Feed intake is a key determinant of dairy cow milk production. Napier grass usually contains a high percentage of neutral detergent fibre (NDF), which limits ruminant DM intake. On average, low-producing dairy cows can consume DM and NDF at 2.1% and 1.8% of their liveweight (LW), respectively, and 42.9 MJ ME/day from Napier grass containing 84 g CP/kg DM, 723 g NDF/kg DM and 7.5 MJ ME/kg DM (Table 1). Napier grass of this quality (i.e., low CP and ME, and high NDF) and intake supported the production of 7.9 kg milk/day/cow (but continued to lose its LW at 36 g/day), a reduction in the average 10.2 kg milk/day before the start of the experiment (Table 1). This suggests that in addition to quality, DM intake from this grass should be increased beyond 2.1% to sustain milk production of cows either by reducing the NDF content of this grass or by supplementing feed which has less NDF content. The total DM intake is limited when NDF intake is greater than 1.2% of LW ^[1][15]. However, it is difficult to manage NDF intake at this level in tropical C₄ grass-based diets, as NDF content from these grasses is generally high, and consequently, NDF intake on grass-based diets is also likely to be high. Cows consume NDF, on average, at 1.6% of LW when the total intake is 20.4 kg DM/day (17.4 kg from forages and 3 kg from grains; DM intake 3.3% of LW) from kikuyu grass (Pennisetum clandestinum) containing 203 g CP/kg DM and 470 g NDF/kg DM ^[2][13]. A similar NDF intake (1.6% on average, 1.2–1.9% of LW) was calculated from elsewhere ^[3][16]. Therefore, feed quality, particularly CP and ME, should be increased, and NDF should be decreased in Napier grass to increase DM intake by cows beyond 2% of LW to sustain milk or meat yield.

,18]. Despite the high intake, this growth (i.e., 390–420 g/day) ^[19][30] was low, since the ARC standard ^[20][31] indicates that grasses containing 120 g CP/kg DM would allow heifers to gain 500 g/day as they would meet the rumen degradable protein requirement of heifers for this growth. Nonetheless, this growth (i.e., 390–420 g/day) from the feeding of a sole Napier grass diet at 56 days HI containing 117 g/kg DM CP ^[19][30] was greater compared to the 210–250 g/day growth of young stocks reported by others who offered Napier grass containing <80 g CP/kg DM ^[21][22][32,33]. However, with similar CP (118 g/kg DM) and NDF (587 g/kg DM) content of this grass which was harvested at 0.5 m height (42 days HI; 8.6 MJ ME/kg DM), a 500 g/day LW gain in Holstein heifers (144 kg LW) with intakes of DM and NDF at 3.5% and 2% of LW, respectively, was reported in an experiment conducted over 104 days ^[23][34]. This growth of the heifers matches with the growth reported by ARC ^[20][31], suggesting cattle solely offered Napier grass containing approximately 12% CP and 55% NDF will grow at 500 g/day.

