The northeast of Brazil accounts for 18.27 % of the country, and most (62.11 %) of this region is semiarid¹. Annual rainfall distribution (500 mm)² is irregular, leading to severe feed shortages in ruminant livestock systems. The cactus species Opuntia sp. and Nopalea sp. can be used as ruminant forage and are adapted to water scarcity, high temperatures and poor soils^3,4,5. They are thus valuable alternative feed sources, especially during the dry season. Opuntia ficus-indica Mill and Nopalea cochenillifera Salm Dyck are the two most widely cultivated cactus species in northeast Brazil⁶.

Nopalea cochenillifera cv. Miúda, commonly known as Nopal Dulce, is a fodder that has the advantage of being resistant to cochineal (Dactylopius opuntiae Cockerell)^7,8. This characteristic, in addition to its 6.2 % crude protein (CP) content, 26 % neutral detergent fiber (NDF) content and 78 % digestibility⁹, makes it a promising alternative livestock feed source. As with any cultivated forage species, N. cochenillifera requires adequate soil fertility to produce properly. Fertilization with organic matter and minerals is common practice in cactus cultivation to compensate for nutrient extraction by this crop and increase forage production efficiency^10,11.

The variables used in forage plant ecophysiology research exhibit different responses depending on plant management. In cacti, cladode morphological characteristics are directly related to fresh and dry matter yields¹², but few studies use morphological characteristics as productivity indicators. The present study objective was to evaluate the effect of organic and mineral fertilization, and cladode harvest frequency on cladode morphological characteristics in Nopalea cochenillifera Salm Dyck cv. Miúda cultivated in the agreste region of Pernambuco, Brazil.

The experiment was done between June 2011 and May 2013 at the Experimental Station of the Pernambuco Agronomic Institute, Caruaru Municipality, in the agreste region of the state of Pernambuco. This is a transition zone between humid and semi-arid tropical forest zones. This region’s stony soil supports sparse (˂40 and ˃20% coverage), low-level (˂1.5 m height) vegetation¹³. It is in northeast Brazil (8°14’ S; 35°55’ W) at an altitude of 575 m asl and contains Neolithic Regolithic soil¹⁴. During the experimental period precipitation at the site was 1,068.3 mm but varied widely from month to month (Figure 1).

Figure 1 Monthly rainfall (mm) and maximum and minimum temperatures in Caruaru Municipality, Pernambuco, Brazil, from January 2011 to June 2013 

Soil samples were collected at the surface and 20 cm depth and chemically analyzed following the soil analysis method of Empresa Brasileira de Pesquisa Agropecuária¹⁵ (Table 1).

Seven fertilization treatments were evaluated: negative control with no fertilization; fertilization with bovine manure at 10,000, 20,000 and 30,000 kg OM ha^-1 yr^-1; and mineral fertilization (with added urea) at 120, 240 and 360 kg N ha^-1 yr^-1. Annual and biennial harvest cutting frequencies were applied. The experimental design was randomized blocks sub-divided into plots, with four replicates. The largest plot (14.4-8.0 m) was used to test organic matter levels; subplots (7.2 x 8.0 m) to evaluate cutting frequencies, and other sub-plots (14.4 x 2.0) to assess nitrogen levels. Each experimental unit consisted of six rows of plants. The two side rows and three plants at the ends were considered edges, leaving an effective sampling area of 33.84 m² containing 282 plants.

Planting was done between April and May 2011 by sowing mature N. cochenillifera cladodes in rows with 1.2 m between rows and 0.1 m between cladodes. Overall density was 83,336 plants per hectare. Organic fertilization was done at the time of planting (June 2011) and after the first annual cut (June 2012) using bovine manure containing 1.1 kg^-1 N, 3.74 kg^-1 P and 16.5 g kg^-1 K (determined using AOAC methods)¹⁶. During the first year of cultivation mineral fertilization was done from 5 June to 19 July 2011, and during the second year on 28 June, 23 July and 19 August 2012. Harvest of cladodes was complete, leaving only the mother plant. Cladode length (cm), width (cm) and perimeter (cm) were traced onto a sheet of white paper (A4 size). The outline of two cladodes per sub-subplot were drawn and examined with a leaf area analyzer (Portable Laser Leaf Area Meter CI -202 Bio-Science Inc.^®). The CAI was evaluated by adding the area of one plant’s cladodes (m²) (considering both sides of the cladode) and dividing it by the soil surface (0.12 m²) occupied by each plant¹⁷. Calculations were done of the correlation coefficient between dry matter (DM) production¹⁸ and the variables of cladode length, width and perimeter, as well as the CAI. The data were analyzed using the MIXED procedure in the SAS software package¹⁹. The Tukey test (P≤0.05) was applied to the factor cut frequency, and polynomial orthogonal contrasts (P≤0.05) were applied to the factors organic and nitrogen fertilization.

