation, indicating that plant development was reduced when rainfall was less than 40mm/month.

The northern region of Antioquia had NDVI values between 0.40 and 0.80 during the period evaluated, and thus the vegetation index was found near the ranges reported by other authors. Roldán and Poveda (2006) assessed the temporal variability of NDVI in five regions of Colombia and found that for the Andean region NDVI values were between 0.38 and 0.69. In turn, for pasture-only zones in Spain, Escribano and Hernández Díaz-Ambrona (2013) reported that NDVI ranged between 0.20 and 0.68.

Figure 2. Temporal variation of vegetation index from 1995 to 2014 and the days 1 to 365 of the year in the municipalities of San Pedro de los Milagros, Donmatías, Entrerrios, Santa Rosa de Osos and San José de la Montaña, Antioquia.

While the lowest NDVI values reported in the current study (between 0.40 and 0.50) are not as low as those of other studies (Escribano and Hernández Díaz-Ambrona 2013; Jobbágy et al 2013; Hernández et al 2014), the temporal variation observed in NDVI showed periods of significantly diminished vegetation cover. Cristiano (2010) found that vegetation decreased between 39 and 45% when it was under conditions of water and nutritional stress, due to a reduction in the growth of stems and leaves. Similarly, Robles (2006) mentioned that when the vegetation index values were low, the growth of the vegetation cover decreased, temperature control was reduced and the presence of eroded areas increased.

Figure 2 shows that in the last 10 years grass growth rate increased and it is important to identify the reasons for this increase, as it may be associated to either increased environmental temperature or greater agricultural input (i.e. fertilizer) usage. While livestock production has been regarded as a process that transforms vegetation cover (Moreno and Cuartas 2015), the temporal variation observed in this study suggests that the vegetation cover has been maintained over time, an important observation due to the broken topography of the region and the different types of use to which that land has been subjected to (Escobar 2006). Due to increased food demand, the percentage of land dedicated to pasture production has increased in this region. Whether this would prompt producers to adopt different agricultural practices to ensure the sustainability of their production remains to be demonstrated. However, it is clear that farm productivity can be ensured through the adoption of improvements in pasture management (Murgueitio et al 2015) or changes in stocking rates (Sossa and Barahona 2015) and that in some cases, this also leads to increased vegetation cover (Cuartas et al 2014).

In the highland tropics, it is possible to obtain increases in vegetation cover through the adoption of different practices which include the application of proper fertilization rates and the implementation of new grazing systems such as silvopastoral systems. As for fertilization, Mejía-Taborda et al (2014) and Ruiz et al (2014) reported that the use of different doses of nitrogen and compound fertilizers (> 50kg / ha) increased grass biomass per hectare between 88 and 170%, respectively. As for silvopastoral systems, Mahecha (2002) and Rivera et al (2015) reported increases of 30% in the availability of forage, ensuring sustainable growth of the vegetation and adequate soil cover (Ojeda et al 2003).

Improvements in vegetation cover also included changes in the critical times for grazing in the region. This shifts in NDVI could be associated to environmental factors and/or management changes (De Bello 2006; Paz-Pellat et al 2009). This is observed between days 300 and 365, that early during the evaluation period (around 1995) was the point of the year when there was lower NDVI, and that has shown noticeable increases in NDVI as the evaluation period progressed towards 2014. The opposite occurred for the year days 35-120, when NDVI has slowly declined in recent years (Figure 2). Thus, the vegetation index evaluated across the different municipalities was affected by the change in environmental variables and the conditions of the terrain (Verhulst et al 2010). As grazing is an activity that leads to temporary increases and decreases of vegetation cover, further analysis of NDVI should be improved by tracking changes in animal numbers in the area.

The effect of the smoothed function of the interaction between longitude and latitude was highly significant (p <0.01), indicating that there are differences in the amount of vegetation due to the geographical position of the municipalities included in the study. As shown Figure 3, the NDVI showed a homogeneous spatial variation with an average value of 0.61, indicating that the distribution of vegetation cover in the monitored region is within the ranges reported in the literature, similar to the value of 0.60 determined by Roldán and Poveda (2006) for the Andean region, while for grassland and plantations areas in Argentina, Jobbágy et al (2013) found NDVI averages of 0.48 and 0.73, respectively.

On Figure 3, those in yellow are areas of low NDVI values, and these were more evident in some areas of the municipalities of Santa Rosa de Osos and San José de la Montaña, suggesting that production conditions within those farms affected the proper development of a healthy vegetation cover. One reason for this could be having greater stocking rates which could be the case of Santa Rosa de Osos, where land is expensive. Another factor that may have limited the development of a healthy vegetation cover in these municipalities can be related to the topographical conditions of the terrain. Indeed, Lasanta et al (2004), Lasanta and Vicente-Serrrano (2006) reported that the slope, altitude and shape of the slope are important variables in the spatial distribution of vegetation, with the areas of gentle slopes being both the most grazed by cattle and where most pressure is placed on the growth of vegetation. By contrast, farms in the municipalities of San Pedro de los Milagros, Entrerrios, and Don Matías had the greater NDVI values, identifying the southern part of the region evaluated as the one that allows better development of the vegetation. In addition, the pig-grasses-milk system frequently found in the northern highlands of Antioquia, presents further development in towns like Don Matías and San Pedro de los Milagros (Quirós et al 1997). This allows obtaining a great amount of organic fertilizers that contribute to the growth and development of kikuyu pastures.

Figure 3. Average NDVI values in the municipalities of San Pedro de los Milagros, Don Matías, Entrerrios, Santa Rosa de Osos and San José de la Montaña, Antioquia.

Conclusions

Although the northern region of Antioquia has been subjected for many years to the agronomic and cultural practices required in dairy production, it has a vegetation index within the range reported for other regions. Analysis of the NDVI data suggests that following the implementation of dairy farming, a stable vegetation index has been maintained over time in that region, and following this approach, no significant reductions in vegetation cover are evident. These findings should be corroborated following an on-site approach which should also be aimed at understanding the main factors underlying this response.

Acknowledgements

This work is part of the project "Fortalecimiento de la producción de la cadena láctea del distrito Norte Antioqueño", agreement No. 2013AS180031 signed between the Ministry of Agriculture and Rural Development, the Department of Antioquia, la Universidad Nacional de Colombia, Medellín and the University of Antioquia, with resources from the Sistema General de Regalías - SGR. The support from the Committee for the development of the investigation CODI (Estrategia para la Sostenibilidad 2016 grupo GaMMA) and from the Jóvenes Investigadores e Innovadores Program of COLCIENCIAS by the first author (call 645 2014) is also gratefully acknowledged.

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Received 31 December 2015; Accepted 29 January 2016; Published 1 March 2016

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