Effect of harvesting strategy of second-cut orchardgrass silage on feed intake, digestion, and milk production in dairy cows

2.1 Preparation of grass silage

Three second-cut orchardgrass silages were prepared from third-year orchardgrass swards in 2022. The first-cut grasses were harvested on 24 May at the early (E) and on 4 June at the heading (H) stage of growth. The two second-cut grasses were harvested from the regrowth area E on 5 July (regrowth interval: 6 w, E6w) and 12 July (regrowth interval: 7 w, E7w), and the one second-cut grass was harvested from the regrowth area H on 22 July (regrowth interval: 7 w, H7w). By the way, the second-cut grass was not harvested from the regrowth area H at 6w intervals, because this harvesting method is not practical to optimize yield and workability. The cumulative temperature, which was calculated from the onset of growth as Σ (daily mean temperature minus 5°C), was 530.6, 666.3, and 701.8°C for E6w, E7w, and H7w, respectively. The N-P-K fertilizers were applied at rates of 60–24-60 kg/ha for the first-cuts on 21 April and for the second-cuts on the day after first-cut grass harvesting, respectively. The second-cut orchardgrass was cut with a mower conditioner and harvested after 6 hours of wilting using a precision-chop harvester. The chopped materials were transported and baled in roll form and wrapped with eight layers of polyethylene film using a chopping combi bale wrapper (TSW2020; IHI STAR Machinery, Chitose, Japan), and then stored outdoors for over 8 months.

2.2 Feeding trial

Six multiparous Holstein cows (2.4 ± 0.4 parity, 694 ± 46 kg body weight, 170 ± 31 days in milk) were randomly assigned to a replicated 3 × 3 Latin square design with the three dietary treatments (E7w, E6w, or H7w). The experimental period was 21 d, with 14 d for treatment adaptation and 7 d for data collection. The experimental total mixed rations (TMRs) contained second-cut grass silage (E7w, E6w, or H7w) at 30.0% of DM, and were supplemented with corn silage, ear corn silage, commercial concentrate, and calcium carbonate. The ingredient composition of the commercial concentrate has been described in our previous report (Miyaji, Yajima, Tada, et al., 2020). All experimental diets were formulated to meet or exceed the energy and protein requirements of the Japanese Feeding Standard for Dairy Cattle (NARO, 2017) for a 700 kg lactating dairy cow producing 38 kg of milk/d. Throughout the experiment, the cows were kept in individual tie stalls and had free access to fresh water and a trace mineral salt block. The cows were offered the experimental TMR for free intake (10% refusals) twice daily (09:30 and 16:30 h) and milked twice daily (09:00 and 18:00 h).

During days 15–20 of each 21-day period, the diets and orts were weighed, and representative samples were collected daily. The diet and ort samples were dried at 60°C in an oven for 48 h. The dried samples were ground through a 1-mm screen using a centrifugal mill (SM2000; Retsch & Co., Haan, Germany) and stored until analysis. Milk yield was measured daily and averaged over the collection period; milk samples were collected at each milking. Milk samples were analyzed for fat, CP, and lactose concentrations using infrared spectroscopy (Lacto-Scope FT-A; Delta Instruments, Netherlands).

On days 15–19 of each 21-day period, fecal grab samples were collected twice daily (10:00 and 16:00 h) and stored in a refrigerator at 5°C during the collection to determine the apparent digestibility. A sample of fresh feces was dried at 60°C for at least 72 h in a forced-air oven followed by ground through a 1-mm screen and stored until analysis.

2.3 Chemical analysis, calculation, and statistical analyses

All procedures for DM, OM, CP, NDF, ADF, iNDF, and starch analyses of feed and fecal samples, in situ effective ruminal NDF degradability (ERND), and fermentation profile of silage analyses were performed as described previously (Miyaji et al., 2022; Miyaji, Yajima, Tada, et al., 2020).

The whole-tract apparent digestibility of the DM and nutrients was determined using iNDF as an internal digestibility marker. All calculations for nutrient digestibility, FCM, and ECM were performed as previously described (Miyaji et al., 2022; Miyaji, Yajima, Tada, et al., 2020).

The data were statistically analyzed according to a replicated 3 × 3 Latin square design using R (version 4.2.2). The model is given by Yijkl = μ + ai + bj + ck (ai) + dl + εijkl, where Yijkl is the dependent variable, μ is the overall mean, ai is the fixed effect of the square (i = 1 to 3), bj is the fixed effect of the period (j = 1 to 3), ck (ai) is the random effect of animal within a square (k = 1 to 6), dl is the fixed effect of dietary treatment (l = 1 to 3), and εijkl is the residual error. Multiple comparisons among the dietary treatments (E7w, E6w, and H7w) were performed using the Tukey method, and significance was set at P < 0.05. We evaluated the effect of regrowth interval by comparing E7w and E6w, and the effect of first-cut timing by comparing E7w and H7w.

3.1 Diet characteristics

Our comparison of the three experimental silages revealed little differences in the fermentation profile and chemical composition, except for iNDF content (Table 1). The iNDF content was the lowest and the ERND was the highest for E6w. The iNDF content and the ERND were similar in E7w and H7w. The fermentation quality was good for all silages as indicated by the low pH values and butyrate content and NH3/TN.

