Association among pasture-level variables and grazing dairy cow responses to supplementary feeds : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Animal Science at Massey University, Manawatū, New Zealand

Abstract

The milk production response to additional feed (i.e., supplement) is dependent on the relative feed deficit (RFD) of the cow. We hypothesized that the relative feed deficit could be defined by post-grazing residual (PGR), with a greater PGR indicating less of a relative feed deficit. We undertook a computerized literature review, utilising key words associated with grazing systems and supplementary feed. Approximately 70 published manuscripts were reviewed. Only those that satisfied predetermined inclusion criteria were retained. A meta-analysis was undertaken across all the studies using random coefficient regression fitted as a mixed-model. In total, we collated data from 26 experiments and 90 treatments, wherein pasture-level variables, supplementary feed variables, and milk production were reported. Due to a lack of reporting of standard errors, two analyses were undertaken; one where responses were weighted against the reciprocal of the standard error of the mean, and one where they were not. On average, pasture DM intake declined (-0.28 kg/kg supplement DM; P = 0.001) and milk, fat, and protein increased (P < 0.001) 0.65 kg, 20g, and 30g/kg supplement DM, respectively. For every kg DM supplement consumed, PGR height and mass increased by 1.4 mm and 42 kg DM/ha. These results were similar in the non-weighted analysis. Associated with every 10 mm increase in PGR height in the control treatment, marginal milk response declined (P < 0.05) by 55 ± 21.6 g. The association between PGR height and pasture DMI at zero supplementary feed intake (i.e., unsupplemented group in experiment) on the PGR and pasture DMI responses to supplementary feed, were however, inconsistent in the weighted and non-weighted analysis. These results will enable farmers to use the change in PGR when feeding supplements, to estimate likely marginal milk production response to supplementary feeds. These results are associations only and need to be tested in controlled, interventionist, experiments. Due the number of variables affecting MR, we cannot conclude that anything is causative.