Cladode length, width and the CAI values exhibited no effects (P>0.05) from mineral fertilization. This could have been due to irregular rainfall during the experimental period (Figure 1), which would affect nutrient absorption, or low soil organic matter content (Table 1). Similar effects have been reported for mineral fertilization on morphological characteristics and biomass production in N. cochenillifera cv. Miúda. In one study these were attributed to the root system, the growth of which responds to rainfall, meaning that irregular rainfall distribution can negatively affect nitrogen absorption efficiency and increase nutrient loss through leaching during excess rainfall or volatilization during its absence²⁰. Positive effects from nitrogen and mineral phosphate fertilization have been observed in the production of other cactus species such as Opuntia lindheimeri, but only two years after crop establishment²¹.

Organic fertilization is known to promote crop growth and production²². In the 30,000 kg ha^-1 year^-1 treatment, cladode width and length increased proportionally to organic fertilization (R²= 0.26 and 0.47; P≤0.001), and increased 9.8 and 17.8 % (respectively) versus the control (P≤0.05) (Figure 2).

Figure 2 Cladode length and width in Nopalea cochenillifera Salm Dyck cv. Miúda fertilized with bovine manure (Agreste region, Pernambuco, Brazil) 

This is similar to studies showing that cladode length in O. ficus-indica cv. Lisa increased gradually over three consecutive years in response to organic fertilization with bovine manure at 20,000, 40,000 and 60,000 kg ha^-1 yr^-1, as well as in the control (0 kg ha^-1 yr^-1 fertilization)²³. In another study using higher fertilization rates (90,000 kg ha^-1 yr^-1 bovine manure) over a shorter period (600 d), cladode length in O. ficus-indica cv. Gigante increased only 8 % compared to the control²⁴. In a previous study cladode width in O. ficus-indica did not change (P>0.05) in response to application of bovine manure at 0, 20,000, 40,000 and 60,000 kg ha^-123.

Organic fertilization, mineral fertilization and an annual cut frequency affected cladode perimeter (P≤0.05). This increase became larger (P≤0.05) in a linear manner in response to organic matter fertilization. Cladode perimeter also depended on the interaction between mineral fertilization and cut frequency (Figure 3). This variable increased at 0.339 cm per 1,000 kg OM and 360 kg N per ha^-1, and 0.211 cm per 1,000 kg OM and 120 kg N ha^-1. The cladode perimeter increase rate was higher with annual cuts (0.211 cm per 1,000 kg OM) than biennials (0.0304 cm per 1,000 kg OM).

Figure 3 Cladode perimeter in Nopalea cochenillifera Salm Dyck cv. Miúda in response to organic fertilization, mineral fertilization and cut frequency (Pernambuco, Brazil) 

In response to organic fertilization level the cladode area index (CAI) exhibited a quadratic trend (P≤0.05) with proportional increases in the 10,000 and 20,000 kg OM ha^-1 year^-1 treatments (Figure 4). The maximum CAI value was reached at 25,970 kg OM ha^-1 year^-1; organic fertilization levels higher than this did not affect CAI values. This plateau in CAI values may be due to the shading effect, which consists of lack of exposure of the photosynthetically active area and can lead to lower production under high plant densities²⁵. For instance, in one study spacing between plants and organic fertilization level affected CAI values in Opuntia ficus-indica cv. Gigante, with higher values at spacings of 1 x 0.5 m and organic fertilization levels between 60,000 and 90,000 kg ha^-1 yr^-124. Both this spacing and fertilization level are higher than those used in the present study. Other types of organic fertilizers produce different responses. When a biofertilizer based on bat guano and chopped up cladodes was used at 15,000, 30,000, 45,000 and 60,000 kg ha^-1 yr^-1 on O. ficus-indica cv. Gigante increases of 30 % in the photosynthetically active area were observed²⁶. However, this 30 % increase in CAI values is notably less than the 68.9 % (versus the control) observed in the present study at the of 25,970 kg ha^-1 yr^-1 fertilization level.

Figure 4 Cladode area index (CAI) values in Nopalea cochenillifera Salm Dyck cv. Miúda in response to organic fertilization (Pernambuco, Brazil) 

Cut frequency did affect CAI (P≤0.05). Plants cut at two years had higher (P≤0.05) average CAI values (4.7) than those cut annually (2.2). This difference can be attributed to a smaller residual photosynthetic area in plants subjected to annual cutting than in those cut biennially since a low CAI value represents light interception and slower plant growth²⁷. The effect of cutting frequency may also be due to other factors. For instance, dry matter production in O. ficus-indica was reported to be higher when secondary cladodes were not cut²⁸, possibly because plants with secondary-level cladodes have a larger CAI which represents a larger photosynthetic area^29,30.

Correlation coefficient analyses between productivity and morphological variables identified a high correlation between the evaluated variables, suggesting that measurement of morphological variables can be used as a productivity indicator (Table 2).

In Nopalea cochenillifera Salm Dyck) cv. Miúda, fertilization with bovine manure increases cladode width and length, and cladode area index (CAI) values, while mineral fertilization with urea has little or no impact on these variables. Cladode area index (CAI) values were higher when using a biennial cut frequency. Correlation was high between morphological and production variables, highlighting the importance of studying morphological variables in this forage crop.