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Revista mexicana de ciencias pecuarias
versão On-line ISSN 2448-6698versão impressa ISSN 2007-1124
Rev. mex. de cienc. pecuarias vol.6 no.4 Mérida Out./Dez. 2015
Articles
Regional nitrogen balance in the milk-forage production system in the Comarca Lagunera, Mexico
a Campo Experimental La Laguna. Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias (INIFAP). Blvd. José Santos Valdez no. 1200 pte. 27440 Matamoros, Coahuila. México. Tel. 871-1823177.
b Campo Experimental Centro Altos de Jalisco. INIFAP. México.
The intensive system of milk production in the Laguna Region is characterized by large farms with more than 1,000 milking cows and irrigated areas for forage production. The objective of this study was to estimate the balance between manure nitrogen (N) added to the soil and the requirement of N for forage crops. SAGARPA statistics were reviewed about dairy cows inventory and forage production during 1999-2011. Manure production and N excretion were estimated using daily milk production, while the crop N requirement was estimated based on crop yield and crop N removal. Forages occupied 69 % of the irrigated area, with 89,500 ha. Forage production was 1.2 million t yr-1 dry matter (DM), while the N requirement was 15,070 t yr-1. The dairy cow inventory was 423,000, with 53 % lactating. Manure production in DM was 842,000 t yr-1, while the N excreted was 44,154 t yr-1. The regional balance expressed as [N incorporated - crop N requirement] was 187 kg ha-1, which represents an excess. Previous studies showed that a balance <150 kg ha-1 allows a sustainable milk production. According to the results of this study, municipalities with a cow density greater than 2.87 cows ha-1 will have a balance >150 kg ha-1.
Keywords: Manure; Cow density; N efficiency; N excretion; Crop N extraction
El sistema intensivo de producción de leche en la Comarca Lagunera se caracteriza por grandes explotaciones con más de 1,000 vacas en lactancia y áreas para la producción de forrajes de riego. El presente estudio tuvo como objetivo estimar el balance entre el nitrógeno (N) del estiércol incorporado al suelo y el requerimiento de N por los cultivos forrajeros. Se revisaron estadísticas de la SAGARPA sobre el inventario ganadero y producción de forrajes de 1999 a 2011. La producción de estiércol y la excreción de N se estimaron en función de la producción diaria de leche, mientras que el requerimiento de N de los cultivos forrajeros se estimó en base al rendimiento y a la extracción de N por los cultivos. Los forrajes ocuparon el 69 % de la superficie de riego, con 89,500 ha. La producción de forraje fue de 1.2 millones t año-1 en materia seca (MS), mientras que el requerimiento de N fue de 15,070 t año-1. El inventario lechero fue de 423,000 vacas, con 53 % en lactancia. La producción de estiércol en MS fue de 842,000 t año-1, mientras que el N excretado fue de 44,154 t año-1. El balance regional expresado como [N incorporado - requerimiento de N por forrajes] fue de 187 kg ha-1, lo cual representa un exceso. Estudios previos muestran que un balance <150 kg ha-1 permite una producción sustentable de leche. De acuerdo con los resultados del presente estudio, los municipios con una densidad mayor de 2.87 vacas ha-1 tendrán un balance >150 kg ha-1.
Palabras clave: Estiércol; Densidad de vacas; Balance de Nitrógeno.
Introduction
A characteristic of the dairy cattle metabolism is low efficiency in the use of nutrients, mainly nitrogen (N). The nitrogen efficiency by dairy cattle is about 30 %, so the remaining 70 % is excreted1. Therefore, inappropriate manure management represents a risk of environmental pollution, being the main routes of nitrate (NO3-N) contamination leaching to groundwater2, nitrous oxide (N2O) emission into the atmosphere and surface water runoff and sediment loaded with N3. The region of the Comarca Lagunera, in the states of Coahuila and Durango, concentrates most of the dairy cattle inventory in Mexico, with an average of 423,000 heads which represent around 20 % of the national herd4. The estimated dry matter production of manure is 619,000 t yr-115, with application rates ranging from 60 to more than 200 t ha-1, causing contamination risk6. In this regard, the excessive nitrate concentration in groundwater of this region has been documented7. Due to the high forage demand for feeding dairy cattle, in the Comarca Lagunera are sown an average of 89,500 ha of forage, which represents 69 % of the agricultural area under irrigation4.
