Introduction

The main source of food for housed cattle in the Comarca Lagunera are forages (corn, alfalfa, sorghum and oats), which occupy more than 70% of the cultivated area (^{SADER, 2021}). Forage corn occupies the largest cultivated area in the region with 48 793 ha under gravity irrigation (13 834 ha) and pumping (34 595 ha) (^{SADER, 2021}).

To increase the productivity of forages, excessive amounts of fertilizers and water are used (^{Reyes et al., 2020}), which is limited and scarce in arid and semiarid regions of the world, in this agroecological zone, the prevailing irrigation system for the cultivation of forage corn is the gravity irrigation system (^{SADER, 2021}), which is very inefficient in water use (^{Reyes et al., 2020}).

Subsurface drip irrigation (SDI) improves water use efficiency and decreases the volume applied by up to 44% with respect to the gravity irrigation system (^{Gutiérrez et al., 2017}). SDI increases yield and water use efficiency in forage crops, since it reduces losses due to evaporation and deep soil percolation (^{Sandhu et al., 2019}).

The use of fertilizers increases crop yields; however, overfertilization and poor soil management accelerate its deterioration. Given this situation, the use of organic fertilizers is a sustainable agricultural practice, since it contributes to maintaining the natural fertility of the soil (^{Bonanomi et al., 2020}), as it is a source of nutrients and an improver of soil (^{Cai et al., 2019}). In the Comarca Lagunera, about one million tonnes of bovine manure are produced (^{Ramírez et al., 2016}), which can be used to reduce or replace inorganic fertilization, as it is an organic source of nutrients for crops (^{Figueroa et al., 2010}).

Various studies have been conducted separately to evaluate different SDI systems at different depths of irrigation tape (^{Salomó et al., 2019}), or different doses of bovine manure (^{Salazar et al., 2010}). However, studies that evaluate the depth of irrigation tape and solarized manure in forage corn in the Comarca Lagunera have not been carried out. For this reason, the objective of the research was: to evaluate the effect of three depths of drip irrigation tape and different doses of solarized bovine manure on the production and bromatological quality in forage corn (Zea mays L.).

Materials and methods

The study was carried out in the La Laguna Experimental Field (CELALA), of the National Institute of Forestry, Agricultural and Livestock Research (INIFAP), located at 25° 32’ north latitude and 103° 14’ west longitude, at 1 150 masl. The texture of the soil is loamy-clayey (28% sand, 40% clay, 32% silt). The climate of the region is classified as dry semi-warm, (Bwh) (^{García, 2004}), with an average temperature of 22.6 °C, average annual rainfall of 227 mm and evapotranspiration (ET) of 2 000 mm (^{Villa et al., 2005}).

The solarization of bovine manure was carried out on mounds 5 m long by 1 m wide with a height of 0.8 m for 90 days. The plastic used was transparent polyethylene 30 microns thick. The mounds were covered in such a way that the edges were sealed, and the plastic was in full contact with the manure, to avoid loss of humidity and temperature.

At the end of the solarization period, the manure was analyzed in chemical form, yielding the following results: pH: 7.79, electrical conductivity (EC): 6.8 dS m^-1, OM: 5.35%, total nitrogen: 0.86%, NH₄: 0.084%.

The preparation of the land consisted of subsoiling, fallow, incorporation of solarized manure (SM), harrowing, leveling and placement of the irrigation tape. The sowing was carried out dry, the distance between plants was 12 cm and 76 cm of separation between furrows to have a population density of 105 000 plants ha^-1. The commercial hybrid 20W41 (Syngenta) was used, which is of intermediate cycle, under conventional sowing, tolerant to lodging, with good plant and cob health.

An experimental design of split plots in random blocks with three repetitions was used. The different doses of solarized manure were considered as large plot (50, 75 and 100 t ha^-1) and a control with inorganic fertilization (200-100-00 kg ha^-1 of N-P₂O₅-K₂O), while the depth of the irrigation tape (0.3, 0.15 and 0.05 m) was the small plot, each small plot was made up of four furrows of 5 m in length and a distance between furrows of 0.76 m (15 m²). One hundred percent of the phosphorus and 50% of the nitrogen dose corresponding to each treatment were applied at the time of sowing to later apply the establishment irrigation. At 34 days after sowing (DAS), a cultivation and hilling work were carried out with a moldboard plow, to break the ‘crust’ of the soil, remove weeds and apply the rest of the nitrogen.

The irrigation tape used was RO-DRIP 8 000 (Rivulis Irrigation Inc., San Diego, CA, USA) with a wall thickness of 0.2 mm, inner diameter of 16 mm, with emitters 0.2 m apart and an expenditure of 0.5 L h^-1 per dripper. The operating pressure of the irrigation system was 8 PSI with a frequency of irrigation every third day. The irrigation time was the same for the three depths. The reference ET was taken from an atmometer (ETgage, model A, marketed by ETgage Company Loveland, Colorado, USA) that was 20 m from the experimental plot.

