Kochia (Bassia scoparia) harvest date impacts nutrient composition, in vitro degradability, and feed value more than pre-harvest herbicide treatment or herbicide resistance traits

Abstract

Kochia is a summer-annual tumbleweed that is drought, saline, and heat tolerant, and often resistant to several herbicides. Glyphosate-resistant kochia exhibits indeterminate growth and remains green during senescence, making it difficult to harvest crops that are contaminated with this weed. Removing dense patches of kochia from contaminated crops can aid in downstream harvest operations while also generating a potential source of livestock feed. This study evaluated the effects of two kochia populations [glyphosate-resistant (GR) and glyphosate-susceptible (GS) kochia] with or without pre-harvest treatment using glyphosate + saflufenacil [untreated (UN) vs treated 10 d pre-harvest (PRE)] on nutrient composition and in vitro degradability when it was harvested at advancing maturities. Kochia (GR-UN, GR-PRE, GS-UN, and GS-PRE) was harvested on 6 dates (HD) corresponding to pre-bloom (HD 1), mid-bloom (HD 2), full-bloom with 50% open flowers (HD 3), developing seeds (HD 4), mid-mature seeds (HD 5) and fully mature seeds (HD 6). In most cases, harvest date affected the chemical composition and in vitro degradability to a greater extent than the kochia population or herbicide treatment. The dry matter (DM) yield increased (P < 0.001) from HD 1–5 and decreased thereafter. The crude protein (CP) content decreased (P < 0.001) while the acid (ADF) and neutral detergent fiber (NDF) content increased (P < 0.001) with advancing HD. Overall, the CP content was higher (P = 0.014), and ADF content lower (P < 0.001) for GS than GR. Total in vitro gas (mL) (P < 0.001) and methane (CH4) production (mL L−1 of gas production) (P < 0.001) after 6 h incubation was greater for GR-UN than GR-PRE across HD. A decrease in acetate (P < 0.001) and an increase (P < 0.001) in propionate concentration with advancing HD led to reduced (P < 0.001) CH4 production after 48 h of in vitro incubation. Dry matter degradability (DMD; %) of GR-UN was greater (P = 0.003) than GR-PRE, while DMD and NDF degradability decreased (P < 0.001) with advancing HD. Pre-harvest application of glyphosate tended to increase the cell wall and decrease non-fibrous carbohydrate fractions, resulting in a slight decline in degradability and an increase (P < 0.05) in in vitro CH4 production. Further in vivo digestibility and growth performance studies using kochia harvested at the optimal maturity are required to define the value of this weed as forage in ruminant production systems.