Effect of ensiling time and exogenous protease addition on nitrogen fractions and ruminal in vitro starch digestibility in whole-plant corn silage from varied hybrid types, maturities and particle size

Abstract Text: The objective of this study was to evaluate the effects of ensiling time and exogenous protease addition on soluble CP (%CP), ammonia-N (%N) and ruminal in vitro starch digestibility (ivStarchD) in whole-plant corn silage (WPCS) of varied hybrid types, maturities and particle size. Samples from 3 hybrids (brown midrib [BMR], dual-purpose [DP], or floury-leafy [FLY]) at 2 harvest maturities (2/3 of milk line [early; ER] or 7 d later [LT]) with 2 theoretical lengths of cut settings on the forage harvester (0.64- or 1.95-cm) were collected at harvest, treated with (PROT) or without (CON) exogenous protease, and ensiled in triplicate in vacuum-sealed plastic bags for 0, 30, 60, 120 and 240 d. Thus, the experiment consisted of 120 treatments (3 hybrids x 2 maturities x 2 particle size x 2 protease treatments x 5 time-points) and 360 mini-silos (3 replications per treatment). Data were analyzed as a split-split-plot design using Proc Mixed of SAS with the Fixed effects of hybrid, maturity, particle size, protease addition, ensiling time and their interaction and the Random effects of hybrid by maturity and hybrid by maturity by particle size. Vitreousness measured on unfermented kernels on the day of harvest was 83.0%, 47.5% and 42.0%, on average, for BMR, DP and LFY, respectively.  Although ammonia-N did not differ (P > 0.10), soluble CP tended to increase (41.4 vs. 42.1% of CP; P = 0.06) and ivStarchD approached a trend for an increase (60.9 vs. 61.8% of starch; P = 0.11) with exogenous protease addition. A protease by maturity interaction was observed (P = 0.01) with protease increasing ivStarchD in LT but not ER. Gradual increases were observed (P < 0.001) over time from 0 to 240 d for soluble CP and ammonia-N concentrations. Likewise, the ivStarchD measurements increased with ensiling time (P < 0.001; 56.2, 58.8, 61.2, 63.3, and 67.3% of starch for 0, 30, 60, 120 and 240 d, respectively). Ensiling time by hybrid interactions was observed (P = 0.001) for ammonia-N and soluble CP concentrations with greater values for FLY than other hybrids only after 120 d of ensiling. Similarly, ivStarchD was greater for FLY than DP only after 60 d of ensiling. Length of the ensiling period did not attenuate negative effects of kernel vitreousness on starch digestibility in WPCS. Results suggest that exogenous protease addition may reduce the negative effects of maturity on starch digestibility in WPCS, and merits further investigation.

Keywords: corn silage, ensiling time, starch digestibility