Effects of corn silage supplementation strategy and grazing intensity on herbage intake, milk production, and behavior of dairy cows

Effects of corn silage supplementation on milk pro-duction of grazing dairy cows depend in part on the sub-stitution rate between the 2 forages, which may be in-fluenced by grazing management. The aim of this study was to compare 2 grazing management strategies for measuring substitution rate between herbage and corn silage, in interaction with grazing intensity. Six treat-ments were compared, with 2 grazing intensities and 3 supplementation strategies investigated at both grazing intensities. The 2 grazing intensities were severe and light grazing, defined by either (1) herbage allowance (HA) of 15 (severe) or 30 (light) kg dry matter (DM)/ cow per d at 3 cm above ground level or (2) postgrazing sward height, depending on the supplementation strat-egy. The 3 supplementation strategies were as follows: (U) an unsupplemented treatment, (A) 5 kg DM/d of corn silage offered at a similar HA as in U, and (H) 5 kg DM/d of corn silage offered at a similar postgrazing sward height as in U. Thirty-six multiparous Holstein cows were used in a randomized complete block design and divided in 2 groups for the entire experiment, one for each grazing intensity. Within each grazing inten-sity group, the corn silage supplementation strategy was studied using a 3 x 3 GrecoLatin square design, with 3 periods of 14 d. Supplementing cows with corn silage increased total DM intake only for severe grazing by 1.7 kg DM/d. The substitution rate between corn silage and grazed herbage was lower for severe than for light grazing, averaging 0.63 and 1.23, respectively. Herbage dry matter intake was lower by 1.2 kg/d for strategy H than A, leading to lower substitution rates (0.81 vs. 0.99, respectively), irrespective of grazing intensity. Milk production increased with silage supple-mentation for severe grazing (+1.0 kg/d milk) and was unaffected by silage supplementation for light grazing (-0.4 kg/d milk). The milk production response to corn silage supplementation averaged +0.23 and -0.08 kg of milk per kg DM of silage for severe and light graz-ing, respectively. Fat-corrected milk production tended to be lower by 0.4 kg/d for strategy H than A, leading to lower milk production response (+0.00 vs. +0.12 kg of milk per kg DM of silage, respectively). Milk protein concentration increased with silage supplementation for severe grazing (+1.0 g/kg) but decreased with silage supplementation for light grazing (-0.6 g/kg). Milk fat concentration did not differ among treatments. On average, daily grazing time (-47 min/d, i.e., -9%) and herbage intake rate (-4.9 g of DM/min, i.e., -14%) decreased when cows were supplemented, with greater grazing time reduction at severe than light grazing, and greater herbage intake rate reduction at light than severe grazing. In conclusion, the greater substitution rate and the lower 4% fat-corrected milk production when corn silage was provided at a similar postgrazing sward height rather than at a similar HA to those of unsupplemented cows explain why supplementing graz-ing dairy cows with conserved forages has no strong effect in practice from a production point of view.

Automated summary

The effects of corn silage supplementation on milk production of grazing dairy cows depend on the substitution rate between herbage and corn silage, which is influenced by grazing management. This study compared two grazing management strategies to measure the substitution rate, in interaction with grazing intensity. The results showed that supplementing cows with corn silage increased total dry matter intake and milk production for severe grazing, but had no effect for light grazing. The substitution rate between corn silage and grazed herbage was lower for severe grazing compared to light grazing. Overall, the study suggests that corn silage supplementation has limited effects on milk production in grazing dairy cows.