A greater DM intake (11.6 kg DM; 2.6% of LW) was reported when 70-day-old grass (containing 80 g CP/kg DM, 540 g NDF/kg DM, 8.2 MJ/kg DM) compared to 105-day-old grass (10.1 kg DM, 2.3% of LW; containing 50 g CP/kg DM, 630 g NDF/kg DM, 7.3 MJ/kg DM) was offered to Holstein steers (445 kg LW) ^[24][35]. Although the NDF contents of 70- and 105-day-old grasses were 540 and 630 g/kg DM, respectively, the NDF intakes between the grass groups were similar (6.4 kg, 1.44% of LW) ^[24][35]. Similarly, a greater DM intake of this grass by buffalo steers (300 kg LW) was reported when 40-day-old grass (8.9 kg DM intake; 3.0% of LW) was offered compared to 60-day-old grass (7.8 kg DM intake; 2.6% of LW) ^[25][36]. The NDF contents of those grasses were 721 and 787 g/kg DM, respectively, but NDF intakes were similar between the groups (6.6 kg, 2.2% of LW) ^[25][36]. This suggest that animals, cattle or buffalo, young or old, cannot consume NDF beyond a certain threshold, and the main driver to increase DM intake from Napier grass is to reduce its NDF content, as animals are not able to consume sufficient DM due to their rumen fill by NDF from mature (older) grasses. Nonetheless, these intakes of NDF by cattle (1.44% of LW) ^[24][35] and buffaloes (2.2% of LW) ^[25][36] were greater than the recommended values ^[1][15], which reported that the total DM intake is limited when the NDF intake is greater than 1.2% of LW. Also, the NDF intake by buffaloes was greater than that of cattle. Thus, Napier grass containing 12% CP and 8.6 MJ ME/kg DM offered to steers/heifers can enable them to grow from 390 to 500 g/day. Overall, intakes of DM and NDF of 2.4% and 1.4% of steer/heifer LW, respectively, from Napier grass containing 594 NDF/kg DM and 626 g/kg DMD (dry matter digestibility) were able to support 445 g LW gain/day/steer (Table 2).

On average, intakes of DM and NDF of 3.3% and 2.0% of goat/sheep LW, respectively, from Napier grass containing 663 NDF/kg DM and 611 g/kg DMD (dry matter digestibility) were able to support 100 g milk or 54 g LW gain/day (Table 3). However, Napier grass (harvested at 60–70 days HI) containing 83 g CP/kg DM, 673 g NDF/kg DM, 570 g/kg DMD and 7.8 MJ ME/kg DM was offered to Boer × Local goats (12 kg LW) over 82 days ^[31][42]. With such quality, these goats were losing 13 g LW/goat/day, although they were consuming DM at 3.4% of their LW (NDF intake 2.2% of LW). However, these goats were gaining 53 g/head/day when Napier grass was supplemented with a concentrate mixture containing 127 g CP/kg DM and 10.5 MJ ME/kg DM. This growth was achieved when the intake of Napier grass was 276 g/day and that of concentrate was 493 g/day (i.e., roughage–concentrate = 36:64; concentrate at 2% of their LW), indicating that a high amount of concentrate is required for growth when Napier grass offered to goats is at high maturity (or HI) ^[31][42]. This, taken alongside the work above, suggests that both the protein and energy contents of Napier grass need to be increased, or supplemented, to support the growth of goats, similar to the needs of cattle. Therefore, Napier grass under the current management cannot support the growth of goats without supplementation. Research is needed to offer this grass at a lower maturity and containing >12% CP and >9 MJ ME/kg DM to investigate the growth potential of different species of animals.

The Napier grass used for milk production had a harvest interval (HI) of between 70 and 91 days and contained 60–80 g CP/kg DM and approximately 7.5 MJ ME/kg DM ^[5][17]. As a consequence, the cows failed to sustain their LW and milk production. Napier grass harvested between 70 and 91 days HI contained 61–72 g CP/kg DM, 714–786 g NDF/kg DM, and 455–503 g ADF/kg DM (7.76 MJ ME/kg DM calculated), and 576 g/kg DMD was offered ad libitum (with ad libitum mineral block lick) to Friesian cows (370–405 kg LW; yielding 15 kg milk/cow/day; body condition score, BCS 3) for 37 days ^[5][17]. Dry matter intake from this grass was 8.4–9.1 kg DM/cow/day (2.3% of LW), and NDF intake was 1.75% of LW. However, cows offered feed of this quality were not able to sustain milk production and LW, resulting in a production of 6.8–7.9 kg 4% fat-corrected milk/cow/day (compared to 15 kg/cow/day at the start of the experiment) alongside a loss of 530–890 g LW/cow/day by the end of the experiment ^[5][17]. Maintaining 15 kg milk and 220 g LW gain per day would have required 1085–1227 g CP/cow/day and 138–145 MJ ME/cow/day, but the cows received 512–655 g CP/cow/day and 71–75 MJ ME/cow/day from Napier grass harvested at 70–91 days HI ^[5][17]. Similarly, Friesian cows (424–441 kg, 4 years old, 210 days in milk, 15 kg milk/cow/day) needed 8 kg concentrate/cow/day (containing CP 160 g/kg DM, ADF 100 g/kg DM, and ash 50 g/kg DM) to maintain 15 kg milk/cow/day when Napier grass was offered after harvesting at 55 days HI containing 86 g CP/kg DM and 410 g ADF/kg DM ^[9][21]. In the same experiment, milk yield dropped to 10.5 kg/cow/day from 15 kg within 4 weeks of the experiment when this grass was offered ad libitum with minerals at 0.05 kg/100 kg LW. These cows needed another 4 weeks to reach a normal state of milk production (i.e., 15 kg milk/cow/day) when 8 kg concentrate/cow/day was offered with Napier grass ^[9][21]. Thus, farmers currently supplement a large amount of imported (to the farm), higher-density feed protein and energy when Napier grass aged 70–91 days is offered to cows to maintain 15 kg milk/cow/day.