The leaf area index (LAI) was determined each week using the PAR/LAI ceptometer, model Lp-80 of ACcuPAR (Decagon Devices, Inc. Pullman, WA, USA). The bar of the ceptometer was placed at an angle of 45° through the crop furrow to measure the interception of photosynthetically active radiation (PAR) along the bar. The interception of photosynthetically active radiation PAR was measured between 12:00 and 14:00 h in each treatment and in each repetition, taking three readings above and below the crop canopy (^{Reyes et al., 2019}).

The readings with the SPAD (Soil Plant Analytical Development) equipment were taken weekly in two plants of each experimental unit. The measurements were made on the youngest completely expanded leaves, in the middle part between the base and the apex of the leaf, at the end the average of three readings per plant was recorded. The final height of the plant was taken at the time of harvest. Two plants per treatment were measured from the base of the stem to the ear with a tape measure. The harvest was carried out manually, at 105 DAS when the maturation of the grain had a third of advance of the milk line.

The production of green forage was estimated by weighing the biomass of three linear meters in the two central furrows of each treatment (4.56 m²), then a sample of 500 g was taken and dried in a forced-air oven at a temperature of 65 ° C for 72 h, to then determine the production of dry forage, by the percentage of dry matter and the production of green forage.

Water use efficiency (WUE) was obtained by dividing the yield of harvested dry forage (kg) by the total volume of water used (m³) in each treatment. For bromatological quality, samples of 200 g of each treatment per repetition, previously ground, were taken, deposited in plastic bags, to be sent to the laboratory and determine the nutritional content by near-infrared reflectance spectroscopy (NIRS).

For this, subsamples of 100 g were taken and ground in a laboratory mill (Arthur H. Thomas Co. Philadelphia, PA) to pass through a 1 mm mesh and obtain the results in the NIRS equipment using preset correlations with wet chemistry analysis (^{Valenciaga and Simoes, 2006}). The parameters evaluated were: neutral detergent fiber of organic matter (NDFom), crude protein (CP), lignin, starch, net energy of lactation (NEL) and neutral detergent fiber digestibility at 30 h of incubation (NDFD-30 h).

The determination of crude protein (CP) was performed following the official methods of the AOAC (2005). The CP was quantified with the microKjeldhal method, while the fat content was by means of the Soxhlet method using a Goldfish extractor (Labconco, USA). The percentages of acid and neutral detergent fibers (ADF and NDF) were quantified with the method of fractionation with detergent and subsequent filtration (^{Van Soest et al., 1978}).

To determine if there were significant differences in the response variables, an analysis of variance was performed on each variable using the GLM procedure of SAS, when statistical differences were detected (p≤ 0.05), the Tukey mean separation test (p≤ 0.05) was applied, using the SAS 9.3 statistical package (SAS Institute Inc., Cary, Nc. USA).

Results and discussion

Leaf area index

The values of LAI were affected by the treatments under study (Figure 1a), in general at the beginning it presents a slow growth, followed by a greater increase in the vegetative stage, until reaching a maximum value and then gradually decreasing due to the senescence of the leaves when the crop reaches its physiological maturity.

The highest LAI was 6.56 for the treatment with 50 t ha^-1 of solarized manure and a depth of the irrigation tape of 0.3 m, exceeding the LAI obtained by the control treatment by 20%, the above is probably due to the fact that in this treatment there was a greater water stress in the plant which induces less transpiration, less CO₂ absorption and reduction in photosynthesis, ^{Montemayor et al. (2012}), due to lower soil moisture; these same authors report similar LAI values, obtaining maximum LAI values of five and six in corn production with subsurface drip irrigation. ^{Reyes et al. (2019}) found low LAI values in the treatments where there was low soil moisture in a corn sown in South Dakota, USA.

Figure 1 Behavior of LAI (a) and SPAD values (b) in three doses of manure and one chemical fertilization and three depths of irrigation tape. The vertical bars indicate the standard deviation of each of the treatments. 

Conclusions

The highest production of dry forage occurred in the treatments with tape buried at 0.15 and 0.3 m deep, this because the water reaches directly to the root area of the crop and the loss of water by evaporation is less than in the surface stratum. The efficiency in the use of water in general was greater in the treatment where the tape was buried at 0.15 m. Regarding the application of solarized manure, the dose with 75 t ha^-1 was the one that had a higher average in the production of dry forage (22.43 t ha^-1). In general, treatments with doses of manure were higher than the control treatment, exceeding it from 14 to 18%. This means that it is possible to replace the chemical fertilizer with the application of solarized bovine manure. In addition, with the use of manure, the efficiency in the use of water was greater. As for the yield of nutritional quality of forage, the treatments where manure and irrigation tape at depths of 0.15 and 0.3 m were applied were the ones that showed the best results.