A support tool to plan manure handling and fertilizers management on forage production, is the N balance, both at regional or farm level8. Moreover, in some countries of Europe the N balance at farm level contributes to regulate the use of nitrogen9. According to the OECD10, Mexico has a positive N (inputs - outputs) balance of 22 kg ha-1, where 40 % of the inputs comes from manure and 20 % from chemical fertilizers. In the European Union, all countries recorded positive (excess) N balances, with values from 30 kg ha-1 in Greece up to 249 kg in Holland11. In this last country, Hilhorst et al12 concluded that in order to optimize the system milk-forage with focus on sustainability, a balance of 158 kg ha-1, has been reached which represented a decrease of 61 % compared to the regional average with conventional management.
A N balance-related management variable of dairy herds is the density in the number of cows per hectare associated to forage production, since it is an indicator of the surface area available for recycling N manure. In Wisconsin, USA, dairy farms estimated that with a density of 1.1 cows ha-1, a farm is self-sufficient in grain and forage production and it has the ability to recycle the manure13.
Therefore, the balance of nutrients is a tool to identify problem areas and to generate management alternatives to recycle more efficiently the manure nutrient content, thus reducing pollution risks to the environment. In addition, in intensive milk-forage systems production, the purchase and application of fertilizers may represent more than 40 % of the production costs14. Consequently, the objectives of this study were: 1) Estimating regional and municipality balance between the potential requirement of N by forage crops and the potential N excreted by dairy cattle; 2) To establish the proper density of dairy cattle heads per hectare harvested for forage and its effect on N balance.
Materials and methods
Description of the study area
The area comprises 10 municipalities of the state of Durango and 5 from Coahuila, with an area of 4.8 million hectares. The climate is characterized by an annual average temperature of 20.9 °C, warm summers with average maximum temperatures of 30.2 °C (±1.9), minimum 10.5 °C (±3.0) and mean annual rainfall of 287 mm (±98), during the evaluation period4. Information obtained by municipalities were: 1) Inventory of dairy cattle, total and lactating animals; 2) Annual milk production; 3) Total harvested area, and forage-harvested area; 4) Annual production of forage crops. The above information was obtained from statistical yearbooks from 1999 to 2011, available in the of SAGARPA Delegation in the Laguna Region4. During the study period, the total agricultural harvested area averaged 130,000 ha under irrigation plus 20,000 under rainfed. The rainfed areas are concentrated in six municipalities of Durango: Mapimí, San Pedro del Gallo, San Luis del Cordero, Rodeo, Simón Bolívar and San Juan de Guadalupe. As none of these municipalities provides more than 1 % of dairy cattle inventory, for purposes of the present study were added in “Others” and the N balance was calculated jointly.
N requirement for the crops
To estimate the N requirements, forage crops with more than 100 ha yr-1 per municipality in the evaluation period were considered. Data of fresh forage production for each crop was transformed to dry matter production (DM), using the DM percentages recorded in Table 1. The requirement of N in forage crops was obtained by N extraction values for ton of DM yield, based on information generated mainly in the Comarca Lagunera region and Mexico15-19. In the case of alfalfa, it was assumed that 75 % of the total N in the forage comes from biological fixation and the remaining 25 % is taken from the soil; this because these soils are manured on a regular basis and with high levels of available N20.
Manure production and nitrogen excretion
Equations generated by Nennich et al21 were used to estimate manure production and N excretion (Table 2), similar to other studies(22,23):
Et = 0.616(DMP) + 46.2
EDM = 0.0874(DMP) + 5.6
NE = 2.82(DMP) + 346
Where Et is the production of total excreta (kg cow-1 d-1), EDM is the production of excreta in DM (kg cow-1 d-1), NE is the total N excreted (g cow-1 d-1) and DMP is the daily average milk production (kg cow-1). As the used equations apply just for lactating cows, the number of cows for calculation purposes was estimated under the following assumptions: 1) From the total inventory of dairy cattle by municipality, 20 % are younger than 1-yr old and were not considered in the balance12; 2) Of the number of non-lactating cows, two animals are equivalent to one lactating, as it is considered by Spears et al24.
