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

The influence of forage supplementation on feed intake and milk production of grazing dairy cows depend on many factors, including pasture characteristics, quality of the supplement, herbage allowance and grazing intensity. The literature describes a wide range of substitution rates between grazed herbage and conserved forage (e.g. the reduction in herbage dry matter intake [DMI] per kg of supplement DMI), but it remains difficult to predict the effects of forage supplementation on total DMI and milk production; one of the reasons being the grazing management strategy followed has a profound effect on the outcomes. To understand this further, the aim of this study was to evaluate short-term effects of two experimental strategies of maize silage supplementation (i.e., herbage allowance or post-grazing sward height similar to that of un-supplemented cows) on herbage intake, milk production and grazing behaviour. The main hypothesis was that maize silage supplementation at a similar post-grazing sward height will induce a higher substitution rate and lower milk production than maize silage supplementation at a similar herbage allowance. The authors also hypothesized that the expected differences in substitution rate and milk production between supplementation strategies would be exacerbated at high grazing intensity, compared with low grazing intensity, taking into account that the substitution rate is well-known to decrease with decreasing herbage allowance.

Six treatments were compared, with two grazing intensities and three ways of supplementation, investigated at both grazing intensities. The two grazing intensities were severe and light grazing: (1) herbage allowance of 15 (severe) or 30 (light) kg DM per cow per day at 3 cm above ground level or, (2) post-grazing sward height, depending on the supplementation strategy. The three ways of supplementation were as follows: an un-supplemented treatment (U), (A) 5 kg DM per day of maize silage offered at a similar herbage allowance as in U, and (H) 5 kg DM per day of maize silage offered at a similar post-grazing sward height as in U. Thirty-six multi-lactation Holstein cows were used in a randomized complete block design and divided in two groups for the entire experiment, one for each grazing intensity. Within each grazing intensity group, the maize silage supplementation strategy was studied using a 3 × 3 Latin square design, with three periods of 14 days each.

The results showed that supplementation with maize silage increased total DMI only for severe grazing, by 1.7 kg DM per day. The substitution rate between maize silage and grazed herbage was lower for severe than for light grazing, averaging 0.63 and 1.23, respectively. Herbage DMI was lower by 1.2 kg per day for H than A, resulting in lower substitution rates (0.81 vs. 0.99, respectively), irrespective of grazing intensity. Milk production increased with silage supplementation for severe grazing (+1.0 kg per day) but was unaffected by silage supplementation for light grazing. The milk production response to supplementation averaged +0.23 and −0.08 kg of milk per kg DM of silage for severe and light grazing, respectively. Fat-corrected milk production tended to be lower by 0.4 kg per day for H than A, resulting in a lower milk production response (+0.00 vs. +0.12 kg of milk per kg DM of silage, respectively). Milk protein concentration increased with supplementation for severe grazing (+1.0 g per kg) but decreased with supplementation for light grazing (−0.6 g per kg). Milk fat concentration did not differ among treatments. On average, daily grazing time (−47 minutes per day, i.e., −9%) and herbage intake rate (−4.9 g of DM per minute, 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 maize silage was provided at a similar post-grazing sward height rather than at a similar herbage allowance to those of un-supplemented cows, explain why supplementing grazing dairy cows with conserved forages has no strong effect in practice from a production point of view.