Enhancing pasture persistence is crucial to achieve more sustainable grass-based animal production systems. Although it is known that persistence of perennial ryegrass is based on a high turnover of tillers during late spring and summer, little is known about other forage species, particularly in subtropical climates. To address this question, this study evaluated survival of grazed tall fescue tillers growing in a subtropical climate. We hypothesized that hard tactical grazing during winter to remove reproductive stems (designated as 'flowering control'), and nitrogen fertilization in spring, would both improve tiller survival over summer, and thus enhance tiller density. This was assessed in two experiments. In both experiments, few tillers appeared during late spring and summer and so tiller density depended on the dynamics of vegetative tillers present in the sward in spring. In Experiment 2, flowering control and nitrogen fertilization both enhanced the survival of that critical tiller cohort, but the effects were not additive. Responses were similar but not statistically significant in Experiment 1, which had a warmer, drier summer and lower overall survival rates. Unlike grasses in temperate environments, persistence of tall fescue in this subtropical site appeared to follow a 'vegetative pathway'; i.e., new tillers were produced largely in autumn, from vegetative tillers that survived the summer.

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Argentina grows the second-largest area of lucerne in the world. Despite its importance, a yield gap exists between potential and measured yields, but factors contributing to it are still unclear. This study aimed to identify management factors and research needs to reduce the lucerne yield gap to improve the livestock systems in this region. We used meteorological data coupled with lucerne crop modelling and measured yields from the National Lucerne Cultivar Evaluation Network (NLCEN) to quantify the lucerne yield gap in nine sites located within the Argentinian Pampas (between parallels 30-45 S and meridians 58-65 W) and three sites outside the Pampas. Specifically, we used the model developed by McCall & Bishop-Hurley (2003), adapted and calibrated for lucerne in Argentina by Berone et al. (2017) to estimate the potential yield (PY) for 12 locations (three irrigated and nine rainfed), and compared those results with measured yields from the NLCEN to calculate yield gaps. We found the average available photosynthetic active radiation (PAR) and temperatures were sufficient to achieve 21.5 ± 3.7 t dry matter (DM) ha À1 yr À1 under rainfed conditions (environments with mean annual rainfall from 400 to 1,200 mm). However, the average measured yield from the NLCEN was 16.8 ± 2.4 t DM ha À1 yr À1 (a 22% gap). Potential yields ranged between 10 and 25 t DM ha À1 yr À1 under rainfed conditions and between 25 and 39 t DM ha À1 yr À1 for irrigated crops. As latitude increased rainfed locations had lower yields, while irrigated locations had higher yields. Adding irrigation was predicted to increase yields to 35.4 ± 2.0 t DM ha À1 yr À1 (a 53% gap) in rainfed sites. For irrigated locations, the gap was smaller (27.3 ± 3.5 vs 32.4 ± 2.2 t DM ha À1 yr À1 for measured vs potential yield, respectively), and most likely linked to nutrient deficits. Also, current grazing management was estimated to achieve approximately 50% less grazing efficiency than optimal grazing management. Our results demonstrated that the livestock industry can potentially increase animal production under current environmental conditions. The four main adjustments to achieve this are increased use of irrigation, increased use of fertilisers, earlier commencement of grazing in spring and increased stocking rates.

Manuscrito recibido el 30 de junio de 2014 y aceptado para su publicación el 10 de octubre de 2014. RESUMENLa producción individual de leche, el consumo de pasturas y suplementos están asociados a la rentabilidad de los tambos. Se analizó información de 40 tambos de la región central de Santa Fe, durante el período 2004-2012, para investigar la evolución de la producción lechera y el consumo de pasturas y suplementos. Los resultados muestran que la cantidad de suplementos consumidos por vaca en ordeño (VO) se incrementó marcadamente (65%) en los ocho años analizados, lo que permitió un incremento moderado de la carga animal (35%) y un leve incremento de la producción individual de leche (9,5%). Sin embargo, el consumo de pasturas por vaca y por hectárea disminuyó en el mismo período. Algunas causas que explicarían la baja respuesta en leche por vaca frente a una alta adición de suplementos podrían ser las siguientes: alta tasa de sustitución, baja calidad de los suplementos, y falta de infraestructura en los tambos. Palabras claves: suplementos, producción de leche, sistemas lecheros, tasa de sustitución. SUMMARYEvolution of supplements and pasture feeding and milk production on dairy farms of Argentina.Individual milk yield and pasture and supplement consumption are factors related to dairy farm's profitability. To investigate the evolution of milk production and supplement and pasture consumption, data of 40 dairy farms of the central region of Santa Fe were analyzed from 2004-2012. Results showed a 65% increase in the amount of consumed supplements per cow, allowing a moderate increase in the stocking rate (35%), and an increase in individual milk production (9,5%).

RESUMENLas variaciones en el precio de la leche y de los alimentos concentrados y el efecto del clima sobre la producción de cultivos afectan la productividad rentabilidad y riesgo de los sistemas lecheros. El objetivo del estudio fue investigar la productividad, el resultado económico y el riesgo de diversos sistemas de producción de leche en la región Centro-Norte de Argentina. Se investigaron sistemas lecheros alternativos al sistema representativo de la región, explorando el efecto de incrementos en carga animal, incrementos en suplementación con alimentos concentrados y el efecto de utilizar diferentes sistemas de alimentación sobre la productividad y la rentabilidad. El enfoque elegido para el análisis fue la simulación con el uso de dos modelos matemáticos: uno para predicción del rendimiento de cultivos y otro para evaluación del sistema lechero completo. Se realizaron simulaciones determinísticas y estocásticas, i.e., con variación aleatoria del precio de la leche, de los concentrados y el rendimiento de los cultivos. Los resultados muestran que el sistema actual de la región no es rentable. Los sistemas alternativos con mayor carga animal y mayor suplementación resultaron en mayor producción de leche por hectárea y mayor resultado económico que el sistema regional actual. Sin embargo, la intensificación a partir del incremento de la carga animal y de la suplementación aumentó la exposición al riesgo. El efecto de la variación de precio de leche y concentrado fue mayor que el efecto del clima sobre el resultado económico.Palabras clave: estocástico, carga animal, modelo, precios, clima, leche. ABSTRACTVariations in milk and concentrate feed prices and the effect of climate on crop production affect productivity and profitability of dairy systems. The objective of this study was to evaluate productivity and economic results, and assess the risk of different dairy systems in the Central-Northern region of Argentina. Alternative dairy systems (other than the traditional system of this region) were studied, evaluating the effects of increases in stocking rate and use of supplementary feed, as well as the effect of using different feeding systems on productivity and profitability. Two mathematical models were used in this analysis: one model to predict crop and pasture production, and the other to evaluate the performance of the whole dairy system. Deterministic and stochastic simulations, i.e., with random variation in milk and concentrate feed prices, and crop yield, were performed. Results showed that the current production system of this region is not profitable. The alternative systems 152-162.

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