Our analyses revealed little difference in chemical composition among the experimental TMRs, except for the iNDF content (Table 2). The iNDF content was lower in E6w than in E7w and was similar between E7w and H7w, reflecting the grass silage characteristics.

3.2 Nutrient digestibility, DMI, and milk production

The whole-tract digestibility of DM, OM, NDF, and ADF was higher for the cows fed E6w TMR than those fed E7w TMR (P = 0.01, 0.01, 0.04, and 0.03, respectively, Table 3). No differences in CP or starch digestibility were detected between E6w and E7w. The timing of the first cut (E7w vs. H7w) did not affect nutrient digestibility.

The DMI, milk yield, FCM, and ECM yields were greater for cows fed E6w TMR than those fed E7w TMR (P < 0.01, =0.03, <0.05 and = 0.02, respectively, Table 3). The replacement of E7w with E6w increased milk CP and lactose yields (P = 0.01 and = 0.02, respectively), but did not affect milk fat yield. Milk composition except for lactose was not affected by the regrowth interval (E7w vs. E6w). The DMI and lactation performance were not influenced by first-cut timing (E7w vs. H7w).

4.1 Effect of regrowth interval (E7w vs. E6w)

In the present study, whole-tract DM and fiber digestibility were increased by the replacement of E7w with E6w silage (i.e., by shortening the regrowth interval), which could reflect degradation characteristics; ERND increased when grass was harvested at short intervals. These results are consistent with those reported by Kuoppala et al. (2008) and Pang et al. (2021). Short-interval harvesting decreases cell wall lignification and increases fiber digestibility (Arnold et al., 2019). Some researchers have suggested that DMI increases when cows are fed more digestible NDF (Huhtanen et al., 2007; Miyaji, Yajima, Tada, et al., 2020), as a result of rapid clearance from the rumen (NRC, 2001). The current results are consistent with those of previous studies. Increased NDF digestibility due to a shortened regrowth interval could lower rumen fill and result in an increase in DMI.

We observed that milk yield increased with the replacement of E7w with E6w silage. Feeding E6w instead of E7w increased DM and fiber digestibility. DMI was also increased by the E6w diet. Thus, increasing DM and nutrient digestibility and DMI by shortening the regrowth interval could increase milk yield. It has been suggested that feeding a diet with higher digestible fiber increases milk fat production as a result of ruminal pH and/or ruminal VFA pattern changes (NARO, 2017; NRC, 2001). However, in the present study milk fat yield was not increased by feeding the E6w diet instead of the E7w diet, although replacing the E7w diet with the E6w diet increased fiber digestibility and DMI. The reasons for the similar milk fat production between E6w and E7w are not clear; however, feeding the E6w- instead of E7w- TMR may have had little impact on ruminal fermentation in this study.

4.2 Effect of first-cut timing (E7w vs. H7w)

Cumulative temperature and silage indigestible fiber content are correlated, with high temperatures increasing lignification and decreasing digestibility (Pang et al., 2021; Van Soest, 1994). In the present study, the cumulative temperature was lower for E7w than for the H7w growing period (666 vs. 702°C). Thus, the high cumulative temperature throughout the second grass growth period for H7w could be expected to decrease nutrient digestibility. However, there were small differences in chemical composition and ruminal fiber degradation between the E7w and H7w silage, and thus the first-cut timing had little impact on the dietary characteristics of second-cut grass silage. Our results are not consistent with those of earlier reports that focused on timothy grass (Kuoppala et al., 2008; Pang et al., 2021). Pang et al. (2021) suggested that despite the same length of the growth period, postponing the first-cut decreased fiber degradability and increased the undigestible fiber content of second-cut timothy silage, as a result of a high cumulative temperature. The orchardgrass had an earlier cutting time for the second grass, thus lowering the temperature throughout the second grass-growing period, compared to timothy grass. In the present study, the cumulative temperature throughout the growing period during early summer was not too high, and then the timing of the first-cut might not have affected the dietary content. In addition, the difference in the cumulative temperature throughout the second-cut grass-growing period between first-cut timings was relatively small. This difference in cumulative temperature between the first-cut periods may have been too small to have an impact on silage dietary characteristics.

Nutrient digestibility, DMI, and milk production were not affected by replacing E7w with H7w silage. The present results are inconsistent with those reported by Kuoppala et al. (2008) and Pang et al. (2021), who found that postponing the first-cut decreased fiber degradability and that feeding second-cut timothy silage after the late first-cut decreased feed intake and milk production in dairy cows. In the present study, first-cut timing did not affect dietary nutrient content or fiber degradation. Thus, the first-cut timing could have had little impact on nutrient digestion, DMI, and lactation performance owing to the similar dietary characteristics between E7w and H7w.

In conclusion, we found that feeding second-cut orchardgrass silage with a short regrowth interval increased feed intake and milk production owing to higher digestibility. However, we also found no significant difference between E7w and H7w in nutrient content and DMI and milk production. Harvesting second-cut orchardgrass at short regrowth intervals can be an effective strategy for improving feed utilization and milk yield.