Starch degradation in rumen fluid as influenced by genotype, climatic conditions and maturity stage of maize, grown under controlled conditions

Starch is the major component of maize kernels, contributing significantly to the feeding value of forage maize when fed to ruminants. The effects of genotype, climatic conditions and maturity stage on starch content in the kernels and on in vitro starch degradability in rumen fluid were investigated. Kernels of six maize genotypes, differing in amylose content and vitreousness, grown under three contrasting day/night temperature regimes during grain filling, and harvested at different maturity stages from two greenhouse experiments were investigated. Starch content was measured using an enzymatic method and the gas production technique was used to assess starch degradation in rumen fluid of dairy cows. The extent of starch degradation at different incubation times (6, 12 and 20h) was calculated from measured gas production data (6, 12 and 20h, respectively) and a published equation. Gas production (ml gas/g OM) showed a positive linear relationship with starch content in the kernels up to a certain level of starch accumulation. At each maturity stage, whole kernel and starch degradation in rumen fluid depended on the genotype (P<0.0001), growing conditions (P<0.0001), starch content (P<0.0001) and starch amount (P<0.0001) in the kernels. While starch content increased with advancing maturity, starch degradation similarly increased up to a certain level of starch content. In vitro starch degradation of the maize kernels in rumen fluid was affected by the starch composition, e.g. amylose and amylopectin content. Starch degradation was inversely related to the amylose content and vitreousness. Higher starch degradation was observed in the waxy (no amylose) and non-vitreous genotypes. The highest starch degradation was observed when plants were grown at intermediate temperatures in both experiments. The difference in starch degradability of each genotype at the same accumulated thermal time, i.e. maturity stage, was due to differences in grain filling rate, caused by the different temperature regimes. This effect of genotype and climatic conditions was consistent for all incubation times (P<0.0001). Rumen in vitro starch degradation is significantly influenced by genotypic, differences in starch content of the maize kernels and their growing conditions.