In another experiment conducted over 14 weeks using Ayreshire/Brown Swiss × Shahiwal cows (430 kg LW, 10 kg/cow/day, 1–4 lactation stage, 14 days in milk), Napier grass harvested either at 1 m (28–42 days HI) or 1.5 m (>42 days HI) plant height was offered to cows supplemented with mineral blocks ad libitum ^[4][10]. The crude protein content of these grasses was 72 and 56 g/kg DM, and ME (MJ/kg DM, calculated) was 7.6 and 7.0, respectively. The 1 m Napier grass treatment had a greater intake (9.1 vs. 6.5 kg DM/cow/day) and milk yield (8.5 vs. 6.1 kg/cow/day) compared to the 1.5 m plants, and the cows in the 1.5 m Napier grass treatment could support 60% of the total milk (i.e., 6.1 kg) compared to their yield before the start of the experiment (i.e., 10 kg) ^[4][10]. In the same work, milk yield decreased by 330 and 220 g/week in cows fed Napier grass harvested at 1.5 m and 1 m, respectively. In addition, cows in both groups lost weight (−420 vs. −665 g/cow/day) ^[4][10]. Supplementation of Leucaena leucocephala even up to 8 kg/cow/day (fresh) was not able to increase the milk yield or LW of the cows, although weight loss was reduced to −20 g/cow/day in the 1 m height group compared to −325 g/cow/day in the 1.5 m height group ^[4][10]. Similarly, Jersey cows offered Napier grass (1.5 m plant height, CP 64–67 g/kg DM, 500–530 g/kg digestible organic matter in DM, DOMD) ad libitum over 50 days were able to produce 4.2–6.4 kg milk/cow/day, and the cows failed to maintain LW (losing 4–29 g/cow/day) ^[8][20]. Supplementation of 300 g CP, either by fish meal (980 g/cow/day) or copra meal (1220 g/cow/day), increased milk yield by 1.2–1.6 kg/cow/day, but the cows were still losing 23–25 g LW/cow/day ^[8][20]. Thus, Napier grass offered to cows when harvested at 28 days or greater HI, and 1 m or greater height, was not able to supply the protein and energy required for moderate levels of milk production.

In contrast, an experiment with kikuyu grass, a C₄ grass and a Pennisetum species that belongs to Napier grass, reported high DM intake (12–18 kg/cow/day) and milk yield (16–27 L/cow/day), despite the high NDF intake (1.6–2.2% of LW) ^[10][22]. Such high DM and NDF intakes were achievable due to high DM digestibility (DMD, 640–780 g/kg), particularly acid detergent fibre (ADF) digestibility (ADFD, 484–688 g/kg DM), as grasses were grazed at an early growth stage ^[11][23]. Cows can eat up to 1.6% NDF of LW when fibre digestibility is high ^[12][24]. However, the intake of excess fibre by ruminants is undesirable as it is associated with energy costs in the digestive process, the consequences of which include greater ME requirements for maintenance ^[13][14][25,26]. The digestibility of NDF is also important as each unit increase in forage NDF digestibility can increase the DM intake of cows by 0.17 kg, which can increase milk yield by 0.23 kg ^[15][27]. These data indicate that grasses offered at an early stage of growth increase fibre digestibility which increases the intake, digestibility and productivity of animals.

