Principal component analysis of steers performance and structural and nutritional characteristics of Mombasa grass

ABSTRACT: There is little information about whether the increased secondary productivity observed in pastures fertilized with high nitrogen rates is attributable to fluctuations in the nutritional value or pasture structural characteristics, or both. This study aimed to identify a set of factors (structural and nutritional characteristics) that best explain the performance of beef steers grazing Mombasa grass pastures under residual effects of nitrogen fertilizer. The data were collected in Mombasa grass pastures fertilized with increasing rates of nitrogen (N) (100, 200 and 300 kg ha-1) from 2015 to 2017. In 2018, nitrogen fertilization was not used in order to characterize a residual effect of the nutrient. Variables related to pasture structural characteristics such as forage accumulation rate (FAR), canopy height, forage mass (FM) and morphological components were evaluated. The study also evaluated the nutritional value of leaf blades and the performance of beef steers based on average daily gain (ADG) and stocking rate. Principal component analysis was performed using the dataset available. Most of the variance (99.6%) was explained by only two principal components (PCs), of which 90.0% corresponded to PC1. The most influential parameters for PC1, in order of priority, were: FAR, FM, leaf blade and stem masses. These variables were positively associated with stocking rate. Conversely, ADG was not associated with any variable. ADG was the most relevant variable for the second PC; however, this PC explained less variance (9.6%). The structural characteristics of the pasture (FAR, FM and morphological components mass) better explain the fluctuations in the performance of cattle on pastures of Mombasa grass under residual effects of nitrogen fertilizer. The stocking rate is an efficient parameter to support decision-making in managed pastures with variable stocking.

摘要：目前针对高氮肥施用牧场中观测到的次生生产力提升现象，其驱动因素究竟为营养价值波动、牧场结构特征变化，还是二者共同作用，相关研究信息仍较为匮乏。本研究旨在明确一组核心影响因子（涵盖结构与营养特征），以最优解释氮肥残留效应下放牧孟巴莎草（Mombasa grass）牧场的肉牛阉牛生长性能。
研究数据采集于2015至2017年间的孟巴莎草牧场，该阶段施用以梯度递增的氮肥（N）用量（100、200及300 kg·ha⁻¹）；2018年未施加氮肥，以表征该养分的残留效应。
本研究评估了与牧场结构特征相关的多项指标，包括牧草积累速率（forage accumulation rate, FAR）、冠层高度、牧草生物量（forage mass, FM）及形态组分；同时还评估了叶片的营养价值，以及以平均日增重（average daily gain, ADG）和载畜率为表征的肉牛阉牛生长性能。
基于可用数据集开展主成分分析（principal component analysis, PCA），结果显示仅两个主成分（principal components, PCs）即可解释99.6%的总方差，其中PC1贡献了90.0%的方差解释量。PC1中影响力由高到低的参数依次为：FAR、FM、叶片生物量及茎秆生物量，上述变量均与载畜率呈正相关。与之相反，ADG未与任何变量呈现显著关联；而ADG是第二主成分中最具相关性的变量，但该主成分仅能解释9.6%的方差。
在氮肥残留效应下的孟巴莎草牧场中，牧场结构特征（FAR、FM及形态组分生物量）可更好地解释肉牛生长性能的波动；载畜率是支持可变载畜量管理牧场决策制定的高效评价指标。