Cows density
The density (cows ha-1) was calculated by dividing the number of animals used for the calculation of the balance between the number of harvested hectares of forage crops by municipality.
N balance
N balance by municipality was calculated as the difference between the manure-N incorporated into the soil and N demand for forage crops .i.e., the value of total N excreted was not used in the N balance. It was assumed that 30 % of the total N excreted is lost through volatilization during handling, storage and manure application25.
Statistical analysis
Reviewed data from statistical yearbooks, as well as the calculated data of manure production and N excretion, were analyzed through descriptive statistics (average and deviation standard). Regression analyses were performed between the harvested crop area and milk production, respect to the assessment years; analysis was also ran between the N balance and dairy cattle density per hectare sown with forage, according to the procedure of Saam et al13.
Results
Crop pattern
The total irrigated area by crops was 130,000 ha on average for the study period, of which 69 % were harvested with forage; the most important crops by harvested area are: alfalfa, maize, sorghum and oats, as well as some areas sown with rye grass, wheat and triticale (Figure 1). During the study period, the rate of increase in the surface harvested with forages was 4,228 ha yr-1, from 64,000 ha in 1999 to 115,000 in 2011. The crop contributing more to this increase was forage sorghum, with an increment of 3,561 ha yr-1 during the period from 2005 to 2011, followed by forage maize, with an increase of 2,584 ha yr-1 in the period from 1999 to 2007.
The regional DM forage production was 1.25 (±0.23) million t yr-1. The municipality that most contributes to the regional production of forage was Gómez Palacio, with around 25 %, of both the harvested area and forage production. Lerdo, Matamoros, and Francisco I. Madero follow it in importance. Forage yield ranged from 12.8 t ha-1 of DM in Francisco. I. Madero, 16.1 t in Viesca, with a regional average of 13.9 t (Table 2).
The nitrogen requirement for forage production was 15,070 t yr-1 on average, which represents a dose of 168 kg N ha-1 at the regional level. The municipality of Gómez Palacio builds up 27.5 % of the regional requirement, for an average rate of 189 kg ha-1, while the lower requirements occurred in Nazas, with 2.5 % of the total and a dose of 142 kg.
Inventory of dairy cattle and milk production
The total number of bovines in 1999 was 363,000, with 52.7 % lactating animals; in the year of 2000 the inventory increased to 415,000 and since then, it was between 400 and 450 thousand, with 53.1 % lactating animals. The milk production (MP) in the region increased significantly at a rate of 80.9 million L yr-1, between 1999 and 2008; the regional production went from 1,535 million L in 1999 to 2.255 million L in 2008; from 2009 the milk production fell down but remained around 2,100 million L yr-1. The above is the result of an increase in production per lactation by 7,220 L cow-1 in 2001 to 9,480 L in 20084.
The distribution of dairy cattle by municipality indicates that 79 % of the inventory was concentrated on 4 of the 15 municipalities (Table 3): Gómez Palacio (28 %), Lerdo (19 %), Matamoros (18 %) and Torreón (13 %). With respect to animal density, the lowest value was recorded in “Other” municipalities with 0.9 cows ha-1, whereas the municipality of Torreón recorded the highest density with 7.72 cows ha-1. The four municipalities which account for 79 % of the dairy herd in the region (Gómez Palacio, Lerdo, Torreón and Matamoros), had an average density of 5.0 cows ha-1, whereas the rest of the municipalities averaged 1.7 cows, with a regional average of 3.2 cows.
Manure production
Total production estimated in fresh weight was 6.624 million t yr-1, with a 12.71 % of DM, resulting in a manure production of 842,000 t DM yr-1 (Table 4). From this previous value, 80 % is provided by lactating cows and 20 % for the rest of the herd (dry cows, heifers, and growing calves). As the amount of manure depends mainly on the number of animals, four municipalities which have 79 % of the inventory of dairy cattle (Gómez Palacio, Lerdo, Torreón and Matamoros), accounted for 78 % of the manure produced. Total N excreted was almost 44,200 t yr-1, with a minimum of 471 t yr-1 in Nazas, to a maximum of 11,785 t yr-1 in Gómez Palacio.