In line with the experience of kikuyu grass, Napier grass grazed (or harvested) by cows at 30 days HI does not need any supplement for most of the year, except in dry seasons for moderate milk production in a smallholder production system. In an experiment ^[6][18] conducted over 1 year, Holstein–Zebu cross cows (483 kg LW, 45–60 days in milk) grazed solely Napier grass at 30 days HI, and at a 4.5 cows/ha stocking rate, produced on average 11.4 kg milk/cow/day. The intake of the cows ranged from 12.9 to 14.9 kg DM/cow/day (2.7–3.0% of LW; calculated NDF intake of 1.8–2.0% of LW), and the consumed grass contained 112–137 g/kg DM CP, 690–750 g/kg DM NDF, 380–440 g/kg DM ADF and 530–580 g/kg IVDMD (in vitro DM digestibility) ^[6][18]. This result is supported by other works ^[16][17][28,29], who reported mid-lactating cows can produce 14–15 L milk/cow/day when well-managed kikuyu grass supplemented with minerals is offered, provided DM intake (13 kg DM/cow/day) is not limited by its NDF content (when NDF intake does not exceed 1.5% of LW). Therefore, more research is required to investigate if cows can produce >10 kg milk/day and maintain LW when Napier grass is managed to contain >14% CP.

Similar to cows, when a sole diet of Napier grass is offered to dairy goats, milk production typically declines. Napier grass harvested at 49 days HI containing 113 g CP/kg DM, 696 g NDF/kg DM, 600 g/kg DMD and 10.2 MJ ME/kg DM was offered to Toggenburg goats (2 years old, 29 kg LW, 9 months of lactation, 3rd month of pregnancy, 430 g milk/goat/day) over 11 days ^[18][11]. The intake of this grass was 1.21 kg DM/day (4.2% of LW). However, the goats were able to maintain 117 g milk/goat/day (out of 430 g milk/goat/day) and a complete drying up occurred during the end of the trial. The calculated protein and digestible energy requirements per goat were 191 g and 20 MJ per day, but each goat was receiving 163 g and 13 MJ per day, respectively ^[18][11]. Thus, Napier grass harvested at 49 days HI was not able to supply the CP and ME required by the lactating dairy Toggenburg goats, despite its high intake (4.2% of goats LW). Thus, Napier grass managed under the current management and offered to lactating cows and goats requires a high amount of expensive supplementation (protein, energy and minerals) to produce milk. These include fish meal, molasses and mineral blocks, which smallholder farmers in the tropics and subtropics may not afford to offer. Information is also lacking as to whether such supplementation is cost effective. Therefore, current Napier grass management for the milk production of cows, goats or other lactating animals needs to be altered towards an improved management strategy to increase protein and energy to limit the requirement for expensive supplements.

Napier grass (56 days HI, 12 t DM/ha/year grown with 110 kg N/ha/year) containing 117 g CP/kg DM, 560 g NDF/kg DM and 704 g/kg DM potential degradability (PD, 96 h; 550 g/kg DM effective degradability, ED) was offered to Friesian or Shahiwal heifers (163–181 kg LW; 1 year old) over 120 days ^[19][30]. Heifers grew at 390 and 420 g/day, respectively, solely on Napier grass. The DM intake was 5.6–6.0 kg/cow/day (3.4% of LW), and the estimated NDF intake was 1.8–1.9% of LW for both groups ^[19][30], similar to the intake of cows per unit of LW reported in the literature (NDF intake 1.6–2.0% of LW) ^[5][6][17