Introduction
In the Arid Chaco, calves are reared on natural pastures or implanted megathermal pastures, mainly Cenchrus ciliaris cv. Texas 4464 (buffel grass). Due to the seasonal growth of grasslands and pastures, it is common to defer the use of forage to winter, with the consequent reduction in forage quality. Nitrogen is the most limiting nutrient in the region, so nitrogen fertilization could be implemented to increase the yield and nutritional value of pastures and/or pastures, prior to deferral. On the other hand, protein supplementation during winter is the recommended practice to correct the nitrogen deficit of megathermal pastures (Balbuena et al., 2000). The interaction of both practices on the animal response has not been evaluated in steer rearing in the region. The objective was to evaluate the effect of protein supplementation, nitrogen fertilization and their interaction on daily weight gain (GDP) and blood urea nitrogen (BUN) concentration during rearing of steers in deferred buffel grass pastures.
Materials and methods
The study was carried out at INTA EEA La Rioja (30°30’27´´S – 66°07’15´´O) between July and November 2020 (130 days). Two levels of fertilization (urea, 46% N) were evaluated: without fertilization (SF) and fertilization with 75 kg N ha-1 (CF), and 2 levels of protein supplementation: without supplementation (SS) and with 0.8% PV supplementation with soybean expeller (CS), defining 4 treatments: SF-SS, SF-CS, CF-SS and CF -CS. The experimental design was completely randomized with 2 plot repetitions (experimental unit=3 ha) for each treatment combination (8 plots in total). In each experimental unit, 3 or 4 Angus steers (146 ± 15 kg LW and 228 ± 16 days of age, at the beginning) were assigned according to the forage production of each plot (forage allocation = 7% LW). Fertilization was carried out prior to the deferral of forage (02/05/2020). The supplement was provided daily in groups per plot, adjusting the amount offered every 30 days. In the fertilized plots, forage availability was 3120 ± 326 and 1060 ± 246 kg ha-1.-1 and in the unfertilized 2645 ± 448 and 885 ± 356 kg ha-1 , at the beginning and end of grazing respectively. At the beginning of grazing, the fertilized plots had 8.3% CP and 49.0% IVMD and the unfertilized plots had 6.3% CP and 47.0% IVMD. In the animals, live weight was evaluated every 14 days, GDP (kg day-1 ) estimated as a regression of weight as a function of time and NUS (mg dl-1 ) every 28 days. The GDP and NUS variables were analyzed using mixed linear models considering supplementation, fertilization and their interaction as fixed effects, and plot as a random effect. P< 0.05 was considered for the separation of means.
Tabla 1. Daily weight gain (GDP, kg d-1 ) and blood urea nitrogen (NUS, mg dl-1 ) of steers reared in buffel grass pastures deferred according to fertilization treatment (SF= without fertilization and F= fertilized) and supplementation (SS= without supplementation and CS= with supplementation).
Results and discussion
In GDP, an interaction was detected between the factors supplementation and fertilization (P< 0.01). In the treatments without supplementation, the ADG of the animals tended to be higher (P=0.0836) in the fertilized pasture, and in the treatments with supplementation, the ADG was higher (P=0.0136) in the unfertilized pasture. . Then, the treatments with supplementation had higher GDP compared to those without supplementation, regardless of the level of fertilization (Table 1). In NUS, a supplementation effect was detected (P< 0.01), with values higher than 15 mg dl-1 in the treatments that included supplementation. A marginal effect (P=0.0716) of fertilization was detected (Table 1). According to Byers and Moxon (1980), values from 11 to 15 mg dl-1 of NUS are associated with maximum gain rates in growing animals.
conclusions
Protein supplementation would be the strategy with the highest GDP (0.800 – 0.900 kg d-1 ) in steers during a pastoral rearing. However, GDP greater than 0.500 kg d-1 would be obtained in steers on fertilized buffel grass pastures, without supplementation. The combination of fertilization and supplementation at the levels used would not be recommended because it would produce an excess of nitrogen not used by the animals, possibly due to lack of energy.