N balance
Figure 1 illustrates the N balance of the manure by municipality. The available N to be incorporated into the soil corresponds to 70 % of the values listed in Table 4, assuming that the remaining 30 % is lost by volatilization25; values range from 154 kg ha-1 at Nazas to 859 kg in Torreon. N requirement by forage crops is less variable among municipalities, with values of 140 kg in Nazas to 190 kg in Gomez Palacio. The N balance illustrated in Figure 3 is the difference between the incorporated manure-N and the N requirement by forage crops. On average, San Pedro, Tlahualilo, and “Others” had a negative balance, while the rest had a positive balance. i.e. in most municipalities manure-N is greater than N requirement for forage crops. The municipalities with the highest concentration of cattle, Gómez Palacio, Lerdo, Matamoros and Torreón, had an average balance of 183, 336, 313 and 691 kg ha-1, respectively, while the other municipalities had balances of less than 150 kg. The regional average balance is positive, with 187 kg (Figure 2).
Analyzing the trend of the N balance (Figure 3), it can be seen that the highest average was recorded in 2001, with 291 kg ha-1; from 2002 the balance value decreases up to 109 kg in 2011 (final year of evaluation). By separating the four municipalities which account for 79 % of heads of cattle (Gómez Palacio, Lerdo, Torreón and Matamoros) from the others, the information shows that those locations reached a maximum balance of 597 kg ha-1 in 2001 to then decrease to 271 kg in 2011, i.e., they were always above 150 kg. The rest of the municipalities always had a balance of less than 150 kg, reaching a maximum of 88 kg in 2002 and a minimum of -16 kg in 2010. The regional average showed values of 158 kg in 2003 and 2005, and less than 150 kg from 2006 to 2011.
By relating the animal density (CD cows ha-1) with the N balance (NB kg ha-1), the resulting linear regression model is presented in Figure 4:
NB = 106.6(CD) - 156
The previous model was significant (r2= 0.98) and predicts that a system in equilibrium (NB= 0) is obtained with a value of CD= 1.6 cows ha-1. The municipalities with the most positive balance, or greater N excess (Figure 2), are also those who have a higher cow density (Figure 4).
Discussion
Crop pattern
During the 13 yr analyzed period, it has been a major change in the crops pattern of the Comarca Lagunera. At the beginning of the period (1999), alfalfa held 53.8 % of the forage area; in that same year, maize along with the sorghum occupied 32.5 % of the area, whereas oats forage harvested 7.9 %. At the end of the study (2011), alfalfa decreased to a 34 %, while maize + sorghum increased to 51.2 % and oats 13.3 % of the crops forage area (Figure 1). These changes in the pattern increased the N demand in the region, by decreasing the N biologically fixed by alfalfa and increasing the crops conventionally fertilized26; it increases also the water use efficiency at the regional level without affecting the milk production27.
The regional average yield of 13.9 t DM ha-1 corresponds to all forage crops, which is greater than the 9.9 t ha-1 found by Hillhorst et al12 in an experimental farm in the Netherlands. At farm level, in the Comarca Lagunera Region DM yield per hectare can be greater, as alfalfa has an average yield of 16 t ha-1. In addition, in irrigated areas with groundwater (64 % of the area) is common to have two or three crops in the same year, with experimental yields of 25 and 18 t DM ha-1 in maize forage planted in spring and summer, respectively, and 12 t DM ha-1 with winter cereals28.
Manure production
There are different constants to estimate manure production in animal species5,29. Fortis et al5 estimated a production of 619,000 t DM yr-1 in the Comarca Lagunera, based on statistics from 2008; the value is less than the average estimated in this study of 842,000 t. In the region of Delicias, Chih., with the use of the Nennich et al equations21 a total manure production of 2.9 million t yr-1 was estimated (freshly excreted weight), with a dairy cattle population of 56,800 heads22. These equations have the advantage that are based on the milk production and can be used at the farm level.
