Supplementing grazing dairy cows with crops: fodder beet and oats, to improve milk production and nitrogen utilization: A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

The objectives of this research were to determine the impact of supplementing pasture-based diets with forage crops, previously used to mitigate winter N losses, on dry matter intake (DMI), milk yield, milk composition, and N utilization from dairy cows. Three supplementation studies integrating fodder beet (FB, Beta vulgaris L) or oats (Avena sativa) with perennial ryegrass (Lolium perenne L.)-white-clover (Trifolium repens L.) pasture were carried out in early or late lactation. Further, modelling studies were conducted to compare the effects of these crops on the productivity and profitability of irrigated dairy farms in Canterbury, New Zealand. The aim of the first experiment was to determine the effect of FB or oats on dry matter intake (DMI), milk production and N utilisation of grazing dairy cows in early lactation. The experiment was a comparative study of four spring feeding regimes with sampling replication via animal (n = 12 cows/treatment). Forty eight early lactation dairy cows were fed 3 kg DM/cow/d of fodder beet, oats forage or oats silage as supplement + 18 kg DM/cow/d of pasture for 21 days using a completely randomized design. Total apparent DMI was greater (P < 0.001) for grazed oats forage (OF) and oats silage (OS) compared with pasture only (Control) or fodder beet (FB). Cows substituted pasture for supplement maintaining a similar metabolisable energy (ME) intake (226± 3) across treatments. Consequently, there was no effect (P = 0.865) of supplement on milk yield or milk solids yield (P = 0.436). However, supplementation with FB resulted in lower (P < 0.001) urinary N concentration (4.7 g N/L) compared with CON, OF and OS (5.9, 6.0 and 5.5g N/L) respectively. The higher urinary N concentration in CON, OF and OS is likely to be due to higher N intake (630, 725 and 657 g N/cow/day respectively) compared to FB (589 g N/cow/day). These results showed there was no increase in milk production due to high substitution rates and similar energy intake. However, low protein supplements such as fodder beet were effective at reducing N intake and improve N use efficiency for milk production. Questions remained regarding the animal response to FB or oats when supplemented to cows with a lower energy demand in late lactation. The aim of the second experiment (autumn) was to determine the effect of FB or silage (ryegrass or oat) supplementation on DMI, grazing behavior, milk production and urinary N excretion on grazing dairy cows in late lactation. Fifty-four late lactation dairy cows were supplemented 4 kg DM of fodder beet (FB), oats silage (OS) or ryegrass silage (RGS) in addition to allocation of 12 kg DM/cow/d above 1500 kg DM/ha residual as perennial ryegrass and white clover pasture. The experiment was carried out over 21 days. Total DM intake was similar across treatments (P = 0.69), but milk production for cows supplemented with OS was lowest (1.01 kg MS/cow/day) compared with the RGS (control) or FB treatment that had similar milk yield (1.11 and 1.13 kg MS/cow/day respectively). The lower milk yield of OS compared with other supplements could be explained by the low ME of the supplement as well as the greater pasture mass offered to those cows. Autumn is an important time of the year to reduce N losses and supplementing with FB in late lactation reduced (P < 0.05) apparent N intake, milk urea N, (37, 40 and 44 g/dL) and spot urine N concentration (2.8 versus 3.9 and 4.0 g/L) compared to feeding OS or RGS respectively. These results demonstrated that cows on pasture offered low CP, and high-energy supplements, such as FB, during late lactation can sustain milk production while lowering N surplus. Further considerations were needed to understand why no milk response to FB was observed and to determine whether the quality of the product (milk composition) alters even though the quantity (milk yield) does not. In the third experiment, the purpose was to more clearly understand the lack of response in the first experiment by including grazing behavior measurements as well as the the effect of milk quality. Thirty-six early lactation dairy cows were fed perennial ryegrass-white clover (control) and supplemented 3 kg DM of FB or OF for 21 days. Again, as with experiment one there was no difference observed in total DMI (16.1 ± 0.52 kg DM/cow/day) or milk solids yield (1.9 ± 0.8 kg MS/cow/d) between treatments. Cows in each treatment spent similar amount of time grazing (413 ± 38.5 minutes/cow/d). There were supplement effects on milk quality. Milk fat and lactose (%) increased by feeding FB. The increase in fat % appeared to be due to increased short chain fatty acids. Though FB reduced the more valued long chain, (and some medium chain) fatty acids compared with the RGS control. Milk protein % was unaffected by supplement, but feeding FB increased casein concentration along with the minerals calcium and phosphorus. This suggests that cows supplemented with FB in early lactation may have been Ca deficient. As with previous experiments, cows that were supplemented with FB had lower N intake and lower urinary N concentration (2.4 g N/L) compared with OF and CON (4.3 and 3.1 g N/L). This resulted in, FB cows having a higher N use efficiency (36.2%) compared with OF and CON (29.5 and 29.8%, respectively). Given that there were little apparent milk yield benefits to supplementing with FB or oats during these short term grazing studies, follow up questions arose regarding the economic value of trying to integrate winter crops on the milking platform. To determine the feasibility and profitability of including winter supplements on the milking platform, results from experiments 1,2 and 3 were incorporated into a commercial decision support tool, FARMAX. Four scenarios were compared to a baseline farm system representing a Canterbury dairy farm. The scenarios were 1. Feeding FB only in spring, 2. Feeding FB (grazed) in autumn and drilling oats after grazing FB, which is grazed in spring, 3. Feeding oat forage only in spring and the last scenario was feeding oat silage in autumn. The results of the simulation showed that FB & OF (Autumn & Spring) scenario had the highest milk production (460 kg MS/ha) compared to (422, 418, 440 and 436 kg MS/ha) for baseline, FB spring, OF and OS scenarios respectively. Also, FB & OF (Autumn & Spring) scenario had lower cost per kg MS ($3.7 kg MS). The most profitable system at a $6.00/kg MS was FB & OF (Autumn & Spring) scenario on the milking platform. Results showed that despite there being no difference in herbage diet quality, supplementing FB & OF (A & S), scenario increased total DM offered and consumed (6.2%), thereby increasing milk production with 8.3% compared to (-1.6, 4.1 and 3.2%) for FB spring, OF and OS scenarios respectively with FB spring scenario that had the lowest milk production. Resulting in an increase of 10.1% in the gross margin profit/ha for FB & OF scenario compared to (6.0 and 4.4%) for OF and OS scenarios with the FB spring scenario having the lowest. In the short term grazing studies, supplementing pasture-fed dairy cows with FB or oats did not increase total DMI in early or late lactation due to high pasture substitution rates (SR) and similar energy intake. However, supplementing FB had effects on milk quality in both spring and autumn experiments. Only FB supplement lowered N intake (in spring) and improved N use efficiency for milk production as well as consistently lowering spot urine N concentration. Oats was more likely to reduce milk production when offered ensiled and may be more appropriate as a dry cow supplement. In both short term feeding and economic modeling, integrating FB supplement has value in sustaining milk production while lowering N losses in urine