With respect to estimation of the excreted N, dairy cattle has a low efficiency in the use of the ingested N, because 70 % of N intake is excreted1. Main factors associated with excretion of N are dry matter intake, content of crude protein in the diet, milk production, live weight and days in lactation21. An excretion of 44,154 t yr-1 was estimated in the present study; however, taking into account the manure production in DM (842,000 t yr-1), with an average total N concentration of 1.61 %30, results a total N content of 13,557 t yr-1. The difference represents a 69.3 % of N losses during handling, storage and manure application. The process that contributes most to the previous losses is ammonia volatilization, estimated as 25 to 50 % from the total N31. In the present study it was assumed a 30 % N loss by volatilization. Part of this high volatilization is because around 38 % of the excreted N, mostly in the urine, is in the form of urea or ammonia6. Urea in the presence of urease is hydrolyzed to ammonium (NH4+) and this turns in to ammonia (NH3) in conditions of alkaline pH32, a common condition in calcareous soils in the North of Mexico. On the other hand, the management of manure by anaerobic digestion (biodigester) can reduce losses by ammonia volatilization to values between 5 and 10 %33; this technology is now in use and extending in the Comarca Lagunera.
N balance
Balance in Figure 2 indicates an excess of N at the regional level of 168 kg ha-1, with a variation of -43 kg (other municipalities) to 691 kg ha-1 (Torreón). These values are comparable with those found in other studies. At farm level, in Australia an average balance of 193 kg ha-1 was estimated, with a variation of 47 to 601 kg34. A study of 120 farms in Belgium35 found an excess balance of 238 kg. The average balance obtained in this study is higher than the estimated at national level of 22 kg10, but close to the maximum value of 150 kg suggested to maintain the sustainability of the production system35. In the present study, the municipalities with the highest cows density had a balance greater than 150 kg ha-1 throughout the period evaluated, although with a clear downward trend, with a maximum of 1,343 kg in 2001, to 508 kg in 2011 (Figure 3). The remaining 11 municipalities recorded a maximum balance of 88 kg in 2002, and -16 kg in 2010.
The decrease of the N balance is due to the increase in the harvested area with forage crops, and that inventory of dairy cattle remained steady from 2000 onwards. A practice to lower the N balance is to move manure surplus to N-deficit municipalities or outside the Comarca Lagunera. This is the approach suggested by FAO36, i.e., integrate livestock areas with neighboring areas of agricultural production where nutrients from manure can be recycled. However, the cost of cattle manure application accounts for 25 to 148 % of the cost for applying fertilizers, depending on the dose required for crops; in addition, the maximum distance to which manure can be moved to match the cost of fertilizer that would apply, can vary from 1 to 92 km37.
Animal density
According to the regression model between animal density (AD) and N balance (NB), the AD value for a balanced system of 150 kg ha-1 is 2.87 cows ha-1. This value is greater than estimations in Wisconsin by Saam et al13, who point out that more than 1.1 cows ha-1 generate an excesses of N. However, Van Horn et al1 in Florida, report that at farm level, 1 ha can recycle manure of 10.4 cows; in this balance the manure incorporation is in liquid form and crops used 55.1 % of excreted N in three crops per year, with 29 t DM ha-1 yield. In the Comarca Lagunera, during the study 66,400 ha were cultivated with irrigation, including 38,000 ha of alfalfa4, so the rest of the 28,400 ha are suitable to obtain two or three harvests per year. By including a second crop (62 % of the area with maize and 38 % with sorghum), the DM production increases from 13.9 to 25.0 t ha-1, increasing the density at 6.7 cows ha-1, while maintaining a N balance of <150 kg ha-1.
Conclusions and implications
The regional N balance between dairy cattle manure and the requirement of forage crops in the Comarca Lagunera Region was estimated; the regional average balance was 187 kg ha-1, which indicates an excess of N. Over the years, the N balance showed a downward trend in all municipalities, with a maximum of 291 kg ha-1 in 2001 to 109 kg in 2011. It was found a positive relationship between the cows density per hectare and the N balance. The municipalities with a density greater than 2.87 cows ha-1 had a N balance of >150 kg ha-1, assuming a single crop per year, which represents higher contamination risk. In this intensive system with two forage crops per year (common in this region), the density could increase to 6.7 cows ha-1, while maintaining a N balance of <150 kg ha-1.
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Received: September 08, 2014; Accepted: